Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface

Safe on MarsPrecursor Measurements Necessary to Support Human

Operations on the Martian Surface

Committee on Precursor Measurements Necessaryto Support Human Operations on the Surface of Mars

Aeronautics and Space Engineering BoardSpace Studies Board

Division on Engineering and Physical Sciences

National Research Council

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NOTICE: The project that is the subject of this report was approved by the Governing Board of theNational Research Council, whose members are drawn from the councils of the National Academyof Sciences, the National Academy of Engineering, and the Institute of Medicine. The members ofthe committee responsible for the report were chosen for their special competences and with regardfor appropriate balance.

This study was supported by Contracts No. NASW-96013 and No. NASW-99037 between theNational Academy of Sciences and the National Aeronautics and Space Administration. Anyopinions, findings, conclusions, or recommendations expressed in this publication are those of theauthor(s) and do not necessarily reflect the views of the agency that provided support for the project.

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Cover: “First Light,” by Pat Rawlings and commissioned by NASA, depicts the first human travelersto Mars exploring the enormous Noctis Labyrinthus canyon system. Just after sunrise, early morningfog masks the canyon floor 4 miles below. These scientists-explorers conduct geological andmeteorological research in order to help us better understand the characteristics of our sister planetand possibly our own Earth. Reproduced courtesy of the artist and NASA.

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COMMITTEE ON PRECURSOR MEASUREMENTS NECESSARY TO SUPPORTHUMAN OPERATIONS ON THE SURFACE OF MARS

FREDERICK H. HAUCK, Chair, AXA Space, Bethesda, MarylandHARRY Y. McSWEEN, JR., Vice Chair, University of Tennessee, KnoxvilleCYNTHIA BREAZEAL, Massachusetts Institute of Technology, CambridgeBENTON C. CLARK, Lockheed Martin Astronautics, Denver, ColoradoVON R. ESHLEMAN, Stanford University (retired), Palo Alto, CaliforniaJOHN HAAS, Applied Research Associates, South Royalton, VermontJON B. REID, Consultant to the Environmental Protection Agency, Cincinnati, OhioJONATHAN RICHMOND, Centers for Disease Control and Prevention, Atlanta, GeorgiaRONALD E. TURNER, ANSER Corporation, Arlington, VirginiaWILLIAM L. WHITTAKER, Carnegie Mellon University, Pittsburgh, Pennsylvania

Staff

DOUGLAS H. BENNETT, Study Director, Aeronautics and Space Engineering BoardSANDRA J. GRAHAM, Senior Program Officer, Space Studies BoardGEORGE LEVIN, Director, Aeronautics and Space Engineering BoardJOSEPH ALEXANDER, Director, Space Studies BoardBRIDGET EDMONDS (from July 2, 2001), Senior Project Assistant, Aeronautics and

Space Engineering BoardMARY LOU AQUILO (February 1, 2001, until July 2, 2001), Senior Project Assistant,

Aeronautics and Space Engineering BoardANNA L. FARRAR, Financial Associate, Aeronautics and Space Engineering BoardVERNELLE C. MENKIR, Financial Associate, Space Studies Board

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AERONAUTICS AND SPACE ENGINEERING BOARD

WILLIAM W. HOOVER, Chair, United States Air Force (retired), Williamsburg, VirginiaA. DWIGHT ABBOTT, Aerospace Corporation (retired), Los Angeles, CaliforniaRUZENA K. BAJSCY, NAE, IOM, National Science Foundation, Arlington, VirginiaWILLIAM F. BALLHAUS, JR., NAE, Aerospace Corporation, Los Angeles, CaliforniaJAMES BLACKWELL, Lockheed Martin Corporation (retired), Atlanta, GeorgiaANTHONY J. BRODERICK, Aviation Safety Consultant, Catlett, VirginiaDONALD L. CROMER, United States Air Force (retired), Lompoc, CaliforniaROBERT A. DAVIS, The Boeing Company (retired), Seattle, WashingtonJOSEPH FULLER, JR., Futron Corporation, Bethesda, MarylandRICHARD GOLASZEWSKI, GRA Inc., Jenkintown, PennsylvaniaJAMES M. GUYETTE, Rolls-Royce North America, Reston, VirginiaFREDERICK H. HAUCK, AXA Space, Bethesda, MarylandJOHN L. JUNKINS, NAE, Texas A&M University, College StationJOHN K. LAUBER, Airbus Industrie of North America, Washington, D.C.GEORGE K. MUELLNER, The Boeing Company, Seal Beach, CaliforniaDAVA J. NEWMAN, Massachusetts Institute of Technology, CambridgeJAMES G. O’CONNOR, NAE, Pratt & Whitney (retired), Coventry, ConnecticutMALCOLM R. O’NEILL, Lockheed Martin Corporation (retired), Bethesda, MarylandCYNTHIA SAMUELSON, Opsis Technologies, Springfield, VirginiaWINSTON E. SCOTT, Florida State University, TallahasseeKATHRYN C. THORNTON, University of Virginia, CharlottesvilleDIANNE S. WILEY, The Boeing Company, Long Beach, CaliforniaTHOMAS L. WILLIAMS, Northrop Grumman, El Segundo, California

GEORGE LEVIN, Director

SPACE STUDIES BOARD

JOHN H. McELROY, Chair, University of Texas at Arlington (retired)ROGER P. ANGEL, University of ArizonaJAMES P. BAGIAN, Veterans Health Administration’s National Center for Patient SafetyJAMES L. BURCH, Southwest Research InstituteRADFORD BYERLY, JR., University of ColoradoROBERT E. CLELAND, University of WashingtonHOWARD M. EINSPAHR, Bristol-Myers Squibb Pharmaceutical Research InstituteSTEVEN H. FLAJSER, Loral Space and Communications, Ltd.MICHAEL H. FREILICH, Oregon State UniversityDON P. GIDDENS, Georgia Institute of Technology/Emory UniversityRALPH H. JACOBSON, Charles Stark Draper LaboratoryCONWAY LEOVY, University of WashingtonJONATHAN I. LUNINE, University of ArizonaBRUCE D. MARCUS, TRW (retired)RICHARD A. McCRAY, University of ColoradoHARRY Y. McSWEEN, JR., University of TennesseeGARY J. OLSEN, University of IllinoisGEORGE A. PAULIKAS, The Aerospace Corporation (retired)ROBERT J. SERAFIN, National Center for Atmospheric ResearchEUGENE B. SKOLNIKOFF, Massachusetts Institute of TechnologyMITCHELL SOGIN, Marine Biological LaboratoryC. MEGAN URRY, Yale UniversityPETER W. VOORHEES, Northwestern University

JOSEPH K. ALEXANDER, Director

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Preface

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Humankind’s fascination with Mars predatesrecorded history. The bright planet with the reddish tintis unique among the other celestial objects. TychoBrahe’s observations of its unpredictable motion weredeciphered by Johannes Kepler in the early 17th cen-tury as he developed his laws of planetary motion.Galileo trained his telescope on Mars and saw it as adisk in 1610. Later in the 1600s, Christiaan Huygensand Gian Cassini drew the first maps of the Martiansurface. In the late 18th century, Sir William Herschel,astronomer to King George III, measured the tilt of theplanet’s axis and noted the Martian atmosphere and itsseasons. As recently as the beginning of the 20th cen-tury, the respected American astronomer PercivalLowell was writing popularly about Martians populat-ing a planet hospitable to a life-form, if not to a humanlife-form.

During the space race of the late 20th century, U.S.and Soviet space programs sent the Mariner, Viking,and Mars probes to study the planet during fly-bys,from orbit, and on the Martian surface. In July 1997,the Mars Pathfinder spacecraft of the National Aero-nautics and Space Administration (NASA) landed onMars and released its tiny rover, Sojourner Truth.Anyone with access to the Internet could monitor itsmeanders, see the Martian landscape through its eyes,and get updates on the Martian weather.

Debate as to which agent, robot or human, is likelyto reap the greatest rewards in the future exploration ofMars is outmoded and has evolved in the last decadeinto a discussion of how the two may complement each

other.1,2 In pursuing answers to this question, NASAhas channeled the energies of the robotic and humanexploration communities to “optimize the use ofhumans and robots to increase the pace of discovery atmultiple destinations.”3 It sponsored the present studyto assist it in validating the requirements identified bythese communities, specifically as they relate to thepreparation for human exploration of Mars. The state-ment of task for this study is included as Appendix A.

The Committee on Precursor Measurements Neces-sary to Support Human Operations on the Surface ofMars was confronted with the dilemma of beingcharged to “emphasize those technological issueswhich are directly relevant to managing environmental,chemical, and biological risks to humans operating onMars” while recognizing that a major objective of suchhuman missions will certainly be to search for (possiblyhazardous) life on Mars. The committee took theapproach of addressing only the earliest human mis-sions to Mars, when the unknowns are the greatest andthe steps taken must be the most cautious.

The members of the committee (see Appendix B)were appointed by the National Research Council

1NRC (National Research Council), 1993, Scientific Prerequi-sites for the Human Exploration of Space, National Academy Press,Washington, D.C.

2NRC, 1994, Scientific Opportunities in the Human Explorationof Space, National Academy Press, Washington, D.C.

3James Garvin, NASA, “Human Exploration Vision,” briefingto the committee on May 30, 2001.

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(NRC). They were chosen for their expertise and abilityto provide independent judgments, thereby fulfillingthe study charter.

This report has been reviewed in draft form by indi-viduals chosen for their diverse perspectives andtechnical expertise, in accordance with proceduresapproved by the NRC’s Report Review Committee.The purpose of this independent review is to providecandid and critical comments that will assist the insti-tution in making its published report as sound aspossible and to ensure that the report meets institutionalstandards for objectivity, evidence, and responsivenessto the study charge. The review comments and draftmanuscript remain confidential to protect the integrityof the deliberative process. We wish to thank the fol-lowing individuals for their review of this report:

Vincent Castranova, National Institute for Occupa-tional Safety and Health,

Christopher Chyba, SETI Institute,Pamela Conrad, Jet Propulsion Laboratory,Ann Druyan, Cosmos Studios,Helen Evans, Case Western Reserve University,Stephen Gorevan, Honeybee Robotics,Noel Hinners, Lockheed Martin Astronautics,Andrij Holian, University of Montana,Glenn MacPherson, United States National Museum

of Natural History,

Jeffrey Streator, Georgia Institute of Technology,Lawrence Townsend, University of Tennessee, andWard Winer, Georgia Institute of Technology.

Although the reviewers listed above provided manyconstructive comments and suggestions, they were notasked to endorse the conclusions or recommendations,nor did they see the final draft of the report before itsrelease. The review of this report was overseen byLouis Lanzerotti (NAE) of Bell Laboratories, LucentTechnologies. Appointed by the National ResearchCouncil, he was responsible for making certain that anindependent examination of the report was carried outin accordance with institutional procedures and that allreview comments were carefully considered. Respon-sibility for the final content of this report rests entirelywith the authoring committee and the institution.

The committee also wishes to thank those in NASAwho were so thorough in informing the committee andNRC staff, who facilitated the entire process. The com-mittee would particularly like to recognize the effortsof the study director, Douglas Bennett, who diligentlykept us on course and on time.

Frederick H. Hauck, ChairCommittee on Precursor Measure-ments Necessary to Support HumanOperations on the Surface of Mars

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EXECUTIVE SUMMARY 1

1 INTRODUCTION 7Study Approach, 8Organization of This Report, 8Reference, 8

2 THE MARS PROGRAM IN CONTEXT 9Scope of This Report, 10Rover Technologies and Robotics, 10Establishing Risk Standards, 12References, 14

3 PHYSICAL ENVIRONMENTAL HAZARDS 15Geologic Hazards, 15Hazards from Atmospheric Dynamics, 20Radiation Hazards, 23References, 26

4 CHEMICAL ENVIRONMENTAL HAZARDS 28Chemical Interaction of Martian Soil and Airborne Dust with Astronauts

and Critical Equipment, 28Toxicity of Martian Atmospheric Gases, 36References, 36

5 POTENTIAL HAZARDS OF THE BIOLOGICAL ENVIRONMENT 37Ensuring the Safety of Astronauts, 37Ensuring the Safety of Earth’s Biosphere, 38Return Vehicle Contamination, 43References, 43

Contents

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APPENDIXESA Statement of Task 47B Biographical Sketches of Committee Members 48C Acronyms and Abbreviations 51

TABLES

2.1 Exposure Limits for Some Respirable Chemical Hazards, 133.1 Compositions of Five Different Martian Regolith Scenarios, 253.2 Effect of Hydrogen and Iron Content on Absorbed Radiation Dose, 254.1 Representative Listing of Reference Concentrations for Cancer-Causing

Compounds and for the Noncancerous Effects of Those Compounds fromEPA’s IRIS Database, 29

4.2 Element Detection Capability on the Viking Landers, 304.3 Toxic Metal Inhalation Risk, 314.4 EPA National Ambient Air Quality Standards for Particulate Concentrations, 33

FIGURE

5.1 Mars biology testing protocol, 42

BOXES

2.1 The Anatomy of a Critical System, 112.2 Exposure Limits for Chemical Hazards, 133.1 Definitions of Martian Regolith, Soil, and Dust, 16

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Tables, Figure, and Boxes

1

This study, commissioned by the National Aero-nautics and Space Administration (NASA), examinesthe role of robotic exploration missions in assessingthe risks to the first human missions to Mars. Onlythose hazards arising from exposure to environmental,chemical, and biological agents on the planet areassessed.

To ensure that it was including all previously identi-fied hazards in its study, the Committee on PrecursorMeasurements Necessary to Support Human Opera-tions on the Surface of Mars referred to the most recentreport from NASA’s Mars Exploration Program/Payload Analysis Group (MEPAG) (Greeley, 2001).The committee concluded that the requirements identi-fied in the present NRC report are indeed the only onesessential for NASA to pursue in order to mitigatepotential hazards to the first human missions to Mars.

THE MARS PROGRAM IN CONTEXT

Even though NASA is actively pursuing a Marsexploration program, it is not yet actively pursuing ahuman mission to Mars, and there is no officiallyselected reference human exploration mission. Accord-ingly, the committee determined that it might best assistNASA by assuming that a long-stay mission to Marswill take place, as such a mission would levy the morestringent demand for the safety of astronauts while inthe Martian environment. The reader should not con-clude that this assumption implies an endorsement ofthe long-stay mission as a baseline mission, nor thatthe committee concluded that the long-stay mission is,in total, the least hazardous option.

Executive Summary

In its review of the Mars robotic program, the com-mittee found that NASA has done an excellent job ofdesigning science rovers capable of operating on thesurface of Mars. The committee believes, however, thatthe engineering knowledge being gained from thescience rover experience will not scale up nor will iteasily apply to human assistant rovers or larger humantransport rovers. Furthermore, the committee notes thatcurrent science rover activities do not provide an ade-quate research base for the development of roversneeded for the human exploration of Mars.

NASA has allocated risk factors and reliabilityrequirements for missions in low Earth orbit and forthe International Space Station but has not done so formissions traveling beyond Earth orbit.

Recommendation: Because NASA has not allocatedrisk factors and reliability requirements formissions beyond Earth orbit, it should establish therisk standards necessary to provide preliminaryguidance to Mars mission planners and hardwaredesigners.

The concept of acceptable risk involves ethical,psychological, philosophical, and social consider-ations. The committee relied instead on standard risksources. In reviewing the toxicology risk estimates fortoxic metals, the committee chose to use an acceptablerisk range (ARR) rather than a single risk level. In thisreport, the ARR for developing cancer as a result ofexposure to toxic metals is between 1 in 10,000 and 1in 100,000. The committee understands certain risksmay overshadow others. Regardless of the large differ-

2 SAFE ON MARS

ence between the risk of getting fatal cancer from radia-tion and the cancer risk from exposure to toxic metals,it is prudent to reduce risk in all areas that are amenableto such reductions. It is important to reduce risks inareas that are reasonably achievable, as there can besynergistic effects of combined hazards.

PHYSICAL ENVIRONMENTALHAZARDS ON MARS

The committee categorized the hazards on Mars bytheir sources, or causes. It specifically defined thephysical hazards on Mars separately from the chemicaland biological hazards, because physical hazards canthreaten crew safety by physically interacting withhumans or critical equipment, resulting, for example,in impact, abrasion, tip-over (due to an unstableMartian surface), or irradiation.

Geologic Hazards

To ensure safe landing and operations on the surfaceof Mars, it is necessary for the landing site and thetopography of the anticipated surface operation zone tobe fully characterized with high-resolution stereoscopicimaging. The operation zone is the area around thelanding site defined by the anticipated range of opera-tions of extravehicular activities (EVAs), including theuse of human transport and/or science rovers. The levelof resolution required of this imaging will be deter-mined by the capabilities of the equipment to be usedon the surface.

Recommendation: NASA should map the three-dimensional terrain morphology of landing opera-tion zones for human missions to characterize theirfeatures at sufficient resolution to assure safe land-ing and human and rover locomotion.

Recommendation: To ensure that humans andcritical rover systems can land on and traverse theMartian surface in a safe, efficient, and timelymanner, NASA should characterize the range ofmechanical properties of the Martian regolith at thelanding site or comparable terrain. Specifically, insitu experiments should be performed to determinethe regolith’s aggregate strength, stability, andsinkage properties, including bearing strength, bulkmodulus, yield strength, and internal friction angle.

Recommendation: NASA should determine, inadvance of human missions to Mars, rock size dis-tribution and shapes in situ, at the landing site or oncomparable terrain, in order to predict human androver trafficability.

The abrasive properties of rocks on Mars, includinghardness and surface roughness (as dictated by rockgrain size and shape), are unknown. The committeebelieves that, even faced with this lack of knowledge,NASA can still design systems by making certain edu-cated assumptions about the rocks on Mars. For thisreason, no further in situ experiments to determine theabrasive properties of Martian rocks are required.

Airborne dust presents a potentially significanthazard to human operations on the surface of Mars.Dust intrusion and accumulation will need to be con-tinuously monitored and will require well-designedfilter systems and periodic housecleaning. Afterreviewing NASA’s experience with dust on the Moonand Mars, the committee is confident that NASA engi-neers and scientists will be able to design and buildsystems to mitigate the hazards posed by airborne duston Mars. Some systems that would be used on the firsthuman mission can be designed either by employingwhat is currently known about Mars dust or by assum-ing a worst-case scenario in the design process.

The present Mars soil simulant that has been devel-oped and characterized by NASA for engineering (JSCMars-1: Martian Regolith Simulant) is not adequate fortesting mechanical systems for human missions toMars. However, the committee does not recommendthat any precursor in situ measurements be taken onMars to characterize the mechanical and abrasive prop-erties of airborne dust. Rather, it expects that an appro-priate simulant would adequately stress the design ofany mechanical and seal systems that will be used dur-ing a human mission to Mars. It is critical, however, tofully characterize the adhesive properties of airbornedust in order to design systems that minimize the riskof failure resulting from dust accumulation.

Recommendation: NASA should determine theadhesive properties of Martian soil and airbornedust in order to evaluate the effects of dust adhesionon critical systems. This characterization must beconducted in situ by means of experiments to mea-sure airborne dust adhesion.

EXECUTIVE SUMMARY 3

Hazards from Atmospheric Dynamics

The dry conditions and uncertainty about conduc-tivity, charging, and discharging rates in the Marsenvironment create uncertainties about electrostaticeffects on human operations in the Mars environment.However, even given the potential hazards, the com-mittee believes that the risk to humans from electro-static charging on the surface of Mars can be managedthrough standard design practice and operational pro-cedures.

The committee believes that in light of the lowdynamic atmospheric pressures experienced on Mars,no further characterization of wind speed on Mars isrequired prior to the first human mission. The surfacewinds are sufficiently characterized to allow systemdesigners to ensure human safety on the planet bymeans of robust designs.

Radiation Hazards

Radiation exposure in space will be a significant andserious hazard during any human expedition to Mars.There are two major sources of natural radiation in deepspace: sparse but penetrating galactic cosmic radiation(GCR) and infrequent but very intense solar particleevents (SPEs) associated with solar storms.

There have been no direct measurements of theradiation environment on the surface of Mars. Rather,the radiation environment is estimated using computercodes that model the transport of the deep space radia-tion through the Martian atmosphere and after its inter-actions with the Martian surface. Because of the cen-tral role of radiation transport and absorbed dosemodels in the planning and design of human missionsto Mars, it is important that the code predictions bevalidated by means of a precursor experiment on thesurface of Mars. Radiation risk mitigation strategieswill be an integral part of overall mission design andplanning. Should the results of the in situ experimentprove that the radiation transport models are flawed,more time will be needed to adjust the models toaccount for the differences between the models and themeasurement.

Recommendation: In order to validate the radia-tion transport codes, thereby ensuring the accuracyof radiation dose predictions, NASA should performexperiments to measure the absorbed dose in atissue-equivalent material on Mars at a location

representative of the expected landing site, includ-ing altitude and bulk elemental composition of thesurface. The experiments should distinguish theradiation dose contribution induced by chargedparticles from that induced by neutrons. Theseexperiments should be made a priority in the Marsexploration program.

CHEMICAL ENVIRONMENTALHAZARDS ON MARS

In addition to the hazards from materials on Marsinteracting dynamically with humans or critical sys-tems, the committee has also assessed hazards associ-ated with the chemical reactivity of materials on Mars.

Chemical Interaction of Martian Soil and AirborneDust with Astronauts and Critical Equipment

Some dust and soil will in all probability be broughtinto the habitat through the airlock by returning astro-nauts, as was the case during the Apollo missions to theMoon. The committee has concluded that Martian air-borne dust could present the same chemical hazards asMartian soil, so soil and dust should be characterizedin the same way. In choosing the “worst” toxic chemi-cal hazards to humans, the committee considered inor-ganic substances separately from organic substances.With respect to inorganic substances, it identified cer-tain toxic metals as the worst threat to humans at thelowest concentrations.

Soil and airborne dust on Mars could contain traceamounts of hazardous chemicals, including compoundsof toxic metals, which are known to cause cancer overthe long term if inhaled in sufficient quantities. IfNASA protects astronauts against the risk of develop-ing cancer in the long term as a result of having beenexposed to particulate matter on Mars, NASA will alsobe protecting astronauts from acute and short-termnoncancer effects that could potentially interfere withmission success. While the committee is confident inits knowledge of the possible concentrations of mosttoxic metals on Mars, the committee believes theuncertainty surrounding the amount of toxic hexavalentchromium (Cr VI) on Mars warrants a precursormeasurement. Hexavalent chromium on Earth is veryrare in natural materials, but the great abundance ofchromium (in unknown form) on the surface of Mars,combined with the high oxidation state of Martian soil,

4 SAFE ON MARS

suggests that hexavalent chromium might be present insmall but potentially hazardous amounts.

Recommendation: In order to evaluate if hexavalentchromium on Mars poses a threat to astronauthealth, NASA should conduct a precursor in situmeasurement to determine if hexavalent chromiumis present in Martian soil or airborne dust at morethan 150 parts per million (ppm). This measurementmay take place anywhere on Mars where well-mixed, uniform airborne dust is present. If such ameasurement is not possible, a sample of airbornedust and fine particles of Martian soil must bereturned to Earth for evaluation.

The committee believes that NASA can providefiltration systems capable of minimizing the hazards ofexposure to toxic elements, including hexavalentchromium, arsenic, and cadmium, that are present atconcentrations of less than 150 ppm.

However, if a filtration system cannot be designedto limit the average astronaut respirable particulateinhalation exposure to 1 milligram of particulate matterper cubic meter of air (mg/m3) in the habitat, then asample of airborne dust, taken from the Martian atmo-sphere, and soil must be analyzed for toxic metal con-centrations. The level of analytical precision requiredfor this measurement will be dictated by the filtrationcapability of the astronauts’ habitat.

It should be very clear to the reader that, in the viewof the committee, the 1 mg/m3 specification is the maxi-mum acceptable respirable particle average concentra-tion to which astronauts should be exposed. Thisconcentration level will protect astronauts from expo-sure to toxic metals, which—of all inorganic chemi-cals—the committee considers to pose the greatesthealth risk to astronauts. Filtering at or below therecommended 1 mg/m3 average with a 1.5 mg/m3 peakconcentration should be readily achievable for NASA.Indeed, to minimize risks from exposure, the commit-tee strongly believes that filtering should be imple-mented below 1 mg/m3, to as low a concentration as isreasonably achievable in the Martian habitat.

It is essential that NASA implement proper humidi-fication in conjunction with the filtration system as partof habitat atmosphere conditioning to mitigate thethreat of strong oxidants in Martian soil and airbornedust. The committee concluded that even if strong oxi-dants are present, there will be negligible risk associ-ated with oxidation on the Martian surface if the proper

humidification systems are in place and the particulatelevel is maintained at 1 mg/m3 or less.

However, even with the filtering systems in thehabitat as discussed above, the filtration level may notbe stringent enough to protect astronaut health and criti-cal mechanical equipment from dust and soil that areextremely acidic. There are high concentrations ofsulfur and chlorine in Martian soil, which implies thepossibility of acidity in both the soil and airborne dust(Clark et al., 1982; Wanke et al., 2001). When inhaledby astronauts, acidic soil and dust could degrade theirlung tissue and, if humidified and allowed to penetratecontrol units inside the habitat, could corrode sensitivecritical equipment, such as control circuits.

Recommendation: In order to evaluate the poten-tial corrosive effects of Martian soil and airbornedust on humans and critical systems in a humidifiedenvironment, NASA should measure the pH andbuffer capacity of soil and airborne dust either viaan in situ experiment or on Earth with returnedsamples of soil and airborne dust collected from theMartian atmosphere.

If NASA decides not to implement the necessaryengineering controls or for other science-related rea-sons chooses to measure the oxidation properties ofMartian airborne dust and soil, then the measurementshould be performed on the surface of Mars rather thanvia a sample return.

Certain organic compounds can be highly toxic tohumans, even if those compounds are not associatedwith a life-form, and the threat should be evaluated inplanning the first human mission to Mars. Any hazardfrom organic compounds would most likely come fromhandling subsurface samples that might contain organiccompounds. The committee concludes that if organiccarbon is present at a concentration of more than150 ppm in soil to which astronauts might be exposed,a possible threat exists. Filtration systems that reduceastronaut exposure to organic carbon to concentrationsless than 150 ppm would mitigate this threat. If experi-ments determine that organic carbon is present in con-centrations greater than 150 ppm, the subsurface soilshould be considered a toxic hazard until proven other-wise. The need to assess the potential threat posed by ahazardous life-form consisting of organic carbonrequires a more stringent measurement of organiccarbon concentration.

EXECUTIVE SUMMARY 5

Toxicity of the Martian Atmospheric Gases

The Martian atmosphere, when mixed in smallamounts with the habitat atmosphere, does not pose atoxic risk for astronauts, and no further characteriza-tion is required before the first human mission takesplace. The primary hazardous components are easilyremoved by standard cabin-atmosphere conditioningsystems.

POTENTIAL HAZARDS OF THE BIOLOGICALENVIRONMENT ON MARS

The committee was charged with addressing issuesof biological risks on Mars from two perspectives:(1) ensuring the safety of astronauts operating on thesurface of Mars and (2) ensuring the safety of Earth’sbiosphere with respect to potential back-contaminationfrom returning human missions.

The probability that life-forms exist on the surfaceof Mars (that is, the area exposed to ultraviolet radia-tion and its photochemical products) is very small.However, as a previous NRC study (NRC, 1997) notes,there is a possibility that such life-forms exist there “inthe occasional oasis,” most likely where liquid water ispresent, and, furthermore, that “uncertainties withregard to the possibility of extant Martian life canbe reduced through a program of research andexploration.”

This charge to the committee results in a dilemma.How can NASA use human ingenuity and creativity onMars to search for life when that life (if it exists) maypose a threat to astronaut health and safety (and there-fore the success of a human mission) as well as toEarth’s biosphere?

Ensuring the Safety of Astronauts

The committee believes it is highly unlikely thatinfectious organisms are present on Mars. Neverthe-less, once an astronaut has been directly exposed tosuch life, it would be very difficult, to the point of beingimpractical, to determine conclusively that the astro-naut would not pose a contamination threat to Earthlife. In such an event, NASA might be faced withrequiring quarantine and surveillance of returningastronauts until it is determined that a threat no longerexists.

Ensuring the Safety of Earth’s Biosphere

While the threat to Earth’s ecosystem from therelease of Martian biological agents is very low, “therisk of potentially harmful effects is not zero” and can-not be ignored (NRC, 1997). NASA should assume thatif life exists on Mars, it could be hazardous to Earth’sbiosphere until proven otherwise. As such, NASAshould ensure proper quarantine or decontamination ofequipment that may have been exposed to a Martianlife-form.

To protect Earth from contamination by Martian life-forms aboard a returning human mission and astronautswhile they are on the surface of Mars, the committeerecommends that NASA employ the concept of zonesof minimal biologic risk (ZMBRs) for astronaut explo-ration. These zones, operational areas on the surface ofMars, would be determined, to the maximum extentpracticable, to be devoid of life or to contain only life-forms that would not be hazardous to humans or Earth’sbiosphere.

The committee recognizes that the requirement toestablish and operate in a ZMBR, while intrinsic to thestudy charter to manage risk to astronauts, may be inconflict with one of the primary goals of the explora-tion of Mars: to find extraterrestrial life.

To establish a ZMRB, NASA should first attempt todetermine whether or not life exists (1) at the physicallocations where astronauts will be operating and (2) inthe Martian material to which astronauts will beexposed. The establishment of a ZMBR might initiallybe based on an in situ testing protocol conducted priorto the first human visit. Once a landing site is estab-lished as a ZMBR, the astronauts can land and freelyoperate within it.

While some have suggested that non-carbon-basedlife might be present on Mars, this committee agreeswith assumptions made by previous NRC committeesthat should hazardous life exist on Mars it would becarbon-based and thus would contain organic com-pounds (NRC, 2002a, 2002b). A search for life shouldtherefore include a search for organic carbon. Thedetection of organic carbon might indicate the pres-ence of life-forms.

If a sample of Martian soil and airborne dust isreturned to fulfill this requirement, the returned sampleshould be considered hazardous and NASA shouldfollow quarantine procedures as outlined in previous

6 SAFE ON MARS

NRC studies (NRC, 2002b). The committee also urgesNASA to set an operational value for the life detectionthreshold limit through a separate advisory processdrawing on a broad range of relevant expertise.

Recommendation: The committee recommends thatNASA establish zones of minimal biologic risk(ZMBRs) with respect to the possible presence ofMartian life during human missions to Mars. Inorder to do so, NASA should conduct a precursor insitu experiment at a location as reasonably close tothe human mission landing sites as possible todetermine if organic carbon is present. The mea-surement should be on materials from the surfaceand down to a depth to which astronauts may beexposed. If no organic carbon is detected at orabove the life detection threshold, the landing sitemay be considered a ZMBR. If no measurementtechnique can be used to determine if organiccarbon is present above the life detection threshold,or if organic carbon is detected above that thresh-old, a sample should be returned to Earth for char-acterization prior to sending humans to Mars.

There has been some concern that if a sample returnis required, the planning for the first human mission toMars may be delayed until a sample can be obtained.The committee believes that, even should a sample berequired because organic carbon has been found, abaseline mission plan for a mission to Mars and evenhardware development may still proceed under theassumption that a sample return will not find anything

significant enough with regard to Martian biology toinvalidate the baseline mission plan.

Return Vehicle Contamination

To prevent contamination of Earth by Martian mate-rial, great care must be exercised to ensure the contain-ment of all material returned from Mars to Earth. Theremust be a sterile, intermediate transfer conducted inspace that ensures Earth’s environment will not beexposed to any Martian material, including dust or soildeposits on the outside surface of the return vehicle.The protocols for such a sterile transfer will be com-plex and, if the transfer is unsuccessful, may requirethat the return vehicle be discarded in space and neverreturned to Earth. Ultimately, however, only containedmaterials should be transported back to Earth, unlesssterilized first (NRC, 1997).

REFERENCESClark, B.C., A.K. Baird, R.J. Weldon, D.M. Tsuasaki, L. Schnabel, and

M.P. Candelaria. 1982. “Chemical Composition of Martian Fines.”Journal of Geophysical Research 87:10,059-10,067.

Greeley, R., ed. 2001. Scientific Goals, Objectives, Investigations, and Pri-orities, Mars Exploration Program/Payload Analysis Group (MEPAG),March 2. Also known as Jet Propulsion Laboratory (JPL) Publication01-7 (2001). JPL, Pasadena, Calif.

National Research Council (NRC). 1997. Mars Sample Return: Issues andRecommendations. National Academy Press, Washington, D.C.

NRC. 2002a. Assessment of Mars Science and Mission Priorities. NationalAcademy Press, Washington, D.C.

NRC. 2002b. The Quarantine and Certification of Martian Samples.National Academy Press, Washington, D.C.

Wanke H., J. Bruckner, G. Dreibus, R. Rieder, and I. Ryabchikov. 2001.“Chemical Composition of Rocks and Soils at the Pathfinder Site.”Space Studies Review 96: 317-330.

7

1

Introduction

NASA is actively pursuing a Mars exploration pro-gram. It is a “science-driven, technology-enabled effortto characterize and understand Mars, including itscurrent environment, climate, and geological historyand biological potential.”1 Every 2 years from 2001 to2011, with the dates dictated by launch windows,another spacecraft, launched by NASA and/or NASA’sinternational partners, is intended to visit Mars. Somespacecraft will orbit the planet, while others will landon the Martian surface. The NASA Mars ExplorationProgram Office (within the NASA headquarters Officeof Space Science) has established the Mars Explora-tion Program/Payload Analysis Group (MEPAG), con-sisting of more than 110 individuals from the Marscommunity, with representatives from universities,research centers and organizations, industry, and inter-national partners. The MEPAG participants propose theobjectives, investigations, and measurements neededfor the eventual exploration of Mars, focusing on fourprincipal exploration goals (Greeley, 2001). Thesegoals fall under four broad categories:

• Life—determine if life ever arose on Mars.• Climate—determine the climate on Mars.• Geology—determine the evolution of the surface

and interior of Mars.• Prepare for the eventual human exploration of

Mars.

While there is currently no funded human missionto Mars, nor even a baseline reference human mission,one of the goals of the MEPAG is to ensure that suffi-cient information is developed in a timely manner tosupport such a mission, once it has been funded.

NASA commissioned this study from the NRC toexamine what measurements must be made on Marsprior to the first human mission. These measurementswould provide information about the risks to humansso that NASA scientists and engineers can design sys-tems that will protect astronauts on the surface of Mars.

The principal objective of the study was to examinehow robotic exploration missions sent to Mars couldaid NASA in assessing the risks to astronauts posed bypossible environmental, chemical, and biologicalagents on the planet. Of critical importance is whetherit will be necessary to return Martian soil and/or air-borne dust samples to Earth prior to the first humanmission to Mars to assure astronaut health and safety.The entire statement of task is contained in Appendix A.The statement of task includes a list of relevant reportsthat were reviewed during the course of the study.

To respond to this task, NRC’s Aeronautics andSpace Engineering Board, together with the SpaceStudies Board, established the Committee on PrecursorMeasurements Necessary to Support Human Opera-tions on the Surface of Mars. Brief biographies of the10 committee members are included in Appendix B.Members were carefully chosen to reflect the expertiseneeded to address the environmental, chemical, andbiological risks to the first humans to set foot on Mars.

1J. Cutts, Jet Propulsion Laboratory, Presentation to the roboticsubcommittee (committee members Breazeal, Hauck, and Whittaker),Pasadena, Calif., on August 27, 2001.

8 SAFE ON MARS

STUDY APPROACH

The committee held three full committee meetings,all of them open to the public. The first meeting washeld in Washington, D.C., in May 2001. At the first2-day meeting, NASA presented its overall strategy forfuture Mars exploration and the current status of theprogram. From that initial briefing the committee thendetermined that it would need to hear further details ofspecific NASA technical capabilities and hazards onMars. The second meeting took place over 3 days at theLunar and Planetary Institute in Houston, Texas, dur-ing early August 2001. Representatives from NASA’sLyndon B. Johnson Space Center in Houston and fromthe Jet Propulsion Laboratory (JPL) in Pasadena pro-vided information on surface operations, space suitsand equipment, detection of life on Mars, radiationhealth effects, lunar dust experience, Martian soil, andin situ instrumentation capability. The issue of whetheror not there is need for a soil or airborne dust sample tobe returned to Earth from Mars was also discussed.

Subsequently, the committee determined that furtherinformation was needed on the Mars robotic program.Three committee members, accompanied by NRCstaff, visited JPL in late August 2001, when they werebriefed on current research and development efforts inrobotics.

The third and final meeting was held in Washington,D.C., in early September 2001; at that meeting the com-mittee finalized the findings and recommendations con-tained in this report.

To ensure that it was including all previously identi-fied hazards in its study, the Committee on PrecursorMeasurements Necessary to Support Human Opera-tions on the Surface of Mars referred to the most recentMEPAG report (Greeley, 2001). The committee con-

cluded that the requirements identified in the presentreport are indeed the only ones essential for NASA topursue in order to mitigate potential hazards to the firsthuman missions to Mars.

ORGANIZATION OF THIS REPORT

Chapter 2 presents the context in which this studywas conceived and conducted. Chapters 3, 4, and 5address the physical, chemical, and biological hazardslikely to be encountered by the first human visitors tothe Martian surface. Each of these chapters addressesthe risks associated with potential hazards. In eachchapter there is a section that details what precursormeasurements, if any, should to be made prior to thefirst human landing on Mars. Appendix C lists theacronyms and abbreviations used in this report.

As a general rule, the committee made recommen-dations only when it determined that a precursor mis-sion to Mars is required to provide information criticalfor the safety of the first human missions. The excep-tion to this rule is the first recommendation, whichdeals with the establishment of risk standards, becausethe risk standard adopted has the potential to greatlyaffect the need for precursor missions. The committeepresented findings if, in its judgment, there are ways ofensuring the safety of astronauts without carrying out arobotic precursor mission to Mars.

REFERENCEGreeley, R., ed. 2001. Scientific Goals, Objectives, Investigations, and Pri-

orities, Mars Exploration Program/Payload Analysis Group (MEPAG),March 2. Also known as Jet Propulsion Laboratory (JPL) Publication01-7 (2001). JPL, Pasadena, Calif.

9

2

The Mars Program in Context

Sending astronauts to the Red Planet, having themland, conduct a mission on the surface, and then returnsafely to Earth will be an enormous undertaking. Themission will be broken into a transit phase, that portionof the journey that takes place in deep space whenastronauts are traveling from Earth to Mars, and thesurface phase, when astronauts are resident on the sur-face of the planet. The time it takes to complete anymission to Mars depends on the relative positions ofEarth and Mars in their orbits. This means that thereare specific launch windows, preferred instances whenMars and Earth are ideally positioned in their respec-tive orbits, for a human mission to take place. Theduration of the mission also depends on the type ofpropulsion used. Depending on the alignment of Earthwith Mars and the amount of propulsive energy avail-able, two basic types of missions can take place, a long-stay mission and a short-stay mission. The names referqualitatively to the amount of time astronauts spend onthe Martian surface.

A long-stay mission would require that astronautsspend 16 to 20 months in orbit around Mars or on thesurface, with total mission duration being 21/2 to 3 years.On a short-stay mission, astronauts would be able toremain in orbit around Mars or on the surface for only30 to 45 days before they would have to embark on thereturn journey to Earth. If they stayed longer, Earth andMars would move out of optimum alignment and thereturn to Earth would require an excessive amount ofpropellant.

There are many safety and mission elements thatmust be considered in deciding whether to spend 30 to45 days (short stay) or 16 to 20 months (long stay) in

orbit around Mars or on the surface. Some of theseinclude the following:

• Transit time. The short-stay mission wouldrequire a longer round-trip transit time in space.A short-stay mission scenario would have astro-nauts traveling in space for 11 to 21 months,versus 10 to 14 months for a long-stay mission.Unless provisions can be made to counter themicrogravity environment (by means of exerciseprotocols or by inducing artificial gravity) andharsh radiation conditions in space, the potentialnegative effects on health of the longer transittime (short-stay mission) may be prohibitive. Infact, the cumulative effects of radiation on theastronauts during the shorter transits involved inthe long-stay mission might be more benign thanthose for the short-stay mission because of theshielding provided by the planetary mass andatmosphere while on the Martian surface(Cucinotta et al., 2001). However, the effects onastronaut health of a long duration stay in the low-gravity Martian environment are unknown.

• Closest approach to the Sun. The short-stay mis-sion would result in transits that bring the space-craft closer to the Sun (inside the orbit of Venus,0.72 AU) than would the long-stay mission(1.0 AU). The closer approach might increase theseverity of the effects of solar particle events.

• Crew exposure to the Martian surface environ-ment. This would obviously be minimized by ashort stay.

• Susceptibility of critical hardware to failure. The

10 SAFE ON MARS

crew would be dependent on hardware used insurface operations for a longer period of time on along-stay mission.

Once the astronauts are on the Martian surface, thereare a variety of operational scenarios that could be con-ducted by NASA. The simplest would be that astro-nauts land and never leave a stationary habitat. Themost complex scenario could include astronauts usinglarge, pressurized rovers to travel long distances froma base habitat to conduct extravehicular activities(EVAs). The committee anticipates that a long-staymission would probably involve the following:

• The use of unpressurized rovers similar to thelunar rover from the Apollo program;

• Walking EVAs of several kilometers (round-trip)from the base camp; and

• Pressurized rovers for transporting the astronautsgreater distances. These could allow walkingEVAs from the rover to take place, extendinghuman presence even farther from the base camp.

The committee determined that it might best assistNASA by assuming that a long-stay mission to Marswill take place, as such a mission would levy the morestringent demand for the safety of astronauts while inthe Martian environment. The reader should not con-clude that this assumption implies an endorsement ofthe long-stay mission as a baseline mission, nor thatthe committee concluded that the long-stay mission is,in total, the least hazardous option.

SCOPE OF THIS REPORT

As dictated by the statement of task (Appendix A),this report examines only those hazards to which astro-nauts will be exposed while on the surface of Mars. Forinstance, the committee did not address the need forso-called pinpoint landing on Mars, nor did it look atwhat technologies must be developed to accomplishpinpoint landing. However, the committee does addressthe terrain issues associated with setting down on theMartian surface.

Also, in accordance with the statement of task thecommittee considered only indigenous risks on Mars—that is, those hazards presented by the Martian envi-ronment itself, not risks based on engineering design.For instance, the committee did not examine the reli-ability of habitat control systems or the likelihood of

their failure, but it did consider the effects acidic air-borne dust or soil might have on such control circuits.

This report does not examine the hazard of forwardcontamination, that is, transporting Earth life to Marsfrom a contaminated spacecraft. There are risks associ-ated with forward contamination of Mars by life fromEarth, including the possibility of generating false posi-tive tests in life-detection experiments (NRC, 1992a,2002). This could certainly be a critical issue whenastronauts on the surface of Mars are looking for life. Afalse positive result could inadvertently require a long-term astronaut quarantine. While this is a topic for con-tinued study and debate, it is beyond the scope of thiscommittee’s charge.

Similarly, this report does not address technologiesassociated with in situ resource utilization (ISRU) ordeep drilling. ISRU is the use of indigenous materialsto produce consumables (e.g., breathable oxygen, pro-pellant), thus reducing the tonnage of materials thatmust be transported to Mars. As such, ISRU does notdeal directly with the “management of environmental,chemical, and biological risks,” as set forth in the state-ment of task. Drilling systems might be used on humanmissions to Mars to explore the subsurface of Mars forscientific purposes. However, these systems are notcritical to human survival. In fact, by using a drill,astronauts might become exposed to other indigenousMartian hazards. The hazards of subsurface probing byastronauts are discussed in detail in this report.

Other potential hazards the committee did notaddress involve the effects on astronaut health of along-duration mission to Mars. A recent Institute ofMedicine report states that the three most importanthealth issues that have been identified for long-durationmissions are radiation, loss of bone mineral density,and behavioral adaptation (IOM, 2001). The committeeacknowledges that these issues are important, as is theneed to ensure a benign social environment during amultiyear voyage, but again such considerations falloutside the scope of this report.

ROVER TECHNOLOGIES AND ROBOTICS

Even though there is no baseline mission defined forhuman missions to Mars, it is likely that rovers of someform will be used to perform functions critical to thesafety of the astronauts. For example, human assistantrovers may carry life support equipment, while othersrobots, such as slow-moving scientific rovers, willlikely perform mission-critical functions.

THE MARS PROGRAM IN CONTEXT 11

BOX 2.1The Anatomy of a Critical System

Critical systems are those upon which a crew’s lives depend.NASA has specific requirements for such critical systems (NASA,1998):

All critical systems essential for crew safety shall bedesigned to be two-fault tolerant. When this is not practical,systems shall be designed such that no single failure shallcause loss of the crew. For the purposes of this requirement,maintenance can be considered as the third leg of redun-dancy so long as missions operations and logistics resupplypermit it.

For long duration missions . . . fault tolerance is notsufficient. For these missions, multiple failures are expected,and the response must include maintenance and systemreconfiguration to restore the failed functions. In the case ofBeyond Earth Orbit vehicles, it is unlikely that resupplyvehicles can supplement the resources aboard the vehicleunless that capability was planned for in advance via pre-positioned spares. Therefore, safe operation of the vehiclerequires that sufficient reliability be achieved through a com-bination of reliable hardware design, installed redundancy,and logistics capability to support maintenance.

It is important to differentiate systems critical for human safetyfrom those critical for mission success. The latter are for the mostpart subjected to less stringent, though still demanding, standards.For instance, any robot operating inside the habitat in a labor-saving capacity, such as EVA suit cleaning, is not consideredcritical. Not all robots that operate outside of a habitat (i.e., incontact with the hostile Martian environment) are critical. Specifi-cally, a human assistant rover that carries oxygen and must keepup with a human walking on Mars is a critical system. A humantransport rover (pressurized or not) is also a critical system, sinceits performance is required to ensure astronaut safety.

However, a science rover that astronauts use to search for lifein a remote area before conducting an EVA to that location is notcritical. Presumably if such a robot failed, an EVA to that locationwould simply not take place, so astronaut safety would not be aconcern. Nor is a machine that drills to look for subsurface life acritical system, in that if it were to fail, humans would not bedirectly at risk. The reader should note that this report deals withissues critical to the safety of the crew, not mission success.

For these reasons, the committee explicitly consid-ered the technologies used in current and plannedMartian scientific rovers and whether such rovers canbe scaled up to support critical tasks for human explo-ration (see Box 2.1).

In its review of the Mars robotic program, the com-mittee found that NASA has done an excellent job ofdesigning science rovers capable of operating on thesurface of Mars. The Mars Pathfinder rover is a suc-cess story that testifies to the skill of NASA scientistsand engineers. And, the Mars exploration rovers nowin development offer some significant advances inrover capabilities.

The committee believes, however, that the engineer-ing knowledge being gained from the science roverswill not scale up nor will it easily apply to human assis-tant rovers or larger pressurized or unpressurizedhuman transport rovers. Furthermore, the committeenotes that current science rover activities do not pro-vide an adequate research base for the development ofrovers needed for the human exploration of Mars.

Typical science rovers generally move very slowlyand for short distances. This is understandable, becauseonce on the surface the rovers do not need to go veryfar to conduct scientific research in most cases. How-ever, the science rovers are not necessarily goodplatforms for testing future rover technology. The Marsexploration rover, scheduled to launch in 2003, will beapproximately 180 kg (~400 lb) in mass and is expectedto have a speed of 1 cm/sec (~0.4 in./sec). The totaldistance the rover is expected to travel in one Martianday is 100 meters. The NASA Smart Lander with rovercapabilities, scheduled to launch in 2009, is expectedto be approximately 1,000 kg (2,200 lb) in mass, with aspeed of anywhere from 2 cm/sec (~0.8 in./sec) to10 cm/sec (4 in./sec).

On the human missions to Mars, rovers will needcapabilities far beyond what is currently planned.Human assistant rovers would have to be able to keeppace with an astronaut walking on the surface of Mars,to operate for a long time, to have an extended range,and to navigate rough terrain quickly. These needswould be especially important for a long-stay mission,where there might be many hundreds of astronautEVAs that would require a robotic assistant to traversehundreds of kilometers over the course of the mission.Such human assistant rovers would require kilowattsof continuous electric power during the EVA. Compactsources of power at that capacity do not exist today.

Human transport rovers have the same performanceissues as human assistant rovers, but on a larger scale.For a long-stay mission, human transport rovers would

12 SAFE ON MARS

need to be able to operate for up to 20 months andtraverse thousands of kilometers over the life of themission. Tens of kilowatts of power might be requiredto accomplish these goals, and it would most likely notbe practical to use solar photovoltaic systems given therelatively large size of those systems.

Even looking ahead to the 2009 rover from theNASA Smart Lander at the optimistic forecast of10 cm/sec, NASA would have to increase the speed ofsuch a rover by a factor of 13 to reach a human walkingspeed of 4.8 km/hr (3 mph). The same rover wouldhave to increase its speed by a factor of 44 to reach the16 km/hr (10 mph) mark that a human transport roverwould probably require.

The mass of the 2009 rover from the NASA SmartLander is comparable to that of the human assistantrovers that may be required on the human missions toMars, but the dynamic nature of the vehicles is entirelydifferent.

In the committee’s judgment, the design and testingof human transport rovers that can traverse long dis-tances and that have long lifetimes and adequate powersupplies may be a pacing item in the mounting of thefirst human mission to Mars.

Finding: NASA’s current focus on small, slowrobotic rovers with short lifetimes and modestpower supplies does not provide an adequate researchbase for the development of the rovers needed forthe human exploration of Mars.

ESTABLISHING RISK STANDARDS

NASA has established detailed requirements andstandards for ensuring the safety and reliability of spaceshuttle and International Space Station operations.There is an ongoing effort to establish a set of require-ments for “human rating” the next generation ofhuman-occupied spacecraft that will permit humans tooperate more efficiently in Earth orbit and to explorebeyond Earth orbit. A human-rated system risk ratingis one that “incorporates those designs, features, andoperational procedures needed to accommodate humanparticipants, allowing NASA to safely conduct humanoperations, including safe recovery of astronauts fromany credible emergency situation” (NASA, 1998).

As stated in the above-mentioned NASA humanmissions requirements document, “many of the detailedrequirements required to implement these . . . missionsare very different. Even with the very high level of the

requirements [contained herein] there are some thatcannot be applied consistently across [all] missions.”The document further states that “the program shall bedesigned so that the cumulative probability of safe crewreturn over the life of the [space flight] program ex-ceeds 0.99” (NASA, 1998).

Using this guideline, a program that involves only10 flights can absorb a greater risk per flight than onethat involves 100 flights. NASA has yet to publish riskallocation guidelines for missions beyond Earth orbit.This report suggests the use of risk factors based onthose established by federal agencies or suggested byother studies. For example, in the absence of NASAstandards for the risk associated with exposure to toxicmetals on Mars, the committee suggests the establish-ment of a risk factor based on a study by the NRC Com-mittee on Toxicology (COT) and on EnvironmentalProtection Agency (EPA) risk estimates. Given thelevel of inherent risk associated with space explorationmissions, NASA may well choose to establish riskfactors different from those proposed in this report,especially if risk estimates change in the future.

Recommendation: Because NASA has not allocatedrisk factors and reliability requirements for mis-sions beyond Earth orbit, it should establish the riskstandards necessary to provide preliminary guid-ance to Mars mission planners and hardwaredesigners.

The concept of acceptable risk involves ethical,psychological, philosophical, and social consider-ations. The committee relied instead on standard risksources. In reviewing the toxicology risk estimates fortoxic metals, which are used extensively in Chapter 4,the committee chose to use an acceptable risk range(ARR) rather than a single risk level. In this report, theARR for developing cancer as a result of exposure totoxic metals is between 1 in 10,000 and 1 in 100,000.

Historically, for the general population, EPA hasconsidered a 1 in 100,000 risk of getting cancer accept-able. The committee based part of the ARR selectionon the fact that the EPA usually (with no overridingregulations or site-specific requirements) takes cleanupaction on a site that has a risk of greater than 1 in 10,000of causing cancer in humans, while the EPA never takesaction at a risk of 1 in 1,000,000 (Travis et al., 1987).Therefore the committee chose the lowest reasonablelevel of acceptable risk for the ARR at 1 in 100,000 ofastronauts getting cancer during their lifetime as a re-

THE MARS PROGRAM IN CONTEXT 13

sult of exposure to toxic metals in Martian soil or air-borne dust.

The 1 in 10,000 maximum risk in the committee’sARR is based on studies by the NRC’s Committee onToxicology (see Box 2.2). It is notable that theCommittee on Toxicology adopted a less stringentstandard than EPA for carcinogens, namely, 1 in

BOX 2.2Exposure Limits for Chemical Hazards

The use of toxic chemical exposure limits serves to protect individuals from excessive health risk due to exposure to harmful chemicals. Exposurelimits or recommendations have been established by a variety of groups; among the most prominent are EPA, the Occupational Safety and HealthAdministration (OSHA), the National Institute for Occupational Safety and Health (NIOSH), and the American Conference of Governmental IndustrialHygienists. There is a fundamental difference in the limits established by EPA and those of other groups, in that EPA regulations are designed to protectthe whole public, including sensitive groups such as the very young and the very old as well as people with respiratory diseases and other illnesses. Incontrast, occupational limits are meant to protect healthy adults in the workplace, usually for an exposure duration of an 8-hour workshift, with asubstantial recovery period at home. For these reasons, occupational standards allow for considerably higher exposure concentrations than EPA riskestimates, as shown in Table 2.1.

For the present study, the committee based its consideration of potential chemical exposure hazards on the conservative risk estimates establishedby EPA for long-term cancer risk at the 1 in 100,000 level and the NRC’s Committee on Toxicology (COT) at the 1 in 10,000 level. The EPA risk estimatesare published in EPA’s Integrated Risk Information System database.

It is clear that NASA has for some time recognized the need to consider exposure of astronauts to potentially hazardous chemicals. It commissionedthe COT to recommend maximum concentrations of spacecraft contaminants three decades ago (NRC, 1972). More recently, COT published fourreports setting spacecraft maximum allowable concentrations (SMACs) for a total of 40 individual contaminants based on guidelines it developed forSMACs in 1992 (NRC, 1992b, 1994, 1996, 1997, 2000). COT has expended considerable effort in developing SMACs, basing its recommendations onan extensive review of toxicity data with application of safety factors. Specifically, SMACs for carcinogenic chemicals are established using toxicelement concentrations that produce an estimated 1 in 10,000 increased lifetime risk of a neoplasm, following U.S. Department of Defense practice. Thedetailed rationale for each recommendation is presented in the related NRC report. In general, these SMACs for 1 in 10,000 risk are comparable to EPAlimits estimated for the 1 in 100,000 risk level.

It is notable that the SMACs have focused on the International Space Station. As such, the 40 airborne chemicals considered to date pertain to operationin a closed-loop environment for a maximum of 180 days, which is a shorter period than many proposed missions to Mars. The 40 contaminantsinclude primarily chemicals outgassing from man-made materials within the space station itself. The contaminant list does not include dust or soilinfiltrating from outside the habitat, which are a concern for planetary exploration. When reviewing the committee’s findings and performing its ownassessment of the risks and acceptable exposure levels of potential chemical hazards, NASA must consider the unique scenarios for operation on Mars.

10,000 (NRC 1992b, 1994, 1996, 1997, 2000). NASA,EPA, and this committee are in general agreement onthe concentration limits for hazardous chemicals pro-ducing acceptable risks for astronauts. The reader isreferred to Box 2.2 for further discussion of thecommittee’s consideration of hazardous chemicalexposure limits.

TABLE 2.1 Exposure Limits for Some Respirable Chemical Hazards (micrograms per cubic meter)

Chemical EPAa OSHAb NIOSHb

As (inorganic as As) 0.002 10 2Cd (inorganic as Cd) 0.006 5 Not listedAcetaldehyde 5 360,000 Not listedBenzene 2.9 3,200 320Ethylene dibromide 0.05 150,000 350Formaldehyde 0.8 900 20Vinyl chloride 2.3 2,560 Not listed

aFrom EPA Integrated Risk Information System database; 1 in 100,000 risk level.bFrom NIOSH (1997).

14 SAFE ON MARS

The committee chose not to use Occupational Safetyand Health Administration (OSHA) exposure limitssince those limits are based on working periods only(8 hours per day, 5 days per week). Once the astronautliving area on Mars is contaminated with soil or air-borne dust, the astronauts may be exposed to low levelsof Martian airborne particulates on a continuing basisfor up to 1.5 to 2 years. Therefore using the riskestimates discussed above for continuous (24-hour)exposure represents a conservative approach for healthprotection.

The committee understands certain risks may over-shadow others. For instance, as discussed, the commit-tee assumes the allowable risk for astronauts gettingcancer (not necessarily fatal cancer) as a result of expo-sure to toxic trace elements is the range between 1 in10,000 and 1 in 100,000. For low Earth orbit, NASAhas established the limit of 3 percent excess risk offatal cancer from radiation exposure, or 1 in 33.

Regardless of the large difference between the riskof fatal cancer from radiation and the risk of gettingcancer from toxic metal exposure, it is prudent toreduce risk in all areas that are amenable to such reduc-tions. It is important to reduce risks in areas that arereasonably achievable, as there can be synergisticeffects of combined hazards. For instance, radiationexposure may weaken the human immune system andmake a person more susceptible to other hazards.Balancing risks from various hazards will be necessaryto allow NASA to make informed decisions regardingrisk.

REFERENCESCucinotta, F., W. Schimmerling, J. Wilson, L. Peterson, G. Badhwar, P.

Saganti, and J. Dicello. 2001. Space Radiation Cancer Risk Projectionsfor Exploration Missions: Uncertainty Reduction and Mitigation. JSC29295, January. Johnson Space Center, Houston, Tex.

Institute of Medicine (IOM). 2001. Safe Passage: Astronaut Care forExploration Missions. National Academy Press, Washington, D.C.

National Aeronautics and Space Administration (NASA). 1998. HumanRating Requirements, JSC 29354. Johnson Space Center, Houston, Tex.

National Research Council (NRC). 1972. Atmospheric Contaminants inManned Spacecraft. National Academy Press, Washington, D.C.

NRC. 1992a. Biological Contamination of Mars: Issues and Recommenda-tions. National Academy Press, Washington, D.C.

NRC. 1992b. Guidelines for Developing Spacecraft Maximum AllowableConcentrations for Space Station Contaminants. National AcademyPress, Washington, D.C.

NRC. 1994. Spacecraft Maximum Allowable Concentrations for SelectedAirborne Contaminants, Vol. 1. National Academy Press, Washington,D.C.




NRC. 2002. Signs of Life: A Report Based on the April 2000 Workshop onLife Detection Techniques. National Academy Press, Washington, D.C.,in press.

Travis, C.C., S.A. Richter, E.A.C. Crouch, R. Wilson, and E.D. Klema.1987. “Cancer Risk Management.” Environmental Science and Tech-nology 21(5):415.

U.S. Department of Health and Human Services, Public Health Service,Centers for Disease Control, National Institute for Occupational Safetyand Health (NIOSH). 1997. Pocket Guide to Chemical Hazards. Alsoavailable at <http://www.cdc.gov/niosh>.

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3

Physical Environmental Hazards

The committee categorized the hazards on Mars bytheir sources or causes. It has specifically definedphysical hazards on Mars separately from the chemicaland biological hazards, because physical hazards canthreaten crew safety by physically interacting withhumans or critical equipment, resulting, for example,in impact, abrasion, tip-over (due to an unstableMartian surface), or irradiation.

It is known that the gravitational force that will beexperienced by humans on the Martian surface isapproximately 3/8 (0.375) that on Earth. The com-mittee notes that very little, if anything, is known aboutthe long-term effects on human health from residing ina 3/8 Earth gravity environment. These long-termeffects could represent a hazard to astronauts on Mars.However, since no further precursor missions arenecessary to quantify the gravity on Mars, the com-mittee has not included the low-gravity environment inthe hazards discussed in this report.

The physical environments that might pose risks tocrew safety on Mars fall into three categories: geologic,atmospheric, and radiation. This chapter elaborates oneach of those categories in light of what is currentlyknown about the hazards and what needs to be knownin order to establish confidence in the safety of humanmissions to Mars.

GEOLOGIC HAZARDS

The geologic features of interest in this study areairborne dust, regolith, and terrain. Airborne dust, withan average diameter of 3.4 microns, is the smallest geo-logical feature (see Box 3.1). The Martian regolith is

the complex outer layer of fractured rock and soil onthe surface of Mars. This is the material that will sup-port astronauts and roving vehicles as they traverse theMartian surface.

The aggregate form of the regolith creates the terrainof Mars. The term “terrain” includes large-scalefeatures such as mountains, hills, valleys, and canyonsas well as smaller-scale features such as craters, dunes,and gullies. The smallest-scale terrain on Mars includesboulder and rock fields.

Terrain Trafficability

When an aircraft operates near the surface of Earth,it is imperative that the pilot know the shape and com-position of the terrain beneath, be it ocean, forest,desert, or a concrete runway. The same is true for amission descending on Mars, except in this case theterrain will not include any prepared landing surfaces.A Mars landing craft will be designed to land on a widerange of terrains, but cost and weight considerations inthe design of the lander will mandate that missionplanners target the most benign landing areas.

Similarly, the terrain around a landing site, specifi-cally including the area in which astronauts will beoperating, must also be studied to ensure that the astro-nauts are provided with the most suitable equipmentfor their operating environment. For example, if theywill be traversing a relatively flat plain scattered withrocks 5 to 8 centimeters in diameter, a human transportrover with fairly narrow wheels, similar to the Apollolunar rover vehicle, may be adequate. If the same plainis covered in boulders one-third of a meter in diameter,

16 SAFE ON MARS

BOX 3.1Definitions of Martian Regolith, Soil, and Dust

For the purposes of this report, the committee developed operational definitions of the solid particulate materials on or near the Martian surface. TheMartian regolith is the complex outer layer of loose rock and soil on the surface of Mars. Owing to repeated meteorite impacts and surface weatheringprocesses, rock fragments of all sizes are mixed with weathered soils in various proportions to produce a regolith of unknown thickness. The compositephysical properties of the regolith are likely to be different from the physical properties measured in Martian soils.

The term “soil” describes deposits of fine-grained, largely unconsolidated materials on the planet’s surface. The planetary, or non-Earth, usage ofthe word “soil” differs from terrestrial usage, which specifies that soil must have an organic component. Martian soils may be mixtures of very smallparticles resulting from deposits of airborne dust and coarser, sand-size particles. The chemical composition of the soil has been measured at threespacecraft landing sites separated by thousands of kilometers, and the soil analysis results are similar.1 The mineralogy of the Martian soil is not wellunderstood at this time, but it is commonly inferred to resemble that of palagonite, a mixture of amorphous and poorly crystalline clays, iron oxides, andother products formed from the weathering of volcanic rocks.2

The terms “dust” and “airborne dust” are used to identify fine particles suspended in the Martian atmosphere. The average grain diameter of airbornedust, as determined from multispectral imaging, is 3.4 microns. For the purposes of this report, dust is characterized as being less than 10 microns indiameter.3

The amount of dust in the atmosphere increases during seasonal dust storms, but even in quiet times there is enough suspended dust to impart asalmon color to the sky. There is a consensus that Martian dust has been globally homogenized by the wind, so that its composition is the sameeverywhere.4 Although its mineralogy is unknown, magnetic experiments have demonstrated that some dust particles contain magnetic iron oxides. Itsred color indicates a high oxidation state.5

1Clark et al. (1982); Rieder et al. (1997).2Bell et al. (2000).3Tomasko et al. (1999).4McSween and Keil (2000).5Madsen at al. (1999).

a vehicle with very large inflatable tires capable ofeasily rolling over large rocks might be the appropriatevehicle in which to transport humans. Simply stated,understanding the shape and form of the terrain at thelanding site on Mars is a critical requirement.

Finally, knowledge of the distribution of larger rocksis needed to plan traverse routes. The rock distributionwill determine if rovers have to surmount objects, thatis, roll over rocks that are small enough, or maneuveraround them.

The Need for Measurements

To ensure safe landing and operations on the surfaceof Mars, it is necessary for NASA to fully characterizethe landing site and the topography of the anticipatedsurface operation zone with high-resolution stereo-scopic imaging. The operation zone is the area aroundthe landing site defined by the anticipated range ofoperations of EVAs, including the use of human trans-port and/or science rovers.

The level of resolution required of this imaging willbe determined by the capabilities of the equipment tobe used on the surface. Presentations to the committeeillustrated human transport rover designs using wheels1 meter in diameter. Vehicles using standard wheelscan typically roll over objects one-third the diameter ofthe wheel being used. This suggests that if human trans-port and scientific rovers will use 1-meter wheels, themission planners will need to know the distribution ofrocks one-third of a meter and larger in the landing andoperation zone. Imaging rocks this size requires a pixelresolution of 10 cm. The committee anticipates that thethree-dimensional mapping would be conducted fromMartian orbit.

Recommendation: NASA should map the three-dimensional terrain morphology of landing opera-tion zones for human missions to characterize theirfeatures at sufficient resolution to assure safe land-ing and human and rover locomotion.

PHYSICAL ENVIRONMENTAL HAZARDS 17

Mobility on the Martian Regolith

The regolith is the complex outer layer of rock andsoil on the surface of Mars. The potential hazards asso-ciated with the regolith include unstable movement orthe inability to move in a timely manner across theMartian surface. These risks will be present whenhumans walk anywhere or when human transportrovers and critical science rovers are in operation onthe surface of Mars.

Stable and Timely Traverse

The greatest threats to the safe movement of humansand critical equipment on the surface of Mars involvethe following:

• Degradation of mobility,• Instability and collision (human or machine),• Mechanical failure, and• Rovers that move too slowly.

Each of these hazards is associated with interactionwith the Martian regolith and the regolith’s ability tosupport planned rovers and humans. It is necessary tounderstand these hazards better to permit the design ofappropriate systems and machines that will be used onthe planet’s surface.

Degradation of Mobility

An emergency could arise if humans or equipmentbecome stranded in dunes or other eolian-depositeddust hazards (loose dust deposits that have a consis-tency of powder snow) because the regolith is unableto support large-mass objects operating on the Martiansurface. Rovers could conceivably bog down in loosesoil or break through a crusty surface. The powerbudgets for the movement of rovers and the life supportconsumables for EVA operations will be based on thedesigner’s understanding of the energy transfer mecha-nisms involved.

Instability and Collision

An astronaut might fall while on an EVA, and it ismore likely that the astronaut would fall on irregularterrain than on smooth terrain. It is also possible thatscience and human transport rovers might collide with

rocks, causing damage to critical systems or injury toastronaut passengers. By understanding the bulk prop-erties of the regolith, NASA should be able to mini-mize the risk of falling through appropriate design andoperational procedures. The characterization of theMartian regolith will also allow science and humantransport rovers to be designed so as to minimizeunexpected mobility-related events that result in tip-over or collision.

Mechanical Failure

The Martian regolith could induce mechanicalfailure not only by causing catastrophic collision or tip-over where mechanical parts are broken, but also byabrading or gouging surfaces that contact the regolith.This hazard would be a concern if long-range roversand extended surface operation times are involved. Thevast majority of wear on critical systems will resultfrom rock abrasion in the form of gouging from pointcontacts on rocks and shear action. Wear in the sub-surfaces of materials, resulting from rolling friction,will also be an issue with which NASA must contend.Rock abrasion and wear will impact space suits androver wheels the most.

For long surface stays with multiple EVAs, spacesuit boots would be worn down by the process of walk-ing over the Martian regolith. Rock abrasion wouldprobably be more apparent on heavy science andhuman transport rover wheels, where sharp rocks mightgouge grooves in the wheels.

Rovers That Move Too Slowly

A hazard will be introduced if humans or theircritical systems cannot move quickly enough on theMartian surface. If there is an emergency caused by anunpredicted solar particle event or a critical system fail-ure, it will be imperative for the astronauts to reachshelter in a timely manner. Critical support rovers needto be able to keep pace with humans walking on thesurface. If a critical rover is left behind because it can-not navigate Martian terrain quickly enough, a safetyhazard would be introduced and human life might be atrisk.

NASA needs to understand the mobility and traf-ficability characteristics of the Martian surface in orderto design systems that can quickly navigate the opera-tions area around the landing site.

18 SAFE ON MARS


It is necessary to understand the bulk physical prop-erties of the Martian regolith that will interact with therover vehicles, which in some scenarios are greater than1,000 kg in mass. In addition to the need to character-ize terrain morphology, as mentioned previously,NASA should also characterize the mechanical proper-ties of the Martian regolith at the landing site orcomparable terrain and rock properties in the landingand operating area. These properties include the fol-lowing:

• Rock distribution and shape,• Rock abrasive properties, and• Regolith sinkage properties, including shear

strength, bulk modulus, yield strength, and inter-nal friction angle.

The experiments required to determine these char-acteristics must be conducted on the surface of Mars.The rocks that would have to be analyzed are simplytoo large to return to Earth, and most rocks in the opera-tions zone will be too small to view from orbit. Also,the bulk properties could only be measured in anundisturbed environment.

Recommendation: To ensure that humans andcritical rover systems can land on and traverse theMartian surface in a safe, efficient, and timelymanner, NASA should characterize the range ofmechanical properties of the Martian regolith at thelanding site or comparable terrain. Specifically, insitu experiments should be performed to determinethe regolith’s aggregate strength, stability, and sink-age properties, including bearing strength, bulkmodulus, yield strength, and internal friction angle.

Rock Distribution and Shape

If high-resolution orbital imaging is not available,the average rock size distribution and shape can bedetermined by in situ observations on the surface ofMars as those observations become available prior tohuman missions to Mars. The committee notes thatrock size is currently determined during all precursorlander and rover missions. It is reasonable to expectthat this activity will continue to be included in allfuture in situ missions, whether at the proposed land-ing site for the first mission or on similar terrain.

Recommendation: NASA should determine, inadvance of human missions to Mars, rock size dis-tribution and shapes in situ, at the landing site or oncomparable terrain, in order to predict human androver trafficability.

Rock Abrasive Properties

The abrasive properties of rocks on Mars, includinghardness and surface roughness (as dictated by rockgrain size and shape), are unknown. The committeebelieves that, even faced with this lack of knowledge,NASA can still design systems by making certaineducated assumptions about the rocks on Mars. Spe-cifically, the committee believes that NASA canadequately design systems by assuming that the rockson Mars have a worst-case hardness similar to basaltand that NASA can test its systems using rocks with avariety of worst-case surface roughnesses. By such test-ing, NASA will be able to ensure that astronauts operat-ing on Mars can work safely on a variety of rocksurfaces. No further in situ experiments to determinethe abrasive properties of Martian rocks are required.

Finding: By testing space suit and rover equipmenton Earth using rocks with the hardness of basaltand a variety of worst-case surface roughnesses,NASA does not need to further characterize theproperties of rocks on Mars.

Regolith Sinkage Properties

To determine the regolith sinkage properties, includ-ing shear strength, bulk modulus, yield strength, andfriction angle, NASA must conduct an in situ conepenetrometer or equivalent experiment, which wouldcharacterize the aggregate properties of the mixture ofrock and soil that composes the Martian regolith. Thisexperiment could be conducted at either the landing siteor on comparable terrain. It should be geared to under-standing how the Martian regolith would react to the pres-ence of large-mass human transport rovers. Cohesion andfriction are intrinsic properties of the most basic soilmechanics models. Friction is commonly expressed as anangle in these models and is referred to as “friction angle.”

Airborne Dust Intrusion and Adhesion

Airborne dust presents a potentially significant haz-ard to human operations on the surface of Mars. The


ubiquity and pervasiveness of the dust will be a con-stant source of concern. Dust intrusion and accumula-tion will need to be continuously monitored and willrequire well-designed filter systems and periodichousecleaning.

The possible hazards associated with airborne dustaccumulation are the following:

• Abrading and wear of mechanical systems,• Degrading of EVA suit seals,• Filter clogging,• Decrease in visibility (optics, space suit visors),• Changing thermal properties, and• Accumulating triboelectric charge (from wheel

movement or wind).

The first two hazards involve dust intrusion; the nextfour, dust adhesion.

Dust Intrusion

The ability of dust to penetrate and damage a systemis a function of the dust grain size, hardness, and shape.Once dust particles have penetrated a system, espe-cially one with moving mechanical parts, the grainscould wear critical moving parts and seals. The hazardpresented by dust intrusion is more crucial for longstays on the Martian surface, where critical systems areexposed to the environment for a much longer time.

Dust Adhesion

Airborne dust on Mars will accumulate on surfacesby a wind-driven process abetted by electrostatic adhe-sion, magnetic attraction, or other adhesive propertiessuch as the adhesion resulting from van der Waals (i.e.,intermolecular) forces. The Martian wind, while beinga cause of dust deposition, should also help limit theamount of dust that will accumulate on an exposed sur-face. However, if habitat filters are not designed prop-erly, they could clog from the dust and soil that enterthe habitat after an EVA return. Dust accumulation onexternal communication antennas and solar panelarrays, for instance, could create very hazardous situa-tions by disrupting critical communications or decreas-ing the efficiency of power generation systems. Duston optical systems and space suit helmet visors couldseriously hinder operations and data measurements.Thermal radiators coated with microlayers of dust willhave different heat exchange properties than originally

intended as a result of the thermal radiative character-istics of the dust coating. Finally, the triboelectriccharge generated by wheel movement in the Martiansoil may cause surface soil and dust to clump onto tires,increasing the amount of dust around driveshaft sealsand reducing wheel efficiency. While this list is notintended to be all-inclusive, it should give some indi-cation of the far-reaching effects of Martian dust.

The committee does note, however, that the threerobot landers that have operated on the surface of Marsto date have all operated successfully in the Martiandust environment by virtue of careful system engineer-ing to prevent many of the effects described above.


After reviewing NASA’s experience with dust onthe Moon and Mars, the committee is confident thatNASA engineers and scientists will be able to designand build systems to mitigate the hazards posed by air-borne dust on Mars. Some systems that would be usedon the first human mission can be designed either byemploying what is currently known about Mars dust orby assuming a worst case scenario in the designprocess, as described below.

Abrasive Properties of Dust

It is known that the average diameter of airbornedust particles is 3.4 microns. However, little is knownof the size distribution, hardness, or shape of the grains.By careful analysis of the abrasive properties of air-borne dust, engineers and scientists should be able todesign systems that account for the potential forMartian dust to cause abrasive wear of moving partsand seals.

To that end, it would be desirable to know the grainsize distribution, hardness, and shape of Martian air-borne dust from the analysis of a sample return of air-borne dust. However, a sample return is not required.The present Mars soil simulant that has been devel-oped and characterized by NASA for engineeringstudies (JSC Mars-1: Martian Regolith Simulant) con-sists of palagonite (glassy volcanic ash altered at lowtemperature) having a bulk chemical composition simi-lar to that of soils analyzed on Mars. While thissimulant is a chemical proxy, it may not represent thephysical properties or mineralogy of Martian airbornedust, so it is not adequate for testing mechanical sys-tems for human missions to Mars.

20 SAFE ON MARS

The committee does not recommend that any pre-cursor in situ measurements be taken on Mars to char-acterize the mechanical and abrasive properties of air-borne dust. Rather, it expects that a simulant having anaverage particle diameter of approximately 3.4 microns(similar to the average size of airborne dust particles)and a hardness similar to that of basalt, would ade-quately stress the design of any mechanical and sealsystems that will be used during a human mission toMars.

Basalt is a common surface rock on Mars (Mustardet al., 1997; Bandfield et al., 2000). It is thought to bethe material from which Martian soils were formed(McSween and Keil, 2000). Alteration minerals—thatis, materials that originated as pure basalt but have beenmodified in some physical way—would be softer thanthe minerals comprising basalt. There is no need for asimulant to be harder than basalt, as spectroscopicsearches have not detected minerals with greater hard-ness (e.g., quartz) (Bandfield et al., 2000). Anymechanical system that withstands testing using such aworst-case basalt simulant will in all likelihood beoverdesigned for use on Mars.

Finding: The present Mars soil simulant developedby NASA does not adequately simulate physicalproperties for engineering purposes.

Adhesion Properties of Dust

It is critical to fully characterize soil and airbornedust adhesion properties in order to design systems thatminimize the risk of failure resulting from soil and dustaccumulation. Also, a full understanding of Martiandust adhesion will allow NASA to predict and plan formaintenance protocols for critical systems such as airfilters, solar panels, EVA suit seals, and communica-tions equipment.

The committee recommends that NASA conductprecursor experiments on the surface of Mars to deter-mine the adhesive properties of Martian soil and air-borne dust. The alternative to conducting these experi-ments on Mars is for NASA to develop an accuratesimulant that is based on a sample of airborne dustcollected from the Martian atmosphere and soil.

Recommendation: NASA should determine theadhesive properties of Martian soil and airbornedust in order to evaluate the effects of dust adhesionon critical systems. This characterization must be

conducted in situ by means of experiments to mea-sure airborne dust adhesion.

HAZARDS FROM ATMOSPHERIC DYNAMICS

The physical dynamics of the Martian atmospherewill offer challenges to habitat and system design notencountered on the Moon or here on Earth. NASAengineers will have to predict the effects of electro-static discharges and high winds to mitigate the hazardscaused by these atmospheric phenomena.

Electrostatic Discharge

When an electrically neutral particle collides withanother neutral particle, electric charge may be ex-changed, so that one particle takes on a positive chargeand the other a negative charge of equal magnitude. InEarth’s atmosphere, lightning is generated by an accu-mulation of charges resulting primarily from collisionsbetween ice particles of different size in clouds. Asmore collisions occur, more charge is built up (Wallaceand Hobbs, 1977). The thermodynamics of the chang-ing states of water (solid, liquid, gas) plays the centralrole in driving the vertical separation of oppositelycharged particles within a cloud. The electric potentialdifferences from one region of the cloud to anotherbuild up to very large values, producing lightningstorms on Earth.

On Mars, the water-driven mechanism for chargeseparation is not present. Charged particles are expectedto be produced by collisions in dust storms, but if theparticles are well mixed in adjacent volumes contain-ing many particles, the total positive charge would beessentially the same as the total negative charge in eachregion. Thus, there would be no large electric potentialdifferences between parts of the cloud and between thedust cloud and the Martian surface (Kolecki andLandis, 1966). Although some electrical activity inMartian dust storms and dust devils should be antici-pated, its intensity is not expected to be comparable tothat of terrestrial lightning storms. The counterpart oflightning bolts on Earth may be small sparks on Mars.

The two Viking landers were enveloped in a globaldust storm soon after landing, and dust devils have beenobserved many times on Mars by Viking orbiters andlanders, the Mars Pathfinder, and the Mars GlobalSurveyor. Windblown dust appears to be a commoncondition on Mars and would cause electrostaticcharging of astronauts’ space suits during operations


on the surface of Mars, as well as of their equipmentand habitat. Despite this phenomenon, there has beenno report of electrostatic damage to delicate electronicson any of the surface systems. Furthermore, neither theViking missions nor the Mars Pathfinder missionexperienced any problems due to electrostatic charging.

While it would be useful to learn more about theelectrical activity in Martian and terrestrial dust stormsand vortices, the committee concludes that suchincreased knowledge is not essential for planning thefirst human mission to Mars.

Probably of greater significance is the charging ofsuch objects even in the absence of airborne dust, dueto the motion of boots and wheels relative to the sur-face material and differential motion of different partsof equipment and material. A glove brushing dust froma space suit may cause more dust to cling to both.

The electrical discharge paths between objects at dif-ferent electric potentials may be through a good con-ductor, resulting in fast charge neutralization, orthrough the atmosphere or another poorly conductingmaterial, resulting in a much slower charge dissipa-tion. Objects on moist ground on Earth are said to begrounded since the conducting ground has an almostunlimited capacity to accept either positive or negativecharge without changing its electric charge potentialfrom what is essentially zero. Such natural dischargeor prevention of charging is not expected to occur onMars, because there is no near-surface liquid water.

The lack of a local electrical ground on Mars may beso electrically isolating that astronauts operating on theMartian surface would build up large potential differ-ences relative to the equipment they will be using orthe habitat in which they will live. For example, anastronaut on an EVA may experience an arc betweenthe space suit and equipment or habitat. Such a dis-charge, if not properly isolated, could damage sensi-tive, unprotected electronic components or the spacesuit. In particular, the committee is concerned with thelevel of charging that might occur as a result of thehigh-velocity movement that is likely to take placewhen using a human transport rover.

The hazards from electrostatic discharge on Marscan range from a simple spark, equivalent to feeling asting here on Earth after walking on certain types ofcarpet and reaching for a doorknob, to potentially morepotent bursts between astronauts and large equipmentor structures on Mars. The principal risk, as the com-mittee sees it, is how these discharges could affect theelectronic equipment that is critical for human survival

on the planet. Here on Earth, the charge generated fromwalking across a carpet is usually more than enough todisable and potentially destroy certain electroniccomponents.


The dry conditions and uncertainty about conduc-tivity, charging, and discharging rates in the Marsenvironment create uncertainties about electrostaticeffects on human operations in the Mars environment.However, even given the potential hazards, the com-mittee believes that the risk to humans from electro-static charging on the surface of Mars can be managedthrough standard design practice and operational pro-cedures. NASA should design accordingly and assumethat no effective local ground is available on Mars.

In one case, the physics of the Martian environmentmay actually help reduce the risk of electrical dischargeto humans or systems. So-called Paschen electrical dis-charge is likely to mitigate the hazard of differentialvoltage buildup on humans and systems due to theatmospheric composition and pressure on Mars. Owingprimarily to its low pressure, the atmosphere ionizesmore easily, which dissipates electrical charges at alower voltage, minimizing the charge buildup. Thismeans that lower overall electric charge should bepresent when humans and equipment are working onthe surface. For a specific case of parallel platesseparated by half a centimeter, on Earth the breakdownvoltage is approximately 7,000 volts, as opposed toslightly over 400 volts on Mars.

The committee does not advocate any specific engi-neering design solution. However, for the sake ofdiscussion, there are many potential solutions for theelectrostatic discharge risk. For example, a device thatallows discharge through a resistive contact to preventelectrical arcing might be used to mitigate the risk ofdischarge occurring between an astronaut and thehabitat when the astronaut returns from an EVA. Acombination of technologies might also be considered,such as point-discharge, needlelike devices or evensmall radiation sources to prevent charge buildup.

The committee believes that no further in situ mea-surements are required to characterize the electrostaticproperties of the Martian environment, including thoseproperties associated with dust devils, for an initialhuman mission to Mars. NASA’s experience with theViking and Pathfinder missions supports this conclu-sion. It should be noted that if or when highly energetic

22 SAFE ON MARS

(i.e., high-speed) rovers are used, the same might notbe true. Such high-speed rovers could conceivably in-duce very strong charges. While no specific in situ ex-periments are required at this time, NASA should payclose attention to electrostatic effects on subsequentscience rover missions to aid in the design of futurefast-moving rovers.

The committee believes it would be helpful forNASA to investigate the design considerations and pro-cedures used at the Siple research station in Antarctica,where there is little to no local electrical ground. Again,as an example of potentially innovative design solu-tions, two crossed-dipole antennas at Siple, each 21.4kilometers long, occasionally charged up to the orderof 20,000 volts when windborne ice particles passedover them. The danger of discharge was removed byconnecting the antennas to the station buildings. Thebuildings prevented a charge from accumulating on theantenna conductors by acting as large capacitors thatstored the charge. The electrostatic voltage on theantennas was reduced to near zero, and since ice is nota perfect electrical insulator, the charge on the build-ings dispersed gradually. Sharp conducting points, theneedlelike devices referred to above, were also usednear the buildings to bleed off the electrical charge.

High Wind Speeds

There has been some concern about the risk to astro-nauts on the surface of Mars from high-speed windfrom either regional or global dust storms or localizeddust devils. Global storms are neither reliably seasonalnor predictable. Nor do we know the upper limit onhow long these storms may last. Most global storms onMars start in its southern hemisphere near the begin-ning of southern summer, when Mars is near perihelion,that is, when it is 17 percent closer to the Sun than ataphelion (Kieffer et al., 1992). The storms occurring atthis time usually last several months. The impairedvisibility caused by these storms could represent ahazard to astronauts on the surface. However, thisphenomenon is well characterized and no further in situmeasurements are required.

The strongest surface winds observed by in situ mea-surements on Mars are believed to be 30 to 50 metersper second (67 to 111 miles per hour) based on eoliandeposits at the Viking I landing site. From a terrestrialperspective, these wind speeds appear to represent asignificant hazard. However, when the lower atmo-spheric dynamic pressure on Mars, resulting from a less

dense atmosphere than on Earth, is accounted for, theEarth-equivalent wind speeds are much less. Dynamicpressure is proportional to the air density times thesquare of the wind speed, so that the following com-parisons can be made:

• For the same wind speed, the dynamic pressureon Mars is less than on Earth by the ratio of airdensities, or a factor of about 82.

• For the same dynamic pressure, the wind speedon Mars must be greater than on Earth by thesquare root of this number, or a factor of about 9.

Simply stated, the wind must blow nine times fasteron Mars than here on Earth to achieve the equivalentdynamic pressure. In the strongest wind case mentionedabove, a 30 to 50 meter per second (67 to 111 mile perhour) wind on Mars is roughly equivalent to a 3.3 to5.5 meter per second (7.4 to 12 mile per hour) wind onEarth.

Another potential hazard associated with wind isabrasion by windblown particles. Suspended dust is sofine that it is unlikely to cause significant abrasion.Although ventifacts (rocks sculpted by windblown par-ticles) were observed at the Mars Pathfinder landingsite, these features do not necessarily indicate higherwind speeds. Ventifacts form by saltation—that is, bysand grains bouncing along the surface. Saltation isunlikely to cause abrasion except at or very near groundlevel, and the time scale of the abrasion that producedventifacts is certainly much longer than the duration ofhuman Mars missions. Lack of evidence that any Marslanders have been affected by this process suggests thatany abrasion by windblown particles can be mitigatedby habitat and equipment design.


The committee believes that in light of the relativelylow dynamic pressures experienced on Mars, no fur-ther characterization of wind speed on Mars is requiredprior to the first human mission. It believes that thesurface winds are sufficiently characterized based onViking and Pathfinder data and atmospheric dynamicmodels with regard to speed (based on Viking and Path-finder experience) and dust devils (based on Pathfinderdata) to allow system designers to ensure human safetyon the planet by means of robust designs. Therefore, nonew in situ experiments to validate global storm or dustdevil wind speeds are recommended. The committee


acknowledges that many precursor lander missions willinclude meteorological instrument payloads, so addi-tional wind speed measurements will in all likelihoodbe gathered, which will further aid designers.

Wind may become a factor in certain areas on Marswith large terrain slope changes, such as on the flanksof volcanoes or inside canyons, depending on the timeof day. The winds on these slopes may need to bestudied more closely if it is determined that humanswill be operating in such areas during the first missionto Mars.1

Finding: The risk to humans on the surface of Marsfrom electrostatic discharge and wind can be man-aged or mitigated through standard design practiceand operational procedures.

RADIATION HAZARDS

Astronauts are by definition radiation workers.Radiation exposure in space will be a significant andserious hazard during any human expedition to Mars.There are two major sources of natural radiation in deepspace: sparse but penetrating galactic cosmic radiation(GCR) and infrequent but very intense solar particleevents (SPEs) associated with solar storms. In addi-tion, many of the scenarios discussed for human mis-sions to Mars involve the use of advanced propulsionsystems that use nuclear power sources. In this eventastronauts will also have to be shielded from this addi-tional radiation source. While on the surface of Mars,the astronauts will be afforded some protection by theplanet itself. Instead of having to contend with radia-tion from all sides, as in space, the astronauts will haveonly radiation from above, and that amount will bereduced somewhat by the Martian atmosphere. How-ever, absorption and reradiation by the Martian regolithwill alter the spectrum of the radiation environment.The radiation dose received by astronauts on the sur-face of Mars will be a significant fraction of the totalradiation exposure for the mission.

The radiation environment on Mars is the result ofcomplex processes of radiation absorption and re-emission. The radiation arriving at the surface of Marsfrom space is a mixture of ions with a wide range ofenergies. This radiation interacts with the regolith and

the atmosphere to create a shower of secondary par-ticles, including recoil nuclei, nuclear fragments,neutrons, electrons, and subatomic mesons. The radia-tion impinging from all directions on a point at theMartian surface is influenced by the density and com-position of the Martian atmosphere and by the compo-sition of the Martian surface as well as the first fewmeters of the subsurface.

Radiation Effects on Humans

The effects of radiation exposure on humans can begrouped into two basic categories, those effects thatoccur very soon after exposure and those effects thatare apparent months or years after exposure. Acuteeffects, those that occur very soon after exposure, canrange from headaches, dizziness, or nausea to severeillness or death. Acute effects of radiation exposure canhave a serious impact on an astronaut’s ability to com-plete the mission. More details on radiation effects canbe found in the NRC reports Radiation Hazards toCrews of Interplanetary Missions, The Human Explo-ration of Space, and Radiation and the InternationalSpace Station and in a report by the National Councilon Radiation Protection and Measurements, RadiationProtection Guidance for Activities in Low-Earth Orbit(NRC, 1996, 1997, 2000; NCRP, 2000). The limits es-tablished by NASA for exposure to radiation duringmissions to low Earth orbit are clearly defined (NASA,1995).

The severity of delayed effects depends on dose. Forthe most part, any long-term effects of radiation expo-sure will not be apparent until well after a mission hasreturned to Earth. These delayed effects may includethe following:

• Cancer,• Cataracts,• Nonmalignant skin damage,• Death of nonregenerative cells/tissue (potentially

including the central nervous system),• Genetic damage,• Impact on fertility, and• Suppression of immune function.

The causes of damage from short-term radiationexposure are fairly well characterized. However, thecauses of long-term effects are poorly understood. Theuncertainty associated with the biological impactscreates problems when trying to quantify the risk of

1Ronald Greeley, Arizona State University, e-mail correspon-dence to the committee, August 10, 2001.

24 SAFE ON MARS

radiation exposure on human missions to Mars.2 Astudy by the National Research Council found that theuncertainty in biological effects caused by radiationrange from 200 to 1,500 percent depending on thespecific effect. In addition, the uncertainties surround-ing the interaction of galactic cosmic radiation, theinterstellar radiation not coming from our Sun, withmaterials on the surface of Mars are believed to be 10to 15 percent. Finally, the radiation transport modeluncertainties are judged to be on the order of 50 per-cent (NRC, 1996).

Developing and Validating Three-DimensionalRadiation Transport Models

In examining the radiation hazard, the committeesought to balance the testing capabilities NASA has atits disposal on Earth with experiments that must beconducted in space or on the surface of Mars. Therehave been no direct measurements of the radiation envi-ronment on the surface of Mars. Rather, the radiationenvironment is estimated using computer codes thatmodel the transport of the deep space radiation throughthe Martian atmosphere and its interactions with theMartian surface. The committee believes that themodels predicting the absorbed radiation dose on thesurface of Mars need to be validated before sendinghumans to Mars. These models will influence the over-all design of a Mars mission, including nominal andemergency operation scenarios and habitat design. Thecommittee was primarily concerned with two issuesinvolved in the development and validation of themodels:

• Developing models that realistically portrayoperational scenarios on Mars and

• Establishing the most direct method of validatingthe absorbed radiation dose models.

The current models that NASA is using to predictastronaut absorbed radiation dose are not designed fordetailed three-dimensional analysis of structures(habitats or vehicles) on the Martian surface. As aresult, the radiation transport models need to beimproved in concert with the development of any in

situ measurement instrument and experiment intendedto verify radiation transport models. The models mustbe applicable to a three-dimensional structure in a com-plex three-dimensional environment. This theory andmodel development will allow NASA to gain confi-dence in their predictions of astronaut radiation doses.

Two classes of simulations (Monte Carlo andanalytic) are used to model radiation transport. Theanalytic approach treats the cascade of particles byestimating the average incident particle energy lossesas those particles travel along a path and the averagebuildup of secondary particles. The Monte Carlomodels compute the path of representative incidentparticles and individual secondary particles. Both tech-niques can be applied to complex shielding geometries.The analytic model computes averages and generatesresults much faster but employs many approximations.The Monte Carlo model uses fewer simplifications, butbecause of the large number of particles that must bepropagated it is slower and less amenable to systematictrade studies.

Both transport models use approximations to char-acterize the physics of nuclear scattering. Theseapproximations are necessary because of the inherentcomplexity of the collisions between large nuclei. Tominimize the error introduced by these approximations,experiments are conducted at accelerator laboratoriesthat help to simplify the process by examining specificparticles and initial energies that impact some types ofsample radiation shielding. The results of such testsprovide feedback to the model developers, who, in turn,refine their models. However, practical considerationsinvolving the amount of time it takes to conduct indi-vidual experiments and the types of ions and energiesavailable with Earth-based testing limit the number ofexperiments that can be performed. For these reasons itis not feasible to experimentally measure the effects ofall the ion species at all energies on all the shieldingconditions that will be experienced on the Martiansurface.

In spite of these limitations, researchers anticipatethat with continued development of both models, trans-port codes will be used to simulate the Martian surfaceradiation environment. These codes will be integral tothe design of the space vehicles, surface vehicles, sur-face habitats, and other shelters. The dose estimatesestablished by these models will be used to set opera-tional rules for surface expeditions. The rules willaccount for such items as the maximum amount of time

2F. Cucinotta and W. Schimmerling, “NASA Strategic ProgramPlan for Space Radiation Health Research,” briefing to the commit-tee, August 2, 2001.


an astronaut may spend on a surface EVA and the maxi-mum distance from a shelter an astronaut may go.

Effect of Localized Hydrogen and Iron

The effective radiation dose that an astronaut willabsorb is strongly affected by the number of secondaryneutrons generated when radiation first impacts amaterial or substance. When radiated material containshydrogen, neutrons are more readily absorbed: a desir-able effect. However, when material contains heavynuclei, more secondary neutrons are generated: adeleterious effect. There has been some concern thatlocalized concentrations of hydrogen in subsurface iceor hydrated minerals or iron within iron-rich rocks inthe Martian regolith could skew the results of in situtesting for absorbed radiation dose if such testing isrestricted to a small, localized area. At the committee’srequest, scientists at NASA Langley Research Centerran model simulations testing the effects of hydrogenand iron concentrations on absorbed dose. The simula-tions were conducted with varying amounts of hydro-gen and iron (Tables 3.1 and 3.2). This preliminaryanalysis indicates that the current understanding of theelemental composition of Martian soil is adequate forradiation transport calculations through bulk Martianregolith. The analysis further suggests that even sub-stantial variations in the amount of localized hydrogenand iron have little effect on the absorbed dose.


Because of the central role that radiation transportand absorbed dose models will perform in the planningand design of human missions to Mars, it is importantthat the code predictions be validated to verify that themodels are providing predictions representative of thereal radiation environment on Mars. The committeerecommends that NASA conduct a precursor experi-ment on the surface of Mars to measure total absorbedradiation dose in a tissue-equivalent material. The mea-surement may take place at one location.

The committee acknowledges that by conductingsuch an experiment, NASA will only be testing for thevalidity of the absorbed dose models. This could leadto some ambiguity in the validation test if the experi-ment design is too simplistic. For example, the modelmay correctly predict a dose rate that the experimentobserves. However, the match between the model andresults may be a result of underestimating one com-

TABLE 3.1 Compositions of Five Different MartianRegolith Scenarios

Nominal High Fe/ High Fe/ Low Fe/ Low Fe/Regolith High H Low H High H Low H

H 0.00 1.00 0.05 1.00 0.05O 44.56 31.52 32.03 47.55 48.05Mg 6.48 4.58 4.66 6.91 6.99Si 27.16 19.21 19.52 28.98 29.30Ca 5.21 3.69 3.74 5.56 5.62Fe 16.59 40.00 40.00 10.00 10.00

NOTE: Values are presented in weight percent. The results are frommodel simulations conducted at the committee’s request by MarthaClowdsley at NASA, “Examination of the Sensitivity of MarsSurface Radiation Exposures to Variations in Regolith Composi-tion of Iron and Hydrogen,” 2001. The nominal regolith elementbreakdown represents expected regolith composition, while theother four categories bound extreme range possibilities of iron andhydrogen in the Martian soil.

TABLE 3.2 Effect of Hydrogen and Iron Content onAbsorbed Radiation Dosea

Shield Nominal High Fe/ High Fe/ Low Fe/ Low Fe/Thickness Regolith High H Low H High H Low H

1 g/cm2 22.9 22.7 22.7 22.6 22.8aluminum221910 g/cm2 22.0 21.8 21.7 21.7 21.8aluminum2219

NOTE: Values represent the dose equivalent for blood-formingorgans (BFOs). The results are from model simulations conductedat the committee’s request by Martha Clowdsley at NASA, “Ex-amination of the Sensitivity of Mars Surface Radiation Exposuresto Variations in Regolith Composition of Iron and Hydrogen,”2001.aCompositions from Table 3.1.

ponent’s contribution to the dose and overestimatingthat of another component, so that the two balance eachother. Since, as discussed earlier, the neutron flux isvery sensitive to the environment, one way to addressthis problem would be to require that the experiment beset up to distinguish the radiation dose contributioninduced by charged particles from that induced byneutrons. In addition to providing more insight into thesource of the absorbed dose, the ability to make thisdistinction will test the value of the models. This infor-

26 SAFE ON MARS

mation will provide some assurance that it is not merelycoincidence if the models’ predicted doses coincidewith the measurement made on Mars, but rather thatthe models are based on sound data.

The full impact of individual variable inputs onmodel performance will not be resolved with this singleexperiment. However, if the measurements of totaldose and dose contribution from neutrons on Marsagree with model predictions, these variable inputsneed not be resolved to conduct the first human mis-sion to Mars.

The in situ test should take place at a location that isrepresentative of potential landing sites for human mis-sions to Mars. Most of Mars appears to have generallythe same composition, with the exception of the polesand certain locations with hematite deposits. The aver-age composition of the test site need not exactly matchthat of the planned landing site.

The experiment should take place at a location withan altitude similar to that of the human missions toMars. The depth of the Martian atmosphere will play asignificant role in astronaut absorbed radiation dose, sothe measurement should be made with an atmosphericthickness similar to that which the human missions willencounter.

If practical, it would be beneficial to have the mea-surements taken at multiple locations separated by tensof meters, such as could be accomplished with a rover.The results of such an experiment could validate thepredictions by models that the absorbed dose is rela-tively insensitive to local variations in the subsurfacecomposition of hydrogen and iron.

Finally, the committee recommends that this in situtest be made a priority in the Mars program and con-ducted as soon as reasonably possible. Radiation riskmitigation strategies will be an integral part of overallmission design and planning. Should the results of thein situ experiment prove that the radiation transportmodels are flawed, more time will be needed to adjustthe models to account for the differences between themodels and the measurement. If the difference is sub-stantial enough to have a significant impact on thedesign and operation of the mission, further in situ testsmay be required.

Habitats designed to protect astronauts against GCRwill also protect them against solar particle events.Operational procedures will be necessary to ensure theastronauts have timely access to these habitats or toother, similarly shielded safe havens such as robust,long-range transports. The committee recognizes that

the proposed approach would not be adequate toexplicitly validate model predictions of the surfacedose during solar particle events. While the GCR fluxvaries slowly within the inner solar system and at Mars,it can be reliably estimated from measurements nearEarth. SPEs, however, can be highly localized, and theflux varies significantly with distance from the Sun andlocation relative to the source of the solar eruption. Theflux of particles at Mars during an SPE cannot bedetermined by measurements taken far from Mars. Todirectly validate a model prediction during an SPEwould require that the surface measurement be sup-ported by a second measurement of the particle fluxabove the Martian atmosphere.

While it would be valuable during an SPE to directlycorrelate measurements of dose on the surface if theparticle flux is available from an orbiting instrument,the committee believes that a validation of the modeledGCR contribution to dose will add sufficient confi-dence to the simulations such that they could be usedreliably to estimate the surface dose during an SPE.The related (and difficult) task of forecasting SPEs atMars in a timely fashion is important, but it is outsidethe purview of this committee.

Recommendation: In order to validate the radia-tion transport codes, thereby ensuring the accuracyof radiation dose predictions, NASA should performexperiments to measure the absorbed dose in atissue-equivalent material on Mars at a locationrepresentative of the expected landing site, includ-ing altitude and bulk elemental composition of thesurface. The experiments should distinguish theradiation dose contribution induced by chargedparticles from that induced by neutrons. Theseexperiments should be made a priority in the Marsexploration program.

REFERENCESBandfield, J.L., V.E. Hamilton, and P.R. Christensen. 2000. “A Global View

of Martian Surface Compositions from MGS-TES.” Science 287:1626-1630.

Bell, J.F., et al. 2000. “Mineralogic and Compositional Properties of MartianSoil and Dust: Results from Mars Pathfinder.” Journal of GeophysicalResearch 105:1721-1755.

Clark, B.C., et al. 1982. “Chemical Composition of Martian Fines.” Journalof Geophysical Research 87:10059-10067.

Kieffer, H., et al. 1992. “The Martian Dust Cycle.” Mars. University ofArizona Press, Tucson, pp. 1017-1053.


Kolecki, J., and G. Landis. 1966. “Electrical Discharge on the MartianSurface,” November, available at <http://powerweb.grc.nasa.gov/pvsee/publications/marslight.html>.

Madsen, M.B., et al. 1999. “The Magnetic Properties Experiments on MarsPathfinder.” Journal of Geophysical Research 104:8761-8789.

McSween, H.Y., and K. Keil. 2000. “Mixing Relationships in the MartianRegolith and the Composition of Globally Homogeneous Dust.”Geochimica and Cosmochimica Acta 64:2155-2166.

Mustard, J.F., S. Murchie, S. Erard, and J.M. Sunshine. 1997. “In Situ Com-positions of Martian Volcanics: Implications for the Mantle.” Journalof Geophysical Research 102:25,605-25,615.

National Aeronautics and Space Administration (NASA). 1995. NASA-STD-3000. Man-Systems Integration Standards, Vol. I, Revision B,July.

National Council on Radiation Protection and Measurements (NCRP). 2000.Radiation Protection Guidance for Activities in Low-Earth Orbit,Report No. 132. Washington, D.C.

National Research Council (NRC). 1996. Radiation Hazards to Crews ofInterplanetary Missions. National Academy Press, Washington, D.C.

NRC. 1997. The Human Exploration of Space. National Academy Press,Washington, D.C.3

NRC. 2000. Radiation and the International Space Station. NationalAcademy Press, Washington, D.C.

Rieder, R., et al. 1997. “The Chemical Composition of Martian Soil andRocks Returned by the Mobile Alpha Proton X-ray Spectrometer:Preliminary Results from the X-ray Mode.” Science 278:1771-1774.

Tomasko, M.G., et al. 1999. “Properties of Dust in the Martian Atmospherefrom the Imager on Mars Pathfinder.” Journal of Geophysical Research104:8987-9008.

Wallace, J.M., and P.V. Hobbs. 1977. Atmospheric Science. AcademicPress, New York, pp. 202-209.

3Three NRC reports reprinted in a single volume: Scientific Pre-requisites for the Human Exploration of Space (1993); ScientificOpportunities in the Human Exploration of Space (1994); andScience Management in the Human Exploration of Space (1997).

28

4

Chemical Environmental Hazards

Chapter 3 presented the hazards from materials onMars interacting dynamically with humans or criticalsystems involved in human missions. In Chapter 4, thecommittee presents hazards associated with the chemi-cal reactivity of materials on Mars. As with radiationexposure standards, NASA standards for astronautexposure to chemical hazards have only been docu-mented for operation in low Earth orbit (NASA, 1995).

CHEMICAL INTERACTION OF MARTIAN SOILAND AIRBORNE DUST WITH ASTRONAUTS ANDCRITICAL EQUIPMENT

As discussed in Chapter 2, the committee assumedthat astronauts will conduct EVAs on the surface ofMars. The following analysis is based on the assump-tion that some dust and soil will be brought into thehabitat through the airlock by returning astronauts, aswas the case during the Apollo missions to the Moon.Even though some space suit designs have been pro-posed that would mitigate the dust and soil intrusionproblem, astronauts will inhale some fraction of thedust and fine particle soil entering the habitat.

The committee was faced with the question ofwhether or not NASA must fully characterize themineralogy of the dust and soil—that is, the chemicalcomposition and the physical form—prior to the firsthuman mission to Mars. The answer to this question iscomplicated by the fact that very little is known aboutMartian airborne dust and soil. The only factual datathe committee was able to use during its deliberationswas based on the analysis of meteorites from Mars thathave landed on Earth (also known as SNC meteorites)

and chemical and spectroscopic measurements of Marssoil taken by the Viking and Mars Pathfinder missions.

The committee’s goal was to identify, if possible,the level below which particulate concentrations in ahabitat must be kept to protect astronauts from dust andsoil whose composition is not well known.

Given the unknown nature of the Martian dust andsoil, the committee believed it was prudent to errconservatively by assuming a worst-case scenario. Inchoosing the “worst” toxic chemical hazards to humans,the committee considered inorganic substances sepa-rately from organic substances. With respect to inor-ganic substances, it identified certain toxic metals asthe worst threat to humans at the lowest concentrations.The committee addressed organic substances in a dif-ferent manner, choosing to look first for the presenceof organic carbon, which will be discussed below.

The chemical health threat from Martian airbornedust and soil can be considered in terms of acute (short-term) and chronic (long-term) effects. Noncancer acuteor chronic effects could result in lung injury in the formof silicosis or in other specific organ damage. Lungtissue could be damaged by the intrusion of acidic dust.Strong oxidants in the soil and dust are also potentiallyhazardous to astronauts. The effects of such damagewould be observable in a short time and could interferewith the completion of a human mission.

Cancer is a potential chronic effect caused by theinhalation of Martian particulate matter containingcertain toxic metals, asbestos-like fibers, or organiccompounds. The committee concluded that the threatof asbestos-like materials being present in Martian dustand soil is not significant based on inferences about

CHEMICAL ENVIRONMENTAL HAZARDS 29

dust mineralogy from spectroscopy, measured soilcomposition, and plausible alteration mechanisms(Morris et al., 1995; Bell et al., 2000).

The committee also concluded that Martian airbornedust could present the same chemical hazards asMartian soil, so soil and dust should be characterizedin the same way. In addition, certain soils could containharmful organic compounds. However, the oxidizingenvironment at the surface would most likely havedestroyed any organic compounds contained in thesurface layers of the soil. Notwithstanding these con-clusions, in the absence of further data, astronautsshould avoid direct skin contact with soil.

Cancer versus Noncancer Risks

Generally, EPA considers that there is no safethreshold (no safe level) for human exposure to certaingenotoxic, cancer-inducing compounds. “Genotoxic”refers to the process by which cancer-causing com-pounds directly interact with DNA. In contrast, there isa defined threshold level above which noncancereffects are induced. An estimate of the safe inhalationconcentration is referred to as the reference concentra-tion (RfC). If hazardous compounds are below the RfCthreshold, noncancer effects are not expected to occur.If the RfC of a toxic compound is exceeded, there couldbe harmful health effects. It should be noted that manycompounds do not have RfCs established.

An examination of the Integrated Risk InformationSystem (IRIS) provided by the EPA reveals that theallowed safe concentrations for cancer are much morestringent than the safe concentrations that give compa-rable risk estimates for noncancer effects (Table 4.1 isa representative sample). Therefore, if NASA protectsastronauts against the risk of developing cancer in thelong term as a result of having been exposed to particu-late matter on Mars, NASA will also be protectingastronauts from acute and short-term noncancer effectsthat could potentially interfere with mission success.

Toxic Metals and Other Inorganic Elements

Airborne dust and soil could contain trace amountsof hazardous chemicals, including compounds of toxicmetals that are known to cause cancer over the longterm if inhaled in sufficient quantities. Soil analysesconducted by the Viking missions established maxi-mum possible concentration limits for a few toxicelements based on the detection capabilities of the

TABLE 4.1 Representative Listing of ReferenceConcentrations for Cancer-Causing Compounds andfor the Noncancerous Effects of Those Compoundsfrom EPA’s IRIS Database (milligrams per cubicmeter)

ReferenceConcentration (RfC) Concentration ThatSafe Dose for Gives a Cancer Risk

Compound Noncancer Effects of 1 in 1,000,000a

Beryllium 2 × 10–5 4 × 10–7

Acrylonitrile 2 × 10–3 1 × 10–5

Acrolein 2 × 10–5 No information givenAcrylic acid 1 × 10–3 No information givenAniline 1 × 10–3 No information givenAntimony trioxide 2 × 10–4 No information givenCarbon disulfide 7 × 10–1 No information givenDichlorobenzene 8 × 10–1 No information givenStyrene 1 × 100 No information givenChromium VI No information given 8 × 10–8

Arsenic No information given 2 × 10–7

Cadmium No information given 6 × 10–7

aHigher dose poses a greater risk.

instruments on the landers (Table 4.2), and Mars Path-finder measurements established that chromium ispresent in Mars soil. Although analogous measure-ments have not been made on airborne dust, soil anddust are commonly assumed to have similar chemicalcompositions (McSween and Keil, 2000).

Based on a survey of Environmental ProtectionAgency (EPA) exposure risk estimates, the elementsthat are toxic at the lowest concentrations are hexavalentchromium (Cr VI), arsenic (As), cadmium (Cd), andberyllium (Be) (see Box 2.2).

Hexavalent Chromium

Chromium contained in naturally occurring geologicmaterials is primarily in the trivalent state (a +3 ion),which is a stable form of chromium and minimallytoxic to humans. Hexavalent chromium (Cr VI, a +6ion), the highly toxic form of chromium, is rarelyencountered in natural geologic materials. In fact,hexavalent chromium is only found naturally on Earthin the rare mineral crocoite (PbCrO4) and is more oftenproduced by humans for industrial purposes (ATSDR,2000).

30 SAFE ON MARS

The committee believes that hexavalent chromiumis not present in abundance on Mars but cannot statethis with absolute certainty. There are three reasons forbeing cautious about the presence of hexavalentchromium on Mars.

• Mars Pathfinder APXS (alpha-proton-x-ray spec-trometer) data indicate that chromium, in anunknown valance state, is present on Mars in thesoil at an average of 0.2±0.1 weight percent(Wanke et al., 2001). If even a modest fraction ofthis amount is hexavalent chromium, it wouldpose a serious health threat to astronauts operat-ing on the surface.

• Hexavalent chromium is derived from trivalentchromium by an oxidation process. The surfaceof Mars is highly oxidizing, which makes it asuitable environment for generating hexavalentchromium.

• Hexavalent chromium reverts fairly easily to thetrivalent state in the presence of organic com-

pounds. The surface of Mars appears to be devoidof organic carbon (Biemann et al., 1977). The lackof organic carbon indicates that few or no organiccompounds are present, so any hexavalent chro-mium present may not easily revert to the non-toxic trivalent state.

To offset these concerns to some degree, it shouldbe noted that the only cation known to combine miner-alogically with hexavalent chromium on Earth is lead(Pb2+), producing the oxide, crocoite (PbCrO4). Basedon SNC meteorite analysis, lead is present on Mars atonly about 72 parts per billion (ppb) (Lodders, 1998).Therefore, if the general mineralogy of Mars is similarto that of Earth, as is indicated by all experiments todate, the amount of hexavalent chromium on Mars islikely to be very small.

The physical and chemical weathering processes thatwould cause rocks on Mars, which the meteorites rep-resent, to be converted into airborne dust and soil couldcause an increase in the concentration of certain toxicmetals. Even given a thousandfold increase in the con-centration of lead (from 72 ppb to 72 ppm) by suchweathering processes, the hexavalent chromium thatwould combine with this lead would not exceed a con-centration of about 18 ppm.

For the purposes of this study, the committee madethe conservative assumption that hexavalent chromiumis present in Martian soil and airborne dust at no morethan 150 ppm. This concentration is equivalent tonearly 10 percent of the chromium detected by Path-finder and well above any expectations for the amountof crocoite present, as discussed above. It must be clearto the reader that all conclusions in this chapter will bebased on this assumption and others as discussedshortly. Should the actual concentration of hexavalentchromium become known, the habitat filtration require-ments discussed in this chapter will change accord-ingly, keeping in mind that other toxic elements, suchas arsenic, might then be dominant.

Given the severe toxicity and uncertainty of theamount of hexavalent chromium, the committee rec-ommends that NASA conduct an in situ experimentprior to the first human mission to Mars to determine ifhexavalent chromium is present in Martian soil and air-borne dust at potentially hazardous concentrations. Asis shown in Table 4.3, a 2-year exposure to Cr VI at aconcentration of 150 ppm in 1 milligram per cubicmeter (mg/m3) airborne particulate matter produces acancer risk of 5 in 100,000, which is in the middle of

TABLE 4.2 Element Detection Capability on theViking Landers

Detection PotentiallyAtomic Lower HazardousNumber Limit Elements Note

1 to 11 No detection Be and F Fluorine amountscapability may be unusually

high on Mars sincehalogen amounts aregenerally high.

12 to 28 Variable Ni, Co, Cr, Si Consists of major andminor elements.

29 to 42 150 ppma As, Se, Rb, Sr, Bromine (Br) wasY, Zr, Mo detected in the

100-150 ppm range.

43 to 75 No detection Cd No ability to placecapability useful lower limits

due to interferencefrom major and minorelements.

76 to 92 300 ppm Hg, Ti, Pb

aBromine is an exception. See note in right column.


TABLE 4.3 Toxic Metal Inhalation Risk

Concentration Maximum Cancer Risk toEquivalent to Astronauts for 2 Years ofLifetime Exposure Exposure at 1 mg/m3

Representing a Particulate MatterRisk of 1 in Containing 150 ppm of

Metal 1,000,000 (mg/m3) the Metal, Roundeda

Chromium VI 8 × 10–8 5 in 100,000b

Arsenic 2 × 10–7 2 in 100,000Beryllium 4 × 10–7 1 in 100,000Cadmium 6 × 10–7 1 in 100,000

NOTE/CALCULATIONS: The concentration of the metal that rep-resents a cancer risk of 1 in 1,000,000 (10–6) is given in the EPA’sIntegrated Risk Information System (IRIS). For instance, forchromium VI, a risk of 1 in 1,000,000 results from a lifetimeexposure (70 years) to a concentration of 8 × 10–8 g/m3. Risks arelinearly related to exposure concentration.

The astronauts will not be exposed to the metal for a lifetime but,in the worst case, for 2 years. Since the risks are proportional to theexposure, one can tolerate a higher concentration for a shorterperiod of time. The concentration equivalent to 2 years exposure togive a 10–6 risk is 70/2 times the lifetime risk.

The following is a sample calculation for arsenic:

• 150 ppm of the metal in the particulate matter in 1 mg/m3 is150 × 10–6, or 1.5 × 10–4 mg/m3 of metal. This is the(conservative) concentration to which the astronauts could beexposed for 2 years.

• The lifetime cancer risk of 1 in 1,000,000 (10–6) for arsenic is2 × 10–7 mg/m3 (from IRIS).

• The concentration equivalent to 10–6 for a 2-year exposure is(70/2) × (2 × 10–7), or 7 × 10–6 mg/m3.

• The ratio of the exposure concentration to the concentrationthat gives a risk of 10–6 represents the risk times 10–6.For instance, 1.5 × 10–4 (exposure concentration) divided by7 × 10–6 (exposure that gives a risk of 10–6) = 0.2 × 102 × 10–6, or2 × 10–5, a risk of 2 in 100,000.

aAssumptions/methods: (1) the metal is uniformly present in therespirable particulate matter at 150 ppm or less and (2) astronautscould be exposed constantly for 2 years at 1 mg/m3 particulates inair (maximum). Rounded to nearest whole number.bRisk of contracting cancer is 5 in 100,000.

the acceptable risk range (ARR) established for theexploration of Mars by the committee. Thus, the com-mittee recommends that the measurement quantifyhexavalent chromium down to a concentration levelthat is 150 ppm or less. This measurement can takeplace at any location on the planet since the airbornedust on Mars is highly mixed and therefore consideredto be uniform in composition.

If this measurement cannot be made in situ, a sampleof airborne dust and fine particles of Martian soil mustbe returned to Earth to determine if hexavalent chromiumwill pose a threat to astronauts operating on the surfaceof Mars.

Recommendation: In order to evaluate if hexavalentchromium on Mars poses a threat to astronauthealth, NASA should conduct a precursor in situmeasurement to determine if hexavalent chromiumis present in Martian soil and airborne dust at morethan 150 parts per million (ppm). This measurementmay take place anywhere on Mars where well-mixed, uniform airborne dust is present. If such ameasurement is not possible, a sample of airbornedust and fine particles of Martian soil must bereturned to Earth for evaluation.

Other Toxic Inorganic Elements

The principal constraint on human missions fromother toxic elements in Martian soil and airborne dustis an upper limit on the abundance of arsenic. This limitis based on Viking mission instrument measurementlimitations (Table 4.2). Based on ratios of the measuredabundances of arsenic, cadmium, and beryllium inMartian meteorites to other elements created by similargeochemical processes, abnormally high concentra-tions of these elements are not expected in Martian soiland airborne dust (arsenic and cadmium data compiledby Lodders, 1998; beryllium analyzed by Lentz et al.,2001). These elements are generally present at a fewtens of parts per billion in Martian meteorites. Martiansoils are similar in major element chemistry to themeteorites, except for enrichment in salts (sulfates andchlorides) (Clark et al., 1982). The geochemicalprocess that introduced salts probably also concentratedarsenic and cadmium, but not beryllium, in soils. How-ever, even at concentrations a thousand times greaterthan the concentrations in meteorites, arsenic andcadmium levels would still be less than 150 ppm.


Based on the predicted concentrations of hexavalentchromium, arsenic, beryllium, and cadmium, as a worst-case scenario, the committee has addressed the risk ofastronauts inhaling particulate matter containing150 ppm of these metals. At this concentration, a 2-yearexposure period, and a maximum average particulate

32 SAFE ON MARS

level of 1 mg/m3, as discussed below, the committeewas able to develop the risk estimates listed in Table 4.3.All of the risks fall within its ARRs. The significanceof this result is that no additional measurements of theconcentrations of toxic elements in Martian soil andairborne dust, except Cr VI, are necessary as precursorsto human exploration. However, this conclusion isdependent on NASA’s ability to maintain particulateconcentration in the habitat at or below the maximumallowable level discussed in this section of the study.

Instead of determining the concentrations of everytoxic element in Martian soil and airborne dust, thecommittee determined that NASA can design aroundthe potential hazard. Simply stated, if the habitat and/or astronauts are equipped with appropriate air filtra-tion systems, then their health will be protected fromthe risk of toxic metal exposure as long as the assump-tions listed below hold true.

Filtration systems that maintain a maximum aver-age concentration of 1 mg of particulate matter percubic meter of air will place astronauts in an ARR ofbetween 1 in 10,000 and 1 in 100,000 of getting cancerduring their lifetime from exposure to toxic elementsin Martian soil and airborne dust (see Table 4.3). Asnoted in Chapter 2, in the section “Establishing RiskStandards,” the committee established the ARR as anappropriate risk for astronauts. The issue is also dis-cussed in Box 2.2.

It is natural that the operation of filtration systemswill result in oscillations in airborne particulate matterconcentration depending on the loading rate. Forinstance, when an astronaut returns from an EVA andintroduces soil and dust into the habitat, the concentra-tion of particulate matter in the air will go up for someperiod of time until returning to less than 1 mg/m3.

It is possible to average a short-term, high-concentration exposure with a low-concentrationexposure over a long period of time and obtain an aver-age concentration meeting toxic concentration limits.However, this is not the intent of the specificationoffered in this report. EPA does not provide short-termor ceiling limits for exposure to chemicals, even thoughthe intention is not to experience high-concentrationfluctuations. The National Institute of OccupationalHealth and Safety (NIOSH) and the OccupationalSafety and Health Administration (OSHA), on the otherhand, do have short-term exposure limits for manychemicals. For most chemicals, this short-term maxi-mum over a period of 15 to 30 minutes is between 1.5and 2 times the specified average daily value. The com-

mittee believes it is reasonable to adopt this precedentin recommending a maximum concentration of respi-rable particles to no more than 1.5 times the averageconcentration for a duration not to exceed 30 minutesper day to protect against toxic elements (NIOSH,2000).

Assumptions

The committee made the following assumptions informulating the recommendation that NASA develop asystem that filters particulate matter in astronaut habitatair to concentrations of 1 mg/m3 or less for the firsthuman mission to Mars:

• Astronaut forays outside the habitat will contami-nate the environmental living space with materialfrom the surface of Mars.

• Astronauts will primarily be exposed to toxicelements by breathing Martian dust, with onlyminuscule amounts of soil or dust contaminatingfood or contacting bare skin.

• Hexavalent chromium, arsenic, cadmium, beryl-lium, and other toxic metal concentrations are 150ppm or less in soil and airborne dust.

• There are few or no negative additive or synergis-tic human health effects of the mixture of toxicmetals and other chemicals, including otherparameters such as acidity, in Mars soil or air-borne dust. Considering metals individually iscommon practice since the quantification ofcombined effects with complex chemical mix-tures is difficult.

• Radiation exposure in combination with exposureto toxic metals has little negative synergisticeffect on human health.

• Toxic metals are not present in higher concentra-tions in the breathable portion of the airborne dust(small particles less than 1 micron in diameter)than in the Martian soil.

• The dust on Mars is homogeneous with respect toits trace element composition.

• Residence time on Mars for the astronauts will beat least 1.5 years. Acute exposures over a fewmonths are outside the scope of this analysis. If ashorter mission takes place, exposure to toxicmetals could be higher and still achieve the sameARR.

• All of the toxic metal is bioavailable—that is, anymetals entering an astronaut’s body would be


completely absorbed. If the metals in Martian dustor soil are encapsulated in a coating of a nontoxicmaterial or are otherwise rendered nonbioavail-able (e.g., by binding to other chemicals), the riskto astronauts will be reduced.

What If Assumed Filtration Levels Cannot Be Attained?

The committee believes, based on current filtrationstandards and the capability demonstrated on the Inter-national Space Station, that the filtration levels requiredto protect astronauts are readily achievable. However,if a filtration system cannot be designed to limit theaverage particulate inhalation exposure of an astronautto 1 mg of particulate matter per cubic meter of air inthe habitat, then a sample of airborne dust taken fromthe Martian atmosphere and soil must be analyzed toestablish concentration levels of all toxic metals. Thelevel of analytical precision required will be dictatedby the filtration capability of the astronauts’ habitat.Although in situ measurements of Martian dust and soilwith a resolution of parts per million are possible inprinciple, the committee judged that limitations on theability of robotic instruments to measure trace elementswould probably necessitate the return of soil and air-borne dust samples to Earth if the specified filtrationlevel cannot be achieved.

The following analyses and/or protocols must beestablished for the soil and airborne dust samples toensure astronaut safety on the surface of Mars if appro-priate filtration levels cannot be attained:

• Trace element abundance must be measured withparts per million resolution to determine the levelsof toxic metals in the soil and airborne dust.

• The chemical and physical form of the toxicmetals may need to be determined, depending ontheir concentrations.

• If the ARR of 1 in 10,000 to 1 in 100,000 due toinhalation of toxic metals cannot be maintainedby using filtration systems to reduce airborne par-ticulate concentration, then animal testing shouldbe conducted to determine the integrated effectsof soil and dust exposure on living systems. Underthese circumstances, the chemical and physicalform of the toxic metals should also be deter-mined, since bioavailability could be an issue.

Airborne Respirable Particulate Matter

It should be very clear to the reader that, in the viewof the committee, the 1 mg/m3 specification is the maxi-mum acceptable respirable particle average concentra-tion to which astronauts should be exposed. This con-centration level will protect astronauts from exposureto toxic metals, which—of all inorganic chemicals—the committee considers to pose the greatest health riskto astronauts. Filtering at or below the recommended1 mg/m3 average with a 1.5 mg/m3 peak concentrationshould be readily achievable for NASA. The 1 mg/m3

particulate level is equivalent to very dirty industrialcity air and is about 20 times greater than the 0.05 mg/m3

average standard set for the International Space Station(Green and Lane, 1964; NASA, 2000). Indeed, tominimize risks from exposure, the committee stronglybelieves that filtering should be implemented below1 mg/m3, to as low a concentration as is reasonablyachievable in the Martian habitat. The committee notesthat there are no risk estimates similar to those pre-sented in the EPA IRIS database for general respirableparticulates, so an analysis similar to the one above fortoxic metals is not possible. However, maintaining aparticulate concentration below 1 mg/m3 would also beconsistent with EPA National Ambient Air QualityStandards (NAAQS). Current and proposed NAAQSfor particulate concentrations are shown in Table 4.4.

TABLE 4.4 EPA National Ambient Air Quality Standards for Particulate Concentrations

Pollutant Measure Value (mg/m3) Type

PM 10—diameter <10 micrometers Annual arithmetic mean 0.05 Primary and secondarya

24-hour average 0.15 Primary and secondary

PM 2.5 (proposed)—diameter <2.5 micrometers Annual arithmetic mean 0.015 Primary and secondary24-hour average 0.065 Primary and secondary

aPrimary standards are intended to protect public health in general, including sensitive populations such as children, the elderly, andasthmatics. Secondary standards are intended to protect the public welfare, including protection of animals, vegetation, buildings, etc.

34 SAFE ON MARS

The mean annual particulate matter (PM) 10 standard(for coarse respirable particles less than 10 microns indiameter) is the same as the International Space Stationstandard for total particulate concentrations. The lowerPM 2.5 standard (for fine respirable particles less than2.5 microns in diameter) is not currently being enforcedby EPA. Although the distribution of Martian dust par-ticle sizes has not been fully characterized, the meanparticle diameter of 3.4 microns suggests that a signifi-cant fraction of the dust will fall in the coarse respi-rable PM 10 size category. It is noteworthy that theNAAQS for particulate concentrations are designatedto serve as both primary and secondary health stan-dards. Primary standards are intended to protect publichealth in general, including sensitive populations suchas children, the elderly, and asthmatics. Secondarystandards are intended to protect the public welfare,including protection of animals, vegetation, buildings,etc. The NAAQS also consider long-term exposure tothe hazards. Thus, because the specifications are set toprotect sensitive populations over long periods ofexposure, NAAQS are conservative for healthy astro-nauts. It would not be unreasonable for NASA to adopthigher concentration standards for human missions toMars with durations of 2 years or less.

NASA’s Advanced Environmental Monitoring andControl Program has recognized the need for generalparticulate monitoring in space habitats, recommendingthat respirable particles less than 10 microns in diameterbe quantified in the range 0.01 to 10 mg/m3 (NASA,1996). This measurement range is well suited for moni-toring particulates at the concentrations the committeehas determined are necessary to protect astronauts fromexposure to toxic elements. If NASA chooses to limitrespirable particulate concentrations to below 1 mg/m3,the cancer risk from toxic elements will also bereduced, since the relationship between the risk of get-ting cancer and the allowable concentration of airborneparticulate matter is taken to be linear. This means thatif NASA allows 10 mg/m3 of particulate matter in thehabitat, the risk range of getting cancer will increasetenfold. On the other hand, if the concentration of par-ticulate matter is one order of magnitude lower, that is,0.1 mg/m3, the protection to the astronauts increases byone order of magnitude.

Biological Degradation and Equipment Corrosion

There are high concentrations of sulfur and chlorinein Martian soil (Clark et al., 1982; Wanke et al., 2001).

This implies that both the soil and airborne dust mightbe acidic, which could pose a hazard if they were intro-duced into an astronaut habitat. When inhaled by astro-nauts, acidic soil and dust could degrade their lungtissue and, if humidified and allowed to penetratecontrol units inside the habitat, could corrode sensitivecritical equipment, such as control circuits. Even withthe filtration systems discussed above, the filtrationlevel may not be stringent enough to protect astronauthealth and critical mechanical equipment from dust andsoil that are extremely acidic.

On the other hand, strong oxidants detected inMartian soil by the Viking biology experiment wouldbe inactivated by humidification inside the astronauthabitat. It is therefore essential that NASA implementproper humidification in conjunction with the filtrationsystem as part of habitat atmosphere conditioning. Thecommittee concluded that even if strong oxidants arepresent, if the dust level is maintained at 1 mg/m3 orless and appropriate humidification systems are inplace, there will be negligible risk associated with oxi-dation on the Martian surface.1


The committee recommends that NASA measure thepH and buffer capacity of Martian soil and airbornedust so that mission planners may better understand thepotential corrosive effects of the soil and airborne duston astronauts and critical systems inside the habitat.This measurement could be made on the surface ofMars or from a sample of soil and airborne dustcollected from the Martian atmosphere and returned toEarth.

Recommendation: In order to evaluate the poten-tial corrosive effects of Martian soil and airbornedust on humans and critical systems in a humidifiedenvironment, NASA should measure the pH andbuffer capacity of soil and airborne dust either viaan in situ experiment or on Earth with returnedsamples of soil and airborne dust collected from theMartian atmosphere.

1It should be noted that the Viking results were unexpected at thetime, and the full nature of the oxidants has not been determined byfollow-up experiments.


As stated above, the committee determined that oxi-dizing agents in soil and airborne dust do not pose ahealth hazard to astronauts if proper filtration andhumidification levels are maintained. However, ifNASA decides not to implement the necessary engi-neering controls or for other science-related reasonschooses to measure the oxidation properties of Martianairborne dust and soil, then the measurement should beperformed on the surface of Mars rather than via asample return. The committee is concerned that theoxidants might dissipate during a sample return transferunless the sample is maintained in near-Martian condi-tions during transit. If NASA chooses to measure theoxidizing characteristics of the Martian environment,the committee recommends exposing a variety ofmaterials, such as space suit material, to the Martianatmosphere and observing the effects of oxidants onthe materials by optical or other measurement tech-niques.

Hazardous Organic Compounds

Organic carbon “includes all compounds of carbon,including straight chain, closed ring and combinations,except such binary compounds as the carbon oxides,carbides, carbon disulfide, etc.” (Lewis, 1997). Organiccompounds are the proverbial building blocks of life.However, their presence does not necessarily indicatethat life is or ever was present. Certain organic com-pounds can be highly toxic to humans, even if thosecompounds are not associated with a life-form. Thisthreat should be evaluated in planning the first humanmission to Mars.

Organic compounds on Mars, if present, could havecome from several sources, including meteorite impact,photochemical synthesis, and Martian biologic activity.At the two Viking landing sites, it was determined thatwithin the limits of the gas chromatography-massspectrometer experiment, the Martian soil contained noorganic compounds to a detection limit of 1 ppb(Biemann et al., 1977). Experimental conditionsrestricted detection to organic compounds that couldbe volatilized and/or pyrolyzed (i.e., released from thesoil) at up to 500 degrees Celsius. While this constraintprecluded the direct detection of living organisms andvery high molecular weight materials such as somepolymers, the temperature was high enough to volatilizeall known organic compounds within the mass detec-tion range of the mass spectrometer (12 to 215 atomicmass units). The temperature was also sufficient to

pyrolyze many larger organic compounds into smallerproducts detectable by the mass spectrometer. Overall,the gas chromatography-mass spectrometer resultsstrongly indicate the absence of appreciable quantitiesof organic carbon on the Martian surface.

This lack of detectable quantities of organic carbonis most likely a result of the abundance of a strong oxi-dizing agent on the surface that is produced by ultra-violet radiation from the Sun. Any hazard would mostlikely come from handling subsurface samples thatmight contain organic compounds. SNC meteoritesamples indicate that there may be very small amounts(in the parts per billion range) of organic compounds inthe subsurface, which would not represent a hazard.The committee also believes that there will not be anythreat from organic compounds in the airborne dust,because oxidants in the atmosphere would have brokendown those compounds.


The committee concludes that if organic carbon isnot detected in Martian soil, there is no hazard fromorganic compounds. If organic carbon is present, it maypresent a hazard to the astronauts through one of twomechanisms: toxicity or infection from a life-form. Theformer is discussed in this chapter, while the latter isdeferred to the discussion of biohazards in Chapter 5.

From a review of the EPA IRIS database, the com-mittee found that organic chemicals pose a risk similarto that posed by toxic metals at the same concentration.Therefore, the committee concludes that if organiccarbon is present at a concentration of more than150 ppm in soil to which astronauts might be exposed,a possible threat exists. Filtration systems that reduceastronaut exposure to organic carbon to concentrationsless than 150 ppm would mitigate this threat.

If experiments determine that organic carbon ispresent in concentrations greater than 150 ppm, thesubsurface soil should be considered a toxic hazarduntil proven otherwise. NASA must then determinewhich compounds constitute the organic carbon byreturning a sample from that specific location to Earth.The reader will learn in Chapter 5 that the need to assessthe potential threat posed by a hazardous life-form con-sisting of organic carbon requires a more stringentmeasurement of organic carbon concentration. For thisreason, the committee’s recommendation on the mea-surement of organic carbon on Mars is deferred toChapter 5.

36 SAFE ON MARS

TOXICITY OF MARTIAN ATMOSPHERIC GASES

Based on previous in situ measurements, the Martianatmosphere has been determined to be composedpredominantly of carbon dioxide (95 percent), withnitrogen, argon, and oxygen (all nontoxic) present inabundances greater than 0.1 percent (Owen, 1992).There is a small amount of toxic carbon monoxide(0.07 percent), as well as traces of ozone up to 0.2 ppm.Scientists have used ultraviolet and infrared remotesensing techniques to search for a variety of candidatetrace gases in the Martian atmosphere. No organicmolecules or toxic gases, such as those containing N,S, P, or Cl, have been detected down to limits of0.01 ppm (Owen, 1992). Even if a habitat were ventedcompletely to the Martian atmospheric pressure of0.6 kPa (6 mbar) and then refilled with the habitat’sbreathable mixture at 100 kPa (1 bar), the dilutionfactor would be over 160. In this scenario, the astro-nauts would be exposed to less than 0.6 percent carbondioxide, 4 ppm of carbon monoxide, and 1 ppb ozoneby volume. All of these amounts are well below thecurrent NASA standard for these toxic gases. In addi-tion, the atmospheric revitalization systems on space-craft include systems for removing carbon dioxide andcontaminants. The committee expects that the samecapabilities would be provided in a human habitat onMars. In addition, any highly reactive species, such ashydroxide radicals or other highly oxidizing species,created by photochemical processes in the Martianatmosphere by ultraviolet radiation would quicklyevolve to less-hazardous chemical forms upon cominginto contact with habitat airlock surfaces. Thus, suffi-cient knowledge is already available to ascertain thatthe Martian atmosphere does not pose a toxic risk forastronauts, and no further characterization is required.Long-term oxidizing effects on materials continuouslyexposed externally is a separate problem, as discussedearlier in this chapter.

If NASA chooses to measure the oxidation proper-ties of the Martian atmosphere, the committee recom-mends—as it did with respect to measuring the oxida-tion properties of soil and airborne dust—that thismeasurement be done on the surface of Mars ratherthan via a sample return. The committee has the sameconcerns—that is, that the oxidants might dissipateduring a sample return transfer unless the sample is

maintained in near-Martian conditions during transit.If NASA chooses to measure the oxidation characteris-tics on Mars, the committee recommends exposing avariety of materials, such as space suit material, to theMartian atmosphere and to assess the effects ofsuperoxidants or other radicals on the materials.

REFERENCESAgency for Toxic Substances and Disease Registry (ATSDR). 2000.

Toxicological Profile for Chromium, 259, September.Bell, J.F., et al. 2000. “Mineralogic and Compositional Properties of Martian

Soil and Dust: Results from Mars Pathfinder.” Journal of GeophysicalResearch 105: 1721-1755.

Biemann, K., J. Oro, P. Toulmin, L.E. Orgel, A.O. Nier, D.M. Anderson,P.G. Simmonds, D. Flory, A. Diaz, D.R. Rushneck, J.E. Biller, and A.L.LaFleur. 1977. “The Search for Organic Substances and InorganicVolatile Compounds on the Surface of Mars.” Journal of GeophysicalResearch 82:4641-4658.

Clark, B.C., A.K. Baird, R.J. Weldon, D.M. Tsuasaki, L. Schnabel, andM.P. Candelaria. 1982. “Chemical Composition of Martian Fines.”Journal of Geophysical Research 87:10059-10067.

Green, H.L., and W.R. Lane. 1964. Particulate Clouds: Dusts, Smokes andMists. E. & F.N. Spon, London.

Lentz, R.C.F., H.Y. McSween, J. Ryan, and L.R. Riciputi. 2001. “Water inMartian Magmas: Clues from Light Lithophile Elements in Shergottiteand Nakhlite Pyroxenes.” Geochimica and Cosmochimica Acta65:4551-4565.

Lewis, R.J., ed. 1997. Hawley’s Condensed Chemical Dictionary, 13th ed.Van Nostrand, Reinhold, New York, p. 823.

Lodders, K. 1998. “A Survey of Shergottite, Nakhlite and ChassignyMeteorites Whole-Rock Compositions.” Meteorite and PlanetaryScience 33: A183-A190.

McSween, H.Y., and K. Keil. 2000. “Mixing Relationships in the MartianRegolith and the Composition of Globally Homogeneous Dust.”Geochimica and Cosmochimica Acta 64:2155-2166.

Morris R.V., D.C. Golden, J.F. Bell, and H.V. Lauer. 1995. “Hematite,Pyroxene, and Phyllosilicates on Mars: Implications from OxidizedImpact Melt Rocks from Manicouagan Crater, Quebec, Canada.”Journal of Geophysical Research 100: 5319-5329.

National Aeronautics and Space Administration (NASA). 1995. NASA-STD-3000, Man-Systems Integration Standards, Vol. I, Revision B,Section 5.1.3.1. July.

NASA. 1996. Advanced Environmental Monitoring and Control Program:Technology Development Requirements. NASA, Washington, D.C.

NASA. 2000. SSP 41000. System Specification for the International SpaceStation, Revision W, Section 3.2.1.1.1.15, Part F, December 20.

Owen, T. 1992. “The Composition and Early History of the Atmosphere ofMars,” in Mars, H.H. Kieffer, B.M. Jakosky, C.W. Snyder, and M.S.Matthews, eds. Tucson, University of Arizona Press.

U.S. Department of Health and Human Services, Public Health Service,Centers for Disease Control, National Institute for Occupational Safetyand Health (NIOSH). 2000. Pocket Guide to Chemical Hazards. Alsoavailable at <http://www.cdc.gov/niosh>.

Wanke, H., J. Bruckner, G. Dreibus, R. Rieder, and I. Ryabchikov. 2001.“Chemical Composition of Rocks and Soils at the Pathfinder Site.”Space Studies, Revision 96:317-330.

37

5

Potential Hazards of the Biological Environment

The committee was charged with addressing issuesof biological risks on Mars from two perspectives:(1) ensuring the safety of astronauts operating on thesurface of Mars and (2) ensuring the safety of Earth’sbiosphere with respect to potential back-contaminationby a Martian organism from returning human missions.

The probability that life-forms1 exist on the surfaceof Mars (that is, the area exposed to ultraviolet radia-tion and its photochemical products) is very small.However, as a previous NRC study (NRC, 1997) notes,there is a possibility that such life-forms exist there “inthe occasional oasis,” most likely where liquid water ispresent, and, furthermore, that “uncertainties with regardto the possibility of extant Martian life can be reducedthrough a program of research and exploration.”

This charge to the committee results in a dilemma.How can NASA use human ingenuity and creativity onMars to search for life when that life (if it exists) maypose a threat to astronaut health and safety (and there-fore to the success of a human mission) as well as toEarth’s biosphere?

ENSURING THE SAFETY OF ASTRONAUTS

The committee believes that it is highly unlikely thatinfectious organisms are present on Mars. The sameNRC study (NRC, 1997) that focuses on the possibilitythat Martian organisms could be agents of infectiousdisease also states on page 21 as follows: “The chances

that invasive properties would have evolved in putativeMartian microbes in the absence of evolutionary selec-tion pressure for such properties is vanishingly small.Subcellular disease agents, such as viruses and prions,are biologically part of their host organisms, and anextraterrestrial source of such agents is extremelyunlikely.” The uncertainty surrounding whether or notlife exists on Mars may not be resolved until afterastronauts arrive on the planet (NRC, 1997).2 Ofcourse, if life does exist on Mars, it could represent abiological hazard to astronauts.

1Discrete entities that can actively utilize the energy and matterof their environment, react to a stimulus, and modify and propagatethemselves.

2Some argue that since no life-form, or no conclusive evidenceof hazardous life, was identified in meteorites from Mars found onEarth, there is no biohazard threat from Mars. These SNCmeteorites are, after all, handled without special precautions in thelaboratory. The committee referenced the analysis of the SNCmeteorites in Chapter 4 in the discussion of toxic metals on Mars.The use of SNC meteorite data is not appropriate to this argumentregarding biohazards on Mars.

Extensive exposure to cosmic rays during a meteorite’s travelsin interplanetary space, during which the meteorite was exposed tosterilizing doses of radiation, would probably have destroyed anylife contained in it (Clark, 2001).

Furthermore, the rocks at the Pathfinder landing site and the soilsat all Mars landing sites have quite different compositions fromthese meteorites. Therefore, the known Martian meteorites are notnecessarily representative of all materials on the surface of Mars.

Finally, it must be acknowledged that there have been biologicalupheavals through the course of paleontological history here onEarth, including millions of species extinctions, with few explana-tions of cause. It is impossible to determine the cause of theseupheavals with absolute certainty, be they physical (meteoriteimpact) or biological (life introduced onto Earth). While the possi-bility that life was introduced on Earth from elsewhere is unlikely,

38 SAFE ON MARS

Martian biological contamination may occur ifastronauts breathe contaminated dust or if they contactmaterial that is introduced into their habitat. If anastronaut becomes contaminated or infected, it isconceivable that he or she could transmit Martianbiological entities or even disease to fellow astronauts,or introduce such entities into the biosphere uponreturning to Earth. A contaminated vehicle or item ofequipment returned to Earth could also be a source ofcontamination.

If an astronaut were infected by a Martian life-form,the infection could potentially be observed and treated,or at least evaluated, before the astronaut lands back onEarth. However, once an astronaut has been directlyexposed to such life, it would be very difficult, to thepoint of being impractical, to determine conclusivelythat the astronaut would not pose a contamination threatto Earth’s biosphere. In such an event, NASA might befaced with requiring quarantine and surveillance ofreturning astronauts until it is determined that a threatno longer exists.

To the degree that NASA has confidence that thelife-support systems and habitats can isolate astronautsfrom hazardous materials, it can more aggressivelyplace those habitats into an environment of unknownconstituency. In the extreme, if 100 percent protectionwere assured by such isolation systems, NASA couldsend astronauts to an area with known biologic hazards.If 100 percent protection against exposure from theunknown cannot be assured, the hazard must bemitigated by gaining confidence that the environmenton Mars poses an acceptable risk.

ENSURING THE SAFETY OFEARTH’S BIOSPHERE

While the threat to Earth’s ecosystem from the re-lease of Martian biological agents is very low, “the riskof potentially harmful effects is not zero” and cannotbe ignored (NRC, 1997). In light of experience gainedduring Apollo missions to the Moon, a previous NRC

report (NRC, 1993) concludes, “It would, however, bevirtually impossible to avoid forward-contamination ofMars or back-contamination of Earth from humanexploration.” This committee understands that thethreat from back-contamination cannot be eliminatedwith all certainty, but it is confident that if NASA takesthe steps outlined in this report, the threat from back-contamination will be minimized.

NASA should assume that if life exists on Mars, itcould be hazardous to Earth’s biosphere until provenotherwise (NRC, 1997). As such, NASA should ensureproper quarantine or decontamination of equipmentthat may have been exposed to a Martian life-form.


For the purposes of identifying precursor missionsthat must take place prior to human exploration ofMars, the committee did not address the general ques-tion of how to detect life on Mars at every location. Forinstance, it does not matter from the standpoint ofhuman safety or Earth ecosystem protection if lifeexists on Mars in places inaccessible to astronauts, suchas in nonfriable layers of rock or in some deep sub-terranean cavern. It does matter, however, if life existsin any material that the astronauts or their spacecraftand equipment might contact, such as Martian airbornedust and surface or near-surface regolith. With theseconsiderations in mind, the committee recommendsthat NASA employ the concept of zones of minimalbiologic risk (ZMBRs) for astronaut exploration. Thesezones, operational areas on the surface of Mars, wouldhave been predetermined, to the maximum extentpracticable, to be devoid of life or to contain only life-forms that would not be hazardous to humans or Earth’sbiosphere.

Establishing Zones of Minimal Biologic Risk (ZMBRs)

To protect Earth from contamination by Martian life-forms aboard a returning human mission and astronautswhile on the surface of Mars, NASA should firstattempt to determine whether life exists (1) at thephysical locations where astronauts will be operatingand (2) in the Martian material to which astronauts willbe exposed.

The establishment of a ZMBR might initially bebased on an in situ testing protocol, to be discussedshortly, conducted prior to a human visit. Once a land-

the uncertainty makes it impossible to prove that meteorites fromanother planet have never had an effect on Earth’s biosphere (NRC,1997). These considerations, and the consideration that a goal offuture human missions will be to search for life-forms in Martianoases, if those oases exist, negate the argument that there is nobiohazard threat from Mars based on the existence of sterile Martianmeteorites.

POTENTIAL HAZARDS OF THE BIOLOGICAL ENVIRONMENT 39

ing site is established as a ZMBR, the astronauts canland and freely operate within it.

The committee recognizes that the requirement toestablish and operate in a ZMBR, while intrinsic to thestudy charter to manage risk to astronauts, may be inconflict with one of the primary goals of the explora-tion of Mars: to find extraterrestrial life.

As stated above, any and all indigenous materialsfrom Mars must be considered as health hazards andecohazards and be contained until proven otherwise. Iflife is discovered on Mars, the committee maintainsthat such life must be considered a risk to humans aswell as Earth’s ecosystem until proven otherwise.

A large variety of chemical and microscopy analyti-cal techniques could be used for detecting extant life,including measurements of biomass, use of specificmolecular probes, and/or detection of growth, meta-bolic activity, or the presence of enzymes. A survey onlife detection techniques was carried out during anApril 2000 workshop sponsored by the NRC but notrelated to this study (NRC, 2002c). The committeebelieves that a search for organic carbon might be thequickest way to establish that the landing site is aZMBR. While some have suggested the possibility ofnon-carbon-based life on Mars, such as silicon-basedlife, this committee agrees with assumptions made byprevious NRC committees that should hazardous lifeexist on Mars it would be carbon-based (NRC, 2002a,2002b).

The committee recommends that NASA conduct aprecursor in situ experiment to determine if organiccarbon is present at a location as close as reasonablypossible to the landing sites selected for humanmissions to Mars. The committee acknowledges that itwill be difficult to land exactly at the location of theanticipated human landing sites. It is beyond the scopeof this report to suggest the parameters that would beused in determining the radius of a ZMBR. However, itis conceivable that NASA can establish criteria so thatthe results of testing one surface material (such as near-surface regolith) can be assumed to apply to all likematerials, in like geologic settings, in nearby regionson Mars.

For instance, an in situ test might determine that theuppermost 10 centimeters of regolith are free of anyindications of life at one location on a large flat plainon the surface of Mars. NASA could then reasonablyconclude that the entire plain is likely to be free of lifeand therefore a ZMBR. If, on the other hand, the plainis scattered with sizable rocks, the entire plain would

not be designated as a ZMBR until tests were con-ducted in both the open regolith and under representa-tive rocks. In that case, the uppermost few centimetersof regolith and the material sheltered under a rock arelike materials (i.e., both are regolith) but are not in likegeologic settings, as one portion of the regolith isexposed and the other portion sheltered.

It is generally believed that any life-form retrievablefrom the surface of Mars, especially any life-form thatwould be a threat to humans or Earth’s ecosystem,would probably be bacterial in nature and size, andpossibly similar to bacterial spores. It is highly unlikelythat viruses hazardous to terrestrial organisms will bepresent on Mars since viruses are highly adapted patho-gens that infect very specific host organisms andrequire those specific host organisms for replicationand survival (NRC, 1997).

On Earth, certain bacteria form spores when stressedby depletion of critical growth factors such as water orother carbon/nitrogen (food) sources. Bacteria survivefor long periods in the spore form until new growthopportunities are available. A typical bacterial spore is1 to 2 microns in major dimension and is made up ofabout 1 × 10–12 grams of organic matter. The amount oforganic material in ecosystems on Earth in nonlivingform, such as foodstock or waste material, is manytimes greater than the amount of organic material con-tained in living organisms.

For purposes of illustration, making the generousassumption that there are at least 10 bacteria-sizeentities containing up to 1 × 10–12 grams of organicmaterial each per gram of soil and that there is 10 timesas much extraneous organic material, the concentra-tion of organics in the soil for this life-detectionthreshold would still be only 0.1 ppb by weight. This islower than the 1 ppb detection limit for organics pos-sible with the gas chromatography-mass spectrometerinstrument flown on the Viking mission (Biemann etal., 1977). The results of those tests were used by manyscientists to infer that life could not be present. Thislife-detection threshold is controversial, however. Thiscommittee did not have the necessary expertise to criti-cally evaluate the lower limit of organic carbon neededfor life detection. It therefore urges NASA to set anoperational value for the life-detection threshold limitthrough a separate advisory process drawing on a broadrange of relevant expertise.

Such a limit should be supported by a rationale forspecifying the least amount of carbon that could signifythe possible existence of an organism or organism

40 SAFE ON MARS

activity as discussed above. Certain classes of organiccompounds in soil or other samples might be specifiedat different life-detection threshold levels. Aminoacids, for example, would probably be considered moreindicative of a possible life-form than highly refractorykerogen-like materials—that is, mixtures of complexorganic polymers of high molecular weight similar topetroleum. The presence of excess optically activeorganic molecules could also be considered indicativeof a life-form.

These analyses should be made on a sample orsamples from the surface and down to a depth at whichastronauts may be exposed. Unless measurement tech-niques or capabilities are advanced significantlybeyond current capabilities, a sample return willprobably be required to establish that the plannedlanding sites are in a ZMBR.

As a further example (and for discussion purposesonly), if NASA’s mission scenario uses rockets forlanding, the exhaust from the rockets will eject theregolith to some depth, exposing subsurface materialthat the astronauts will be walking on during EVAs.Also, some of the ejected materials will most certaintycontact the outside of the return spacecraft, potentiallycontaminating the vehicle surfaces with some life-formthat might exist below the Martian surface. If such alanding scenario is planned, the precursor organiccarbon measurement must take place down to the samedepth as that from which materials will be ejected.

It is conceivable that NASA might generate apassive soft-landing capability that disturbs the regolithvery little. If that is the case, then the precursor in situorganic carbon test needs to be conducted only on theMars surface and near surface.

NASA will need to establish procedures for astro-nauts to expand the ZMBR. One such procedure mightinvolve the use of an iterative robotic testing processon the surface and subsurface. In the process ofexpanding the zones, NASA should take appropriatecaution to ensure that astronauts will not come into per-sonal contact with surface and subsurface soil samplesbefore the soil has been fully characterized with regardto the possible presence of Martian life-forms.

Some of the tests used during this pre-excursionprocess may be fairly straightforward and may onlyrequire that the robots determine if organic carbon ispresent. However, more complex experiments mayrequire human interaction—for example, robots mightbring samples back to the habitat laboratory for morecritical examination by humans. Astronauts might then

use chemistry, microscopy, and biological challenge(interaction with animals, plants, and/or cell cultures)in the testing process to determine if a life-form existsand if that life-form is potentially hazardous to humansor to Earth’s biosphere.

A life-detection experimental laboratory in the habi-tat will be a complex and critical component of Marsexploration, since it must in effect isolate the astronautsfrom all test materials, using appropriate biologicalcontainment techniques (Richmond and McKinney,1999). The committee suggests that NASA plan for thedevelopment, deployment, and design of a facility forconducting life-detection tests when the astronauts landon Mars should the astronauts plan to explore new areasthat have the potential to harbor life. Such a life-detection facility would be important since the pre-cursor missions, including in situ and sample returnmissions, will not conclusively prove that life does notexist somewhere in an oasis on the Martian surface orsubsurface. Alternatively, NASA could implement aset of safeguards to ensure that astronauts and returnequipment are never directly exposed to new, uncharac-terized materials during the entire mission.

Recommendation: The committee recommends thatNASA establish zones of minimal biologic risk(ZMBRs) with respect to the possible presence ofMartian life during human missions to Mars. Inorder to do so, NASA should conduct a precursor insitu experiment at a location as reasonably close tothe human mission landing sites as possible todetermine if organic carbon is present. The mea-surement should be on materials from the surfaceand down to a depth to which astronauts may beexposed. If no organic carbon is detected at or abovethe life-detection threshold, the landing site may beconsidered a ZMBR. If no measurement techniquecan be used to determine if organic carbon ispresent above the life-detection threshold, or iforganic carbon is detected above that threshold, acontained sample should be returned to Earth forcharacterization prior to sending humans to Mars.

If No Organic Carbon Is Detected

If a precursor in situ organic carbon experiment candetermine that no organic carbon is present above thelife-detection threshold, the committee recommendsthat NASA judge the near surface on Mars at the land-ing site to be a ZMBR. Once this initial minimal risk


zone is established, the human mission may land andoperate on the surface in the region around the site ofthe precursor organic carbon test (with like materials,in like geologic settings) and return to Earth reason-ably confident that there is no biological hazard thatcould harm human health or Earth’s ecosystem. In thisinstance, no sample return is required prior to the firsthuman visit, at least from the perspective of protectingastronauts or Earth’s biosphere. It should be noted,however, that there are other scientific reasons for asample return, as discussed in other NRC reports thatcall for a sample return (NRC, 2002a).

As stated above, there are currently no measurementtechniques or capabilities available for such in situ test-ing. If such capabilities were to become available, oneadvantage is that the experiment would not be limitedby the small amount of material that a Mars samplereturn mission would provide. What is more, with theuse of rovers, an in situ experiment could be conductedover a wide range of locations.

Positive or Inconclusive Organic Carbon Measurementand Precursor Sample Return

If a precursor in situ organic carbon experimentindicates the presence of organic carbon on Mars abovethe life detection threshold, the committee recommendsthat a sample must be returned to Earth from the loca-tion and depth where the organic carbon is discoveredif no suitable in situ life-form confirmation technolo-gies are available, as is the case now. The returnedsample should be considered hazardous, and NASAshould follow the same quarantine procedures as out-lined in previous NRC studies (NRC, 2002b). Thesequarantine procedures should be applied to all materialsreturned from Mars, both precursor samples andsamples returned from human missions. However,NASA should also attempt to ensure that any possiblelife-form in the sample would survive the trip to Earth.

The location and depth of such a sample returnwould be dictated by NASA mission operations. IfNASA determines that rocket exhaust on landing willuncover half a meter of Martian regolith and organiccarbon is discovered to exist in material exposed at thatdepth, then a sample from that depth should be returnedto Earth prior to human exploration of Mars.

If a precursor mission sample is returned to Earthsubsequent to a positive organic carbon test result, thefirst priority should be to determine if the organiccarbon constitutes a life-form. If the organic carbon is

not a Martian life-form, then the location from whichthe sample was taken may be considered a ZMBR. Onthe other hand, if the organic carbon in a sample returndoes contain a life-form, there are varying levels ofsafety that can be established in the region from whichthe sample was taken, following a determination ofwhether such an organism is in fact a biohazard.

The findings and recommendations in this report arerelevant to activities represented above the dotted linein Figure 5.1, a decision diagram offered by thecommittee as a preliminary guideline. If life is indeeddiscovered on Mars, the strategy for exploiting that dis-covery will certainly develop based upon the specificnature of the life identified. The open questions thatremain below the dotted line can be answered by firsttesting the life-bearing sample for hazards to humans,such as infectivity, and then evaluating it for hazards toEarth’s biosphere. The committee does believe that if aMartian life-form is proven to be harmless to humansand to Earth’s biosphere, then the astronauts mayoperate in the presence of the life-form.

There has been some concern that if a sample returnis required, the planning for the first human mission toMars may be delayed until a sample can be obtained.The committee believes that, even should a sample berequired because organic carbon has been found, abaseline plan for a mission to Mars and even hardwaredevelopment may still proceed under the assumptionthat a sample return will not find anything significantenough with regard to Martian biology to invalidatethe baseline mission plan.

Sample Return: Additional Benefits

The Viking lander experiments were criticizedbecause life-detection tests gave presumably false posi-tive experimental results in some cases and clearlyunexpected results in other cases, apparently due to thecomplex chemistry of the Martian soil. A secondary,though not mandatory, benefit of a precursor samplereturn mission is that a returned sample would allowresearchers to establish effective and reliable testingmethods that would reduce the risk of false positive orfalse negative results. Soil chemical analyses con-ducted on returned samples would enable improvedlife-detection protocol testing. False positives or falsenegatives in testing for life on the surface of Mars couldcreate serious problems for astronauts. For instance, ifa test produces a false negative, that is, the test resultsindicate that Martian life is not present when it actually

42 SAFE ON MARS

FIGURE 5.1 Mars biology testing protocol.

protocol.

Precursor Organic Carbon Test: CriticalDetection Limit to Be Determined by NASA

Negative result

• Zone of minimal biological risk established for like materials in like geologic settings • No precursor sample return required

• Minimal biological risk zone not established• Possible life-form present

• Precursor sample return from specific location required

Positive or inconclusive result

Test for Martian life-form

Negative result Positive result

• Zone of minimal biological risk established

• No life-forms present

Test for life-form infectivityto humans

Hazardous to humans

Not hazardous to humans

• Astronauts may operate safely in presence of life- form

• Life-form still represents an ecohazard to Earth

Test for life-formimpact on Earth'sbiosphere

Possible impactNo impact

• Astronauts may not operate safely in presence of life-form • Astronauts may

operate safely in presence of life-form

Ecohazard present


is, NASA could improperly establish a ZMBR. In thiscase, the risk would indeed not be minimal, and astro-nauts could be exposed to hazardous Martian life-forms. If a false positive is returned, it may be believedthat astronauts have been exposed to a Martian life-form when in reality no life-form is present.

RETURN VEHICLE CONTAMINATION

Great care must be exercised to ensure the contain-ment of all material returned from Mars to Earth. AllMartian material returned to Earth must be quarantined.There must be a sterile, intermediate transfer conductedin space that ensures that Earth’s environment will notbecome exposed to any Martian material, includingdust or soil deposits on the outside surface of the returnvehicle. The protocols for such a sterile transfer will becomplex and, if the transfer is unsuccessful, mayrequire that the return vehicle be discarded in spaceand never returned to Earth. Ultimately, however, onlycontained materials should be transported back toEarth, unless sterilized first (NRC, 1997).

Other protocols may need to be established if, forunforeseen reasons, it is not possible to isolate thereturn vehicle. In that case, life-detection tests must beconducted within the near-surface Martian regolith that

could adhere to the outside of the return vehicle whenit lifts off from the Martian surface, unless precursormission measurements (in situ or sample return) haveshown that such material is not biologically hazardous.

REFERENCESBiemann, K., J. Oro, P. Toulmin, L.E. Orgel, A.O. Nier, D.M. Anderson,

P.G. Simmonds, D. Flory, A. Diaz, D.R. Rushneck, J.E. Biller and A.L.LaFleur. 1977. “The Search for Organic Substances and InorganicVolatile Compounds on the Surface of Mars.” Journal of GeophysicalResearch 82:4641-4658.

Clark, B.C. 2001. “Planetary Interchange of Bioactive Material: ProbabilityFactors and Implications.” Origins of Life and Evolution of theBiosphere 31:185-197.

National Research Council (NRC). 1993. Scientific Prerequistes for theHuman Exploration of Space. National Academy Press, Washington,D.C.

NRC. 1997. Mars Sample Return: Issues and Recommendations. NationalAcademy Press, Washington, D.C.

NRC. 2002a. Assessment of Mars Science and Mission Priorities. NationalAcademy Press, Washington, D.C.

NRC. 2002b. The Quarantine and Certification of Martian Samples.National Academy Press, Washington, D.C.

NRC. 2002c. Signs of Life: A Report Based on the April 2000 Workshop onLife Detection Techniques. National Academy Press, Washington, D.C.,in press.

Richmond, J.Y., and R.W. McKinney, eds. 1999. Biosafety in Microbio-logical and Biomedical Laboratories. Department of Health and Hu-man Services, Centers for Disease Control and Prevention and the Na-tional Institutes of Health, Washington, D.C.

Appendixes

47

Appendix A

Statement of Task

The principal objective of the study will be to exam-ine the role of robotic Mars exploration missions inassessing the risks to human exploration of Mars due topossible environmental, chemical, and biological agentson the planet. The fundamental question for the studywill be to review the environmental, chemical, andbiological risks to humans operating on Mars and toconsider how the Mars robotic program can provideanswers to mitigate those risks prior to a human mission.In the course of the study, the committee will review:

1. Prior studies by the NRC1 and others regardingthe goals, objectives, and requirements of NASA’sMars exploration activities, both human and robotic;

2. Requirements identified by the NASA HumanExploration and Development of Space Enterpriseto be levied on the robotic exploration program;

3. Plans of the robotic Mars exploration program inthe context of how it can support preparations foreventual human exploration missions; and

4. NASA plans and requirements for validation ofcritical technologies in the actual Martian envi-ronment prior to committing to a program ofhuman exploration. The review of critical tech-nology validation will emphasize those tech-nological issues that are directly relevant tomanaging environmental, chemical, and biologicalrisks to humans operating on Mars.

Among the questions that the committee shouldconsider in conducting its review are the following:

• How well are the environmental, chemical, andbiological risks to humans on Mars characterizedand understood?

• What additional measurements or data from Marsare necessary to properly characterize the risks?

• Are there technologies which must be demon-strated in the Mars environment in order to ensurethat environmental, chemical, and biological riskscan be managed on a human mission to Mars?

• Must samples from Mars be returned to Earthprior to sending a human mission for any reason,including:1. Gaining sufficient confidence in understand-

ing chemical and biological risks to humans onMars, or

2. Ensuring the safety of the Earth’s biospherewith respect to potential back-contaminationfrom a returning human mission?

1Relevant NRC reports:

Microgravity Research in Support of Human Exploration andDevelopment of Space and Planetary Bodies, Space StudiesBoard (SSB), 2000.

Certification and Curation of Martian Samples, SSB, in review.Signs of Life: A Report Based on the April 2000 Workshop on

Life-Detection Techniques, SSB and Board on Life Sciences,in preparation.

Assessment of the Strategy for Mars Exploration, SSB, in prepa-ration.

Space Technology for the New Century, Aeronautics and SpaceEngineering Board (ASEB), 1998.

Advanced Technology for Human Support in Space, ASEB,1997.

Mars Sample Return: Issues and Recommendations, SSB, 1997.

FREDERICK H. HAUCK (Chair) is a member of theAeronautics and Space Engineering Board and ispresident and chief executive officer of AXA Space, acompany that specializes in providing insurance forlaunching and operating space systems. Before comingto AXA Space, Mr. Hauck was director of the NavySpace Systems Division in the Office of the Chief ofNaval Operations. Before that, he was a test pilot and amember of the astronaut corps. As an astronaut, he flewin space three times and was commander of the firstshuttle flight after the Challenger accident. He has beena member or chair of numerous panels and advisorygroups on national and international space activities.He holds degrees in physics and nuclear engineeringfrom Tufts University and the Massachusetts Instituteof Technology. Mr. Hauck is a fellow in both theSociety of Experimental Test Pilots and the AmericanInstitute of Aeronautics and Astronautics, and is anational associate of the National Academies.

HARRY Y. McSWEEN (Vice Chair) is a professor andformer head of the Department of Geological Sciencesand Distinguished Professor of Science at the Univer-sity of Tennessee. He holds degrees from the Citadel,the University of Georgia, and Harvard University(Ph.D.) and has been a member of the University ofTennessee faculty for 23 years. He recently was presi-dent of the Meteoritical Society and chair of the Plan-etary Division of the Geological Society of Americaand has served on numerous advisory committees forNASA, including the Mars Pathfinder and Mars GlobalSurveyor spacecraft. Dr. McSween has served as acommittee member on the NRC Committee on Plan-

etary and Lunar Exploration from 1995 to 1998 and theCommittee on Human Exploration from 1998 to 2000.

CYNTHIA BREAZEAL specializes in robotics andartificial intelligence with an emphasis on human-robotic interaction. Her past work has included behavior-based control for autonomous planetary microrovers,rough terrain locomotion, and fault-tolerant behavior.Dr. Breazeal is currently a professor at the MIT MediaLab. She has published extensively in books, maga-zines, and journals on a wide range of topics, includingautonomous robots, planetary microrovers, leggedlocomotion, fault-tolerant behavior, visual attention,affective speech recognition, humanoid robotics, com-putational models of emotion and motivation, expres-sive speech synthesis, facial animation, behaviorarbitration, social interaction between humans androbots, and learning. She has coauthored a graduate texton embodied intelligence and has authored anotherbook, Designing Sociable Robots, from MIT Press. Herwork has appeared extensively in the popular press,including Business Week, Time, U.S. News and WorldReport, Scientific American, Wired, the New YorkTimes, and the Washington Post and on NBC’s NightlyNews and NPR’s Morning Edition, as well as in variousinternational publications.

BENTON C. CLARK is chief scientist for flightsystems, Lockheed Martin Astronautics (LMA) inDenver, and has over 40 years experience in futuremission design, spacecraft design and operations, spacescience, and development of advanced space instru-mentation. He is director of the Advanced Planetary

48

Appendix B

Biographical Sketches of Committee Members

APPENDIX B 49

Studies group, where flight designs for Discovery andMars missions are conceived and developed. Dr. Clarkhas over 55 publications and 90 reports, abstracts andpresentations in instrumentation, planetary missions,radiation, space science, planetary geochemistry, exo-biology, and other fields of research and development.Dr. Clark has a B.S. in physics from the University ofOklahoma, an M.A. in physics from the University ofCalifornia, and a Ph.D. in biophysics from ColumbiaUniversity.

VON R. ESHLEMAN (NAE) is a professor emeritusat Stanford University. His main publications relate toelectromagnetic remote sensing, with particular empha-sis on spacecraft radio and radar systems. Dr. Eshlemanhas authored or coauthored approximately 175 publi-cations. He has been a team leader or member for manyNASA exploration spacecraft, including Mariner,Pioneer, Viking, Voyager, and Galileo. The missionswere designed to study planetary atmospheres, iono-spheres, magnetospheres, surfaces, rings, and moonsof the solar system. He was also a member of the advi-sory board for NASA’s lunar and planetary missions.Dr. Eshleman received several distinguished alumniawards from George Washington University andStanford, as well as several NASA medals for excep-tional scientific achievement as a result of his work onthe atmosphere of Mars in 1965 and the atmospheres ofJupiter, Saturn, and Titan in 1981. He is also a found-ing member of the Planetary Society.

JOHN HAAS is group leader for the New Technolo-gies Group in Applied Research Associates’ NewEngland Division, located in South Royalton, Vermont.Dr. Haas is currently working on the development ofsensors and analytical methodologies for process,environmental, biotechnical, and geotechnical monitor-ing applications, including planetary exploration.Dr. Haas received his Ph.D. in analytical chemistryfrom the University of Massachusetts and has beenprincipal investigator on nearly two dozen researchprograms in the areas of field analytical chemistryinstrumentation, detection of chemicals of concern tohuman health, sensor development, geochemistry/geophysics, in situ sampling and measurement tech-niques, remote fiber-optic sensing, laser spectroscopy,and miniature devices. Among his achievements arethe invention of various Raman, fluorescence, absor-bance, and refractive index fiber-optic probes, aminiature fluorescence sensor, and a unique Raman

spectrograph. Dr. Haas has also developed an array ofsmall chemical, radiation, and geophysical sensors andsamplers for use in the cone penetrometer, a subsurfacegeophysical and geochemical characterization tool.Several of his current research projects are directed atthe detection of hazardous chemicals, including bio-logical endotoxins, metallocyanides, perchlorate, andradionuclides.

JON B. REID is a professor at the University ofCincinnati Medical College’s Department of Environ-mental Health. Dr. Reid has over 20 years of experi-ence in toxicology and human health risk. He alsoworks for the National Council on Aging and isassigned to the Environmental Protection Agency’sNational Center for Environmental Assessment, wherehis work includes preparation of a methodology fordeveloping a comprehensive pathogen risk assessmentprocedure. His other consulting activities includeresearch in exposure and risk from chemicals in theenvironment and the workplace.

JONATHAN RICHMOND is the director of the Officeof Health and Safety at the Centers for Disease Controland Prevention in Atlanta, Georgia, and is an inter-national authority on biosafety and laboratory con-tainment design. Dr. Richmond was trained as ageneticist, worked for 10 years as a research virologist,and has been involved in the field of biosafety for thepast 25 years. He has authored many scientific pub-lications in microbiology, chaired many nationalsymposia, edited numerous books, and is an inter-national consultant to ministries of health on labora-tory safety and training. He also serves as a director ofa World Health Organization collaborating center onapplied biosafety.

RONALD E. TURNER is a principal scientist atANSER Corporation. Dr. Turner has extensive experi-ence in radiation effects on humans in space; specifi-cally, he has more than 20 years of experience in spacesystems analysis, space physics, orbital mechanics,remote sensing, and nuclear and particle physics. Hisrecent research has included risk management strate-gies for solar particle events during human missions tothe Moon or Mars. He has been an invited participantat NASA workshops looking at space radiation/biologymissions, life science mission requirements for Mars2001 and 2003, and the impact of solar particle eventson the design of human missions.

50 SAFE ON MARS

WILLIAM “RED” L. WHITTAKER is the FredkinProfessor of Robotics at the Robotics Institute ofCarnegie Mellon University. He is the director ofCarnegie Mellon’s Field Robotics Center, which hefounded in 1986, and cofounder and chief scientist ofRedZone Robotics. His research centers on walkers forplanetary exploration, mobile robots in unpredictablefield environments such as work sites and naturalterrain; computer architectures to control mobilerobots; modeling and planning for nonrepetitive tasks;

complex problems of objective sensing in random ordynamic environments; and integration of completerobot systems. Projects under Dr. Whittaker’s direc-tion include unmanned robots to explore planetarysurfaces and volcano interiors, automation of miningmachines and farm equipment, remote work systemsfor nuclear facility decommissioning, mobile robots forhazardous waste site investigation, and autonomousland vehicle navigation.

APXS alpha-proton-x-ray spectrometer (experi-ment on Mars Pathfinder)

ARR acceptable risk range (for toxic metals)COT Committee on Toxicology (NRC)EPA Environmental Protection AgencyEVA extravehicular activityGCR galactic cosmic radiationIRIS Integrated Risk Information System (EPA)ISRU in situ resource utilizationJPL Jet Propulsion Laboratory (NASA)MEPAG Mars Exploration Program/Payload

Analysis GroupNAAQS National Ambient Air Quality Standard

(EPA)NASA National Aeronautics and Space Adminis-

tration

51

NIOSH National Institute for Occupational Safetyand Health

NRC National Research CouncilOSHA Occupational Safety and Health Adminis-

trationPM particulate matterppb parts per billionppm parts per millionRfC reference concentrationSMAC spacecraft maximum allowable concentra-

tionSNC from Mars having landed on Earth (with

reference to a meteorite)SPE solar particle eventZMBR zone of minimal biologic risk

Appendix C

Acronyms and Abbreviations

Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface

Documents

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