The Physics, Biology, and Environmental Ethics of Making Mars ...

ASTROBIOLOGYVolume 1, Number 1, 2001Mary Ann Liebert, Inc.

Research Paper

The Physics, Biology, and Environmental Ethics ofMaking Mars Habitable

CHRISTOPHER P. MCKAY1 and MARGARITA M. MARINOVA1,2

ABSTRACT

The considerable evidence that Mars once had a wetter, more clement, environment motivatesthe search for past or present life on that planet. This evidence also suggests the possibilityof restoring habitable conditions on Mars. While the total amounts of the key molecules—carbon dioxide, water, and nitrogen—needed for creating a biosphere on Mars are unknown,estimates suggest that there may be enough in the subsurface. Super greenhouse gases, inparticular, perfluorocarbons, are currently the most effective and practical way to warm Marsand thicken its atmosphere so that liquid water is stable on the surface. This process couldtake , 100 years. If enough carbon dioxide is frozen in the South Polar Cap and absorbed inthe regolith, the resulting thick and warm carbon dioxide atmosphere could support manytypes of microorganisms, plants, and invertebrates. If a planet-wide martian biosphere con-verted carbon dioxide into oxygen with an average efficiency equal to that for Earth’s bios-phere, it would take .100,000 years to create Earth-like oxygen levels. Ethical issues associ-ated with bringing life to Mars center on the possibility of indigenous martian life and therelative value of a planet with or without a global biosphere. Key Words: Mars—Terraform-ing—Planetary ecosynthesis—Greenhouse warming—Environmental ethics. Astrobiology 1,89–109.

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INTRODUCTION

THE SURFACE OF MARS is cold and dry, the re-golith appears to contain dilute but powerful

oxidants that have destroyed any organic mate-rial, and solar UV light down to wavelengths of190 nm reaches the ground. There is no organismon Earth that could grow or reproduce undermartian conditions (e.g., Clark, 1998). Table 1 isa comparison of selected environmental andphysical properties of Mars and Earth.

Interestingly, Mars was not always this way.

There is direct evidence that early in martian his-tory, liquid water was stable and present at thesurface. The sinuous canyon in Fig. 1, Nanedi Val-lis, shows what may be the best evidence for sus-tained liquid water flow on Mars (Carr and Ma-lin, 2000). The large flood features on Marsindicate that the total inventory of water was con-siderable (Carr, 1986, 1996), possibly creating anocean early in martian history. Recent altimeterdata indicate shorelines consistent with an ocean(Head et al., 1999). Subsurface liquid water mayeven persist to the present time as indicated by

1Space Science Division, NASA Ames Research Center, Moffett Field, CA 94035.2Massachusetts Institute of Technology, Cambridge, MA 02139.

groundwater seepage and surface runoff featuresthat are ,1 million years old (Malin and Edgett,2000). Liquid water is the quintessence of life, andthe past presence of liquid water on Mars is theprimary basis for the search for evidence of pastmartian life (McKay, 1997).

The early wetter, warmer climate of Mars alsomotivates the question of purposefully restoringMars to habitable conditions. This process isknown as “terraforming” or “planetary ecosyn-thesis.” The latter term is more accurate since itavoids the misleading comparison with Earth.Humans are demonstrably capable of altering theenvironment of a planet—we are doing so onEarth primarily through changes in the composi-tion of the atmosphere. Alterations in the physi-cal properties of a planet are far beyond currenttechnology. However, as seen in Table 1, manyof the physical properties of Mars are similar tothose of Earth. The day length and obliquity(which controls the seasons) are very similar tothe values for Earth. The major differences are thelower gravity, the lower average sunlight, and thelonger year and season length. None of these dif-ferences is likely to present fundamental obsta-cles to the creation of a biosphere on Mars. ThusMars, unlike all the other worlds of our solar sys-tem, can be made habitable just by changing thecomposition of its atmosphere—the approachconsidered in this paper.

The study of planetary ecosynthesis on Mars isnow part of the Astrobiology program. In Octo-ber 1997, NASA Headquarters released a Coop-erative Agreement Notice soliciting participationin the new Astrobiology Institute. To frame the

scientific questions that would be addressed bythis Institute, the Notice listed six questions. Thefinal question was: What is the potential for sur-vival and biological evolution beyond the planetof origin? This question represents a new per-spective for the NASA research program. For thefirst time the question of life beyond the Earth isnow expanded to more than the mere survival ofastronauts. “Biological evolution” is a processthat is not a function of single individuals or evena single species. Biological evolution is a functionof many species interacting together and chang-ing their environment. Mars is the logical site fora first test of the possibilities for biological evo-lution beyond the Earth. Thus we consider as partof the Astrobiology program the question: Canecosystems be established on Mars?

While the question of the habitability of Marsis just recently entering the research agenda of as-trobiology, it has been discussed in the scientificliterature for a number of years. The first scien-tific paper dealing with planetary ecosynthesis onany planet was the seminal work of Sagan (1961)suggesting the introduction of algae into the at-mosphere of Venus. Early suggestions for mak-ing Mars habitable by altering its presumed nat-ural climate cycle were due to Sagan (1971, 1973)and Burns and Harwitt (1973). The first compre-hensive look at the habitability of Mars was thereport of an interdisciplinary summer study byAverner and MacElroy (1976). This report was toset the stage for combined biological and physi-cal approaches to the questions of restoring hab-itability to Mars. McKay (1982) followed this ap-proach and suggested that the re-creation of a

MCKAY AND MARINOVA90

TABLE 1. COMPARISON OF MARS AND EARTH

Parameter Mars Earth

Surface pressure 0.5–1 kPa 101.3 kPaAverage temperature 260°C 15°CTemperature range 2145°C to 20°C 260°C to 50°CComposition 95% CO2 78% N2

2.7% N2 21% O21.6% Ar 1% Ar

Normalized sunlight 0.43 1UV light .190 nm .300 nmSurface gravity 0.38 g 1 gRotation rate 24 h 37m 24 hObliquity 25.2° 23.5°Orbital eccentricity 0.093 0.017Year length 687 days 365.25 days

thick, warm CO2 atmosphere would be more eas-ily accomplished than the creation of “Earth-like”conditions with high O2 pressures. Based on en-ergy requirements and efficiencies, he estimated100 and 100,000 years, respectively, for these twotransformations. McKay (1982) also proposedthat biological stability (Gaia) could play a role inmaintaining a future biosphere on Mars. Love-lock and Allaby (1984) proposed a practical andspecific plan for how to warm Mars using supergreenhouse gases, in particular, chlorofluorocar-bons (CFCs), carried from Earth. The use of su-per greenhouse gases remains at the center of cur-rent studies, although subsequent analysis hasshown that perfluorocarbons (PFCs) produced onMars are in fact more practical than transportingCFCs. McKay et al. (1991) published an overviewin Nature of how Mars might be made habitableand brought the subject wide scientific exposure.They used climate models developed for theEarth to compute the amount of super green-house gases needed to warm Mars. Since 1990,numerous specialized papers have appeared inthe Journal of the British Interplanetary Society. [SeeFogg (1995a, 1998) for excellent overviews ofthese papers and a review of the basic science andproposed engineering schemes for creating hab-itable environments beyond the Earth.] It is in-teresting to note that several published papersconsider the environmental ethics of planetaryecosynthesis (e.g., Haynes, 1990; McKay, 1990;Fogg, 2000).

The purpose of this paper is to review previ-ous and ongoing work in planetary ecosynthesisfrom the perspective of how this topic fits withinthe overall field of astrobiology and thereby pro-vide a basis for future research.

HABITABLE CONDITIONS FOR LIFE

The logical first step in considering makingMars habitable is to define precisely the limits ofhabitability. McKay et al. (1991) defined two lim-iting cases of habitability: with and without highO2 levels (Table 2). The main requirement for a

MAKING MARS HABITABLE 91

FIG. 1. Mars Global Surveyor image showing NanediVallis in the Xanthe Terra region of Mars. Image coversan area 9.8 3 18.5 km; the canyon is , 2.5 km wide. Photofrom NASA/Malin Space Science Systems.

http://www.liebertonline.com/action/showImage?doi=10.1089/153110701750137477&iName=master.img-000.jpg&w=228&h=642








habitable planet is a surface temperature in therange to allow for liquid water. In Table 2 theminimum mean surface temperature is listed as0°C. This could be a reasonable value lacking anaccurate determination of how cold Mars couldbe and still maintain a hydrological cycle suffi-cient to support life. The upper limit of 30°C(Dole, 1970) is listed for completeness, but is un-likely to be relevant to Mars. In a warm environ-ment with high CO2, but without high O2 levels,microorganisms, plants, and possibly some in-vertebrates could survive. In this case the mainrequirements on the atmosphere is that the totalpressure be higher than the water vapor pressureplus the minimal levels of N2, CO2, and O2needed by the living organisms. These add up to, 1 kPa—only slightly more than the currentpressure on Mars. However, to make Mars warmwith the greenhouse effect of CO2 requires thatthe total amount of CO2 on Mars be , 200 kPa.Lower levels of CO2 can be compensated for byartificial greenhouse gases such as PFCs or by bi-ologically produced methane.

Oxygen and CO2 levels for algae and plants

Seckbach et al. (1970) reported on photosyn-thetic algae that thrive under pure CO2. It is pos-sible that many plants would be able to adapt toan atmosphere dominated by CO2 with low lev-els of O2. Experimental studies of plant growthunder twice the present atmospheric CO2 onEarth indicate generally increased productivity

for many species (Bazzaz, 1990; Jones et al., 1998;DeLucia et al., 1999). In general, the tolerance ofplants to high CO2 and low O2 varies betweenplant and among C-3, C-4, and CAM plants (Sal-isbury and Ross, 1985). Experimental studieshave focused on doubling of CO2, and it is notclear whether these results can be extrapolated tovery high partial pressures of CO2.

McKay et al. (1991) pointed out that someplants actually prefer O2 levels well below thecurrent value. The requirements vary dependingon the plant, but, in general, net primary pro-ductivity increases as O2 is reduced from the am-bient level until a point is reached where the O2

concentration is low enough (2 kPa) to causemetabolic complications (e.g., Salisbury and Ross,1985). It may be possible to adapt plants to accepteven lower O2 levels (0.1 kPa) since the mito-chondrial enzyme that requires O2 can functionat O2 levels of 0.01 kPa (e.g., Salisbury and Ross,1985). Some viable strategies (Fogg, 1995b) in-clude plants that retain their own photosynthet-ically produced O2 or have more efficient trans-fer of O2 to the sites of O2 utilization.

The current Mars atmosphere contains 0.1%(0.0006 kPa) O2 (and 0.07% CO), produced by thephotochemical breakup of CO2. If this mixing ra-tio of O2 and CO is assumed constant and the to-tal atmospheric pressure was increased to 100kPa, then the concentration of O2 would be 0.1kPa, close to the levels needed for plants. Unfor-tunately, the levels of CO would be 0.07 kPa—seven times the concentration that produces


TABLE 2. HABITABILITY

Parameter Limits Note

Global temperature 0°C–30°C Earth 5 15°CComposition for plants, algae, microorganisms

Total pressure .1 kPa Water vapor pressure plus O2, N2, CO2CO2 .0.015 kPa Lower limit set by photosynthesis,

no clear upper limitN2 .0.1–1 kPa Nitrogen fixationO2 .0.1 kPa Plant respiration

Composition for breathable airTotal pressurePure O2 .25 kPa Lung water vapor plus CO2, O2Air mixture .50 kPa Based upon high elevation

,500 kPa Buffer gas narcosisCO2 ,1 kPa Set by toxicityN2 .30 kPa Buffer gasO2 .13 kPa Lower limit set by hypoxia

,30 kPa Upper limit set by flammability

Adapted from McKay et al. (1991).

clinical symptoms in humans; plants and mi-croorganisms are unaffected by CO. Photochem-ical models of O2 production on early Earth, witha CO2-rich atmosphere, do not directly relate tothe photochemistry on a future thick CO2 mart-ian atmosphere, since on the early Earth, unlikepresent-day Mars, the level of O2 was kept lowbecause of the presence of reductants in the en-vironment and outgassing from volcanoes (Kast-ing, 1993; Rampino and Caldeira, 1994). On Mars,the surface is oxidizing, and there appears to beno source of reducing material (Hartman andMcKay, 1995). The only photochemical modelsthat have addressed this condition are those ofRosenqvist and Chassefière (1995). To determinethe upper limit on abiotic O2 on the early Earth,they considered models without any sinks for O2due to outgassing or surface reductants. Theyfound that the O2 partial pressure for a thick CO2atmosphere was , 0.2 kPa. Detailed photochem-ical models are capable of addressing this ques-tion directly, and such results are a good area forfuture research. The lower solar flux on Marsmight reduce the expected O2 concentration;however, the lower temperatures and hencelower H2O concentration would raise the ex-pected O2 concentration. Models specifically ad-dressing the photochemical state of a thick CO2atmosphere on Mars are still needed and wouldbe important for understanding the progressionof plants and animals that might be able to sur-vive on Mars with the increasing atmosphericpressure.

Lower limit for nitrogen fixation

Nitrogen is required for all organisms, and theamount of atmospheric N2 must be high enoughto provide for biological nitrogen fixation. Pre-liminary work suggests that bacteria can fix N2at levels of 0.5 kPa (Klingler et al., 1989). The cur-rent pressure of N2 on Mars is just under 0.02 kPa.A more accurate determination of the minimumrequired N2 pressure is needed.

Pressure and oxygen levels needed for invertebrates

Previous studies on habitability have focused onthe environmental requirements for microorgan-isms and plants on one end and humans on theother. These extremes are shown in Table 2. How-ever, it is important to consider the CO2 tolerance

and O2 requirements of invertebrates. Small ani-mals play important roles in land ecosystems.Perhaps the two most prominent examples arethe role of earth-moving animals, such as earth-worms and termites, in soil processes and the roleof flying insects in plant pollination. The only experiments that we are aware of are those ofCockell et al. (1999) in which a range of insects(ladybugs, beetles, termites, flies, ants, and drag-onflies) were subjected to pressures of air fromambient down to ,0.1 kPa. They found that allinsects could tolerate pressures down to 10 kPa(O2 partial pressure of 2 kPa) but that water lossat low pressure was an important issue. They con-cluded that on a terraformed Mars the total pres-sure would need to be 20 kPa to minimize waterloss with an O2 partial pressure of at least 1 kPafor breathing. However, their experiments usedonly an air gas mixture and over a short periodof time; the experiment did not allow for ac-climatization or adaptation of the insects, asmight occur over sustained exposure. Multigen-eration studies of invertebrates under a range ofO2 and CO2 levels will be needed to determineaccurately at what point invertebrates could be-gin to play a constructive role in a future mar-tian ecology.

Buffer gas requirements for breathing in humans

A key uncertainty in the current understand-ing of the requirements for a human-breathableatmosphere is the minimum level of buffer gasneeded in addition to O2 for long-term habitation.There is general agreement that pure O2 atmo-spheres are not adequate because of both theflammability hazard and long-term oxygen toxi-city. However, the minimum amount of buffergas needed (for which N2 is the only viable solu-tion) is not well established. Natural experimentsin breathing at reduced pressures (high-altitudepopulations and mountain climbers) are con-ducted in air gas composition. The effect of buffergas or O2 partial pressures on flammability is alsonot well determined for realistic environmentalconditions (cf. Watson et al., 1978 and Robinson,1989; discussed in Berner et al., 2000).

The best available long-term studies of the ef-fect of lowered buffer gas concentration on phys-iology have been space missions. The Apollo mis-sions to the Moon were conducted with pure O2atmospheres at a total pressure of 34.5 kPa (5 psi).However, the longest mission was only 12 days.


Skylab astronauts lived for up to 84 days in a mix-ture of , 70% O2 and 30% N2 at a total pressureof one-third Earth-normal (34 kPa). While thisgave a partial pressure of O2 equivalent to sealevel air, the reduced N2 level required specialprecautions against flammability. For example,paint could not be used on surfaces, and clothinghad to be made of plastic. The long-duration Mirmissions as well as the Space Shuttle use stan-dard air mixture and Earth-normal pressure(101.3 kPa). The role of gravity in the amount ofbuffer gas required to retard flames has not beendetermined. From flammability and breathingconsiderations, Dole (1970) and McKay et al.(1991) estimated that 30 kPa of buffer gas wouldbe required, and Fogg (1995a) similarly suggesteda minimum of 28.5 kPa. These values are muchhigher than the Skylab value of 10 kPa of N2.Given the Skylab experience, one might concludethat the level of buffer gas must at a minimum bebetween 10 and 30 kPa.

Earth history lesson

In a general way, the two habitable states listedin Table 2 correspond to the early Earth beforethe rise of O2 and the present Earth. For the pres-ent Earth, the functioning of the biosphere interms of temperature regulation, plant and ani-mal physiology, and biogeochemical cycles, if notcompletely understood, can at least be directlyobserved. For the early Earth this is not the case,and many aspects remain unclear. Of particularrelevance to planetary ecosynthesis on Mars ishow the temperature on early Earth was sus-tained when it appears that the level of CO2 wasnot sufficient (Pavlov et al., 2000). Current mod-els suggest that biogenic methane may have pro-duced the needed greenhouse effect (Pavlov et al.,2000), a possibility that may be relevant to Marsas well.

THE RIGHT STUFF: CO2, N2, AND H2O

Having identified two possible habitable stateswe must next consider if Mars has the ingredi-ents necessary to construct these states. Onemight wonder if it is possible to import fromEarth or from the outer solar system enough ma-terials to construct an atmosphere. The masses in-volved are too large for foreseeable technology.The mass of just 0.1 Pa of atmosphere on Mars is

4 3 109 tons—much larger than the few hundredton payload of a large rocket to low Earth orbit.Volatiles in sufficient quantities may exist incomet-like objects in the outer solar system, butthere is no foreseeable technology for movingsuch an object onto a trajectory where it wouldimpact Mars in a way that would contribute itsvolatiles to the martian atmosphere rather thancause a net removal of mass due to the impact ex-plosion (Melosh and Vickery, 1989). Thus theonly approach to planetary ecosynthesis consid-ered here is based on compounds present onMars.

In addition to the main volatiles discussed be-low, life must have sulfur, phosphorus, sodium,chlorine, potassium, and a range of trace elements(e.g., selenium). Not all of these have been directlydetected on Mars but are all thought to be therebased on theories of planet formation (e.g., dis-cussed in McKay et al., 1991). Stoker et al. (1993)have prepared a complete analysis of the proba-ble elemental composition of the martian surfacebased on the Viking results and analyses of themartian meteorites and concluded that the minorelement composition on Mars is adequate for life.

The essential ingredients needed to build abiosphere are the volatile compounds: CO2, N2,and H2O. There are only trivial quantities of thesein the present martian atmosphere, but theoreti-cal considerations and observations of past flu-vial activity suggest that Mars must have hadmuch larger quantities of all three compounds.Unfortunately, we do not yet know how much ofeach of these key ingredients Mars has located inthe subsurface in nonvolatile form: H2O frozen asground ice, nitrogen in the form of nitrate, andCO2 in the polar caps and absorbed in the regolithand as carbonates.

There are a variety of ways to estimate howmuch thicker the early martian atmosphere was.Estimates have been made based on the abun-dance of the rare gases, theories of atmosphericevolution, the size of the fluvial channels onMars, and simply by scaling from Earth. Not sur-prisingly, the various estimates do not agree onhow much of each of the volatiles were, or arestill, present on Mars. Table 3 shows a compari-son of the amount of material needed to makeMars habitable compared with the range of esti-mates of the initial abundances for CO2, N2, andH2O. For CO2 and N2 the partial pressure of thegas is given, while for H2O the thickness of a plan-etary-wide layer is given.


As discussed above in the context of Table 2,creating a habitable state only for plants and mi-croorganisms requires a minimal amount ofgases. However, the amount of CO2 needed inthis case is set not by the physiological require-ments of plants, but by the need to warm Marsenough to raise the average temperature abovefreezing. The amount listed in Table 3, 200 kPa,is enough CO2 that augmentation of the green-house effect by other greenhouse gases is not nec-essary (McKay et al., 1991). If the total availableinventory of CO2 was less than this amount, thenadditional greenhouse warming from artificialgreenhouse gases such as PFCs or from biogenicgreenhouse gases such as CH4 would be required.The amount of nitrogen needed is set by the abil-ity of organisms to fix atmospheric N2 (1 kPa).This total amount of nitrogen would not limit themass of a global biosphere. If all the nitrogen inthe current martian atmosphere were convertedto biomass with the typical nitrogen concentra-tion of 5% by weight (Davies and Koch, 1991),then the amount of biomass per unit area wouldbe 87 kg m22, or , 25 times the average biomassper unit area on the Earth [3.6 kg m22 (Krebs,1985)].

For a breathable atmosphere the requirementsare much different. Here the main need for CO2is as a source of O2, and thus only 20 kPa is re-quired. Nitrogen is needed not just to provide fornitrogen fixation, but also as a buffer gas inbreathing. The upper limit on CO2 consistent witha breathable mixture ensures that the greenhouseeffect of CO2 and H2O in such an atmospherewould be insufficient to warm the surface, andsignificant amounts of additional greenhousegases (PFCs and CH4) would be needed (McKayet al., 1991).

For all habitable states a hydrological cycle isrequired, and we estimate that this impliesenough water to create large bodies of water. IfMars had a layer of water 500 m thick over the

entire surface and if this water were to collect intothe current topographic shape of Mars, thenabout one-third of the planet would be coveredwith water.

Interestingly, the estimates of the amount ofvolatiles initially present on Mars overlaps therange needed for habitability (McKay and Stoker,1989). However, it is not clear whether thesevolatiles are still present on Mars and present informs that are accessible, even if the initial quan-tities were large.

Present reservoirs of CO2

It is believed that the primary loss mechanismfor the early dense CO2 atmosphere was the for-mation of carbonate rocks (e.g., Fanale, 1976;Kahn, 1985; Pollack et al., 1987; McKay andNedell, 1988). Other processes, such as CO2 lossfrom the top of the atmosphere by solar wind andnonthermal escape mechanisms, do not seem ableto explain the loss of a putative thick early at-mosphere (Jakosky et al., 1994). Models of car-bonate formation can account for the coincidenceof the observed atmospheric pressure on Marsand the triple point pressure of water (Kahn,1985). Carbonate formation would have pro-ceeded on Mars as long as liquid water was pre-sent and ceased once the pressure dropped lowenough to render liquid water unstable (Kahn,1985). The timescale for decreasing atmosphericCO2 from 100 kPa (1 bar) to its present value bycarbonate formation is estimated to be as short as107 years (Fanale, 1976; Pollack et al., 1987). Morecomplex models (Schaefer, 1993), however, givelonger timescales. Thus, in the absence of CO2 re-cycling, the lifetime of a warm early atmospherewould have been limited. On an active planet likethe Earth, subduction of ocean sediments at plateboundaries followed by thermal decompositionof carbonates deep below the surface is the pri-mary mechanism for completing the long-term


TABLE 3. INVENTORY OF IMPORTANT VOLATILES ON MARS

CO2 (kPa) N2 (kPa) H2O (m)

Required for plants .200 (for warming) .1 .500Required for humans .20 (for oxygen) .30 .500Present Mars atmosphere 1 0.02 7 3 1026

Earth scaling 270 30 1,200Range of current estimates 20–200 0.2–30 6–1,000

Adapted from McKay and Stoker (1989).

geochemical CO2 cycle (Walker et al., 1981). Marsdoes not have, nor is there any sign that Marsever had, crustal dynamics akin to plate tecton-ics; rather, Mars’s features are consistent with aone-plate planet (Solomon, 1978; Head andSolomon, 1981; but cf. Sleep, 1994). Without theseprocesses there appears to be no long-term geo-logical mechanism on Mars to recycle CO2-sinkmaterials back into the atmosphere.

If the early martian atmosphere were thickenough to warm the surface above freezing andif this atmosphere were primarily converted tocarbonate, then massive carbonate depositsshould cover much of the martian surface. Oneatmosphere of CO2 on Mars converted to car-bonates with a specific density of 3 would forma layer 20 m thick over the entire planet. Sur-prisingly, carbonate has not been clearly detectedon Mars either from Earth-based telescopes orfrom spacecraft in orbit over Mars (e.g., Bandfieldet al., 2000). Carbonates are not expected to haveuniformly precipitated over the planet but wouldform in lake basins and drainage areas. One in-teresting suggestion is that the intracanyon lay-ered deposits in the Valles Marineris system arecarbonates (McKay and Nedell, 1988). Figure 2shows Hebes Chasma and the central plateauwithin that canyon, which appears to have beendeposited when the canyon contained a lake gen-erated by regional groundwater flow. Because thecanyon has no inflow or outflow channels, chem-ical precipitation is a likely explanation for thesediments. McKay and Nedell (1988) estimatedthe mass of the plateau corresponds to 3 kPa ofatmospheric CO2.

The fact that massive carbonate deposits havenot yet been detected on Mars may be an indica-tion that our understanding of the compositionand fate of the early martian atmosphere is in-correct. On the other hand, there may be factorsthat obscure carbonates from remote viewing. Forexample, even a thin layer of dust would hide thecarbonate spectral features. In addition, Mukhinet al. (1996) have conducted experiments in whichthey determined that calcite was not stable underUV irradiation at wavelengths between 200 and400 nm in vacuum. This result appears to be in-consistent with previous work (Booth and Kief-fer, 1978), which showed that carbonates form un-der conditions similar to those on the surface ofMars (0.6 kPa of CO2) even with UV light pres-ent. A more promising way to destroy exposedcarbonates may be by acid weathering. Banin et

al. (1997) has proposed that acid fog composed ofchlorine and sulfur gases produced by volcanoescould be the primary weathering agent on Mars.If so, any carbonate exposed at the surface wouldbe destroyed. Thus the absence of spectral evi-dence for carbonates is troublesome but not a de-cisive argument against the presence of massivecarbonate deposits on Mars.

Even given ample carbonates on Mars, releas-ing CO2 from this source is difficult (e.g. Fogg,1989). Carbonate decomposes to form CO2 at hightemperatures (1,000°C) or low pH (,5). On Earth,carbonates are exposed to high temperaturewhen they are carried deep below the surface atsubduction zones. On Mars, the only feasible wayto generate CO2 from carbonates appears to be byacidification. Fortunately, there are some knownmicroorganisms that seem to be able to decom-pose carbonates (Friedmann et al., 1993). How-ever, while this biological source of CO2 may beof importance in the ultimate functioning of amartian biosphere, it cannot be a source of CO2as Mars is warmed from its present cold state,which is too dry and cold to support the growthof microorganisms.

The sources of CO2 that would contribute tothe warming of the present Mars are the ex-changeable reservoirs, that is, the CO2 that is inthe atmosphere, frozen on the polar caps, or ab-sorbed in the regolith. The CO2 in these sourcesis in equilibrium with the surface temperature,and if the surface temperature were raised, thenCO2 would move from the condensed reservoirs(polar caps and the regolith reservoirs) to the at-mosphere, thereby abetting the warming. Thepositive feedback of this CO2 degassing, trig-gered by artificial greenhouse gases, is essentialto warming Mars (McKay et al., 1991; Marinovaet al., 2000).

Theoretical studies of climate evolution onMars (McKay and Davis, 1991; Haberle et al.,1994) suggest that up to 50 kPa of CO2 may haveremained after the main phase of carbonate for-mation. Presumably this CO2 would have beenmostly stored in the polar caps and regolith reser-voirs. It was once thought that the polar caps onMars were composed of massive amounts of CO2in equilibrium with the atmosphere. However,the existence of a permanent CO2 cap can be ruledout for the North Pole (e.g., Fanale et al., 1974),and there is unlikely to be significant CO2 in thelayered deposits (Mellon, 1996). In the southernhemisphere there is evidence for a year-round


CO2 reservoir at the South Pole (Fanale and Can-non, 1974). The size of this reservoir is not deter-mined, but the residual cap is small—only , 350km in diameter—and basal melting should limitthe thickness of CO2 it could contain. A 1-km-thick cap of this size could provide , 10 kPa ofCO2; hence a more detailed assessment of the ex-istence and stability of a permanent CO2 SouthPolar Cap on Mars would be of interest. Thus the

potential release of the 10 kPa of CO2 from theSouth Polar Cap may be a key first step in thewarming of Mars (McKay et al., 1991).

Another possible source of CO2 is the martianregolith. Various studies have estimated there tobe from five to 50 times as much CO2 absorbedin the regolith as is present in the atmosphere(e.g., Zent et al., 1987; Kieffer and Zent, 1992; Zentand Quinn, 1995). McKay et al. (1991) modeled


FIG. 2. Viking orbiter image of Hebes Chasma (0°S, 75°W). This canyon is a box canyon , 280 km long. The mesain the center of the canyon shows layered sediments that may be carbonates (McKay and Nedell, 1988) illustratingwhat is thought to have been, and would be, the fate of martian CO2. Decomposing these carbonates to create at-mospheric CO2 requires either high temperatures or low pH. Microorganisms are capable of lowering pH sufficientlyto release CO2 from carbonate and could complete the carbon cycle on Mars (Friedmann et al., 1993).














the desorption of the regolith CO2 upon heatingand found that it may be an important positivefeedback in the warming of Mars, but because theCO2 may be tightly absorbed onto the regolith, itwould not vent to the atmosphere as readily asthe polar cap CO2.

Present reservoirs of H2O

The initial inventory of water on Mars is un-certain, with estimates ranging from only a fewmeters thick to almost a kilometer, in terms of alayer covering the entire planet (Table 3). In gen-eral, estimates based on geological features, suchas the fluvial features, imply higher levels of wa-ter—up to 1 km. Estimates based on scaling fromobserved abundances of noble gases or based onisotopic data suggest lower levels—closer to afew meters. Some water might have been lost tospace from Mars, but the current loss rate is in-sufficient to deplete any significant initial reser-voirs. If the current rate of atmospheric escapehas remained constant for water (as 2H and O) at6 3 107 cm22 s21 (Liu and Donahue, 1976; McEl-roy et al., 1977), the total loss over the past 4.5 bil-lion years would be 2.5 m of water. Recently thedebate on water levels has focused on the D/Hratio on Mars; it is a factor of 5 more enriched inD than the terrestrial standard (e.g., Owen et al.,1977; Yung et al., 1988; Donahue, 1995; Jakoskyand Jones, 1997). Initially, this was interpreted toimply that Mars had lost most of its water andhad an initial inventory of only a few meters.However, the D/H ratio in the martian meteoriteshas shown that the current amount of crustal wa-ter on Mars must be large and that the initial in-ventory must have been more than several hun-dred meters deep (Donahue, 1995). Geochemicalevidence from the martian meteorites also im-plies significant quantities of martian water,200 3 106 years ago (McSween et al., 2001). TheD/H in the atmosphere appears to correspond tofractionation by escape of just the atmosphericand near surface reservoirs.

Most current models suggest that the bulk ofthe martian water is tied up in the subsurface, ei-ther as permafrost or as deep aquifers. The evi-dence for permafrost is from craters in polar lat-itudes, which show a morphology that indicatesground ice (Squyres and Carr, 1986). The argu-ments for an extensive subsurface aquifer systemare more indirect, but this could also be a signif-icant reservoir of H2O on Mars (Clifford, 1993).

Fogg (1995a) tallied up the polar cap and groundice reservoirs on Mars and concluded that , 25and 500 m of water, respectively, could be ex-pected from these two sources. Mars thus appearsto have sufficient water, if warmed, to form largebodies of liquid water and therefore once againbe capable of harboring life.

Present reservoirs of N2

Perhaps the key issue in assessing the poten-tial of Mars for long-term human habitability isthe abundance of nitrogen. As seen in Table 3 theamount of nitrogen needed to make a breathableatmosphere is at the upper limit of the range ofestimates of the total initial inventory of nitrogenon Mars. If Mars did start out with that much ni-trogen and it is still on the planet, the only pos-sibility is that it is tied up as nitrate in the regolithand subsurface. On Earth nitrate does not usuallyaccumulate in the soil because it is a very solublesalt easily removed by water and it is readily con-verted by microorganisms into N2. The only sig-nificant accumulations of nitrate on Earth occurin the Atacama Desert. The nitrate in the Atacamais of atmospheric (lightning) origin based on thestable isotope work (e.g., Böhlke et al., 1997), andthe implied production rates are not unusuallyhigh. What is unusual is that there are no removalmechanisms due to the lack of water activity andresultant lack of microbial denitrification. Overthe age of the Atacama Desert (10–15 3 106 years)the accumulation has resulted in large concen-trated deposits. Mars has less water and, likely,less microbiological activity than the AtacamaDesert, and consequently nitrogen may havebeen slowly converted to nitrate by lightning, vol-canic-induced electric discharge, or meteors. Thenitrates would have accumulated in sedimentswith no recycling mechanism. Following this sce-nario, most of Mars’s initial nitrogen may be ni-trate in the sediments, possibly colocated with thecarbonates. Preliminary experiments with deni-trifying bacteria by Hart et al. (2000) suggest thatnitrate would be rapidly metabolized to N2 in aCO2 atmosphere if liquid water and organic ma-terials were present. Thus a key target for futuremissions will be to assess the amount and loca-tion of nitrates on Mars. Ferrying nitrogen fromEarth is clearly impractical, since the amountneeded to reach a pressure of 0.3 atmospheres is.1015 tons (compare with the Shuttle lift capa-bility of 20 tons). Importing this amount of ni-


trates would equivalently require over severalhundred thousand comets with a radius of 1 kmcomposed of pure nitrogen. If Mars does not havethe necessary nitrogen, it is not within near-termcapabilities of humans to import it. Nitrogencould be the factor that prevents the creation ofa human-breathable atmosphere on Mars, but isvery unlikely to be limiting for a plant-dominatedbiosphere.

TIME AND ENERGY ANALYSIS

Assuming that Mars has adequate reservoirs ofavailable CO2 in the polar caps and regolith reser-voirs, sufficient amounts of H2O as ground ice inthe permafrost regions, and enough nitrates inthe sediments, then it is logical to next considerhow much energy it would take to transformthese compounds into the desired warm, high-pressure state. Table 4, modified from McKay etal. (1991), shows an energy analysis for warmingMars and altering its chemical state. We dividethe process into three parts: surface warming,deep warming, and chemical alteration. The ini-tial state refers to Mars as it is today with thevolatiles locked up in the frozen state. The finalstate refers to the desired warmer state and, forthe last row, the state with high O2 levels.

In Table 4 the entries under Surface warminginclude the sublimation of 2 atmospheres of CO2,warming the top 10 m of the surface, melting ofa 10-m layer of water ice, and moistening the at-mosphere. Since sunlight is the largest and onlyplausible large-scale energy source, the requiredenergies are converted into equivalent years of

martian sunlight. The total energy difference be-tween the initial and final states corresponds to, 10 years of martian sunlight; most of this en-ergy is associated with vaporizing CO2 from thepolar cap and regolith reservoirs. Thus, if everysingle photon reaching Mars from the sun wereused to warm Mars, it would have a warm sur-face in only 10 years. However, it is not possibleto trap solar energy with 100% efficiency. Evenwith a strong greenhouse effect, a much smallervalue is more probable due to two factors. First,the frozen volatiles are in the polar regions, andthey receive only a fraction of the total solar en-ergy that reaches Mars. Second, Mars continuesto radiate infrared energy from the top of its at-mosphere, so the heating of the planet is the ab-sorbed solar less this radiated infrared. Withouta time-dependent model of the coupled polarcap/regolith/atmosphere system we cannotevaluate the efficiency with which Mars wouldbe warmed; however, we estimate that it wouldbe of the order of 10%. Thus we assign the totaltime for warming the surface of Mars to 100 years.

To melt the deep ice layers on Mars fully wouldtake much longer because of both the energy re-quired and the time for heat to penetrate the deeplayers below the surface. As listed in Table 4, tomelt enough ice to provide a 500-m-thick layer ofwater over the surface of Mars requires the equiv-alent of 56 years of martian sunlight. The processwould probably be limited by the rate of heattransport into the regolith. If thermal diffusionwere the only method of warming the subsurface,this would require on the order of 105 years; how-ever, as the permafrost melts, liquid water wouldallow for the convective transport of heat by the


TABLE 4. ENERGY REQUIREMENTS FOR TERRAFORMING MARS

Energy SolarInitial state Final state Amount (J m22) energy (yr)a Time (yr)

Surface warmingCO2(s) at 125°C CO2(g) at 15°C 200 kPa; 5.4 3 104 kg m22 3.7 3 1010 7.9Dirt at 260°C Dirt at 15°C , 10 m; 2 3 104 kg m22 1.2 3 109 0.3H2O(s) at 260°C H2O(l) at 15°C 10 m; 1 3 104 kg m22 5.5 3 109 1.2H2O(s) at 260°C H2O(g) at 15°C 2 kPa; 5.4 3 102 kg m22 1.6 3 109 0.33Total 10.0 100

Deep warmingH2O(s) at 260°C H2O(l) at 15°C 500 m; 5 3 105 kg m22 2.8 3 1011 56.0 500

Making O2CO2(g) 1 H2O CH2O 1 O2(g) 20 kPa; 5.4 3 103 kg m22 8 3 1010 17.0 100,000

Adapted from McKay et al. (1991).aEnergy divided by the total solar energy reaching Mars in a year, 4.68 3 109 J m22 yr21.

percolating liquid. This could greatly acceleratethe melting. We estimate the total time as 500years.

The final entries in Table 4 deal with the con-version of atmospheric CO2 to O2. From a simpleenergy balance, the energy equivalent to 17 yearsof martian sunlight must be used to transform theCO2 into O2. At present, the only mechanism thatcan generate planetary-scale oxygen is a bio-sphere. McKay et al. (1991) estimated the lengthof time it might take to achieve this by consider-ing a biosphere on Mars with an efficiency of1024—the same energy efficiency as the bio-sphere on the present Earth. With this efficiencyit would take $100,000 years to produce a breath-able atmosphere on Mars if all the organic mate-rial produced were buried and not allowed to re-cycle into the atmosphere as CO2. On Earth, onlya small fraction of the total productivity of thebiosphere is sequestered in sediments. It wouldseem that the production of a breathable oxygenatmosphere on Mars is not feasible with near-term technologies. While genetic engineeringcould make individual plants that are more effi-cient at producing oxygen, these technologies areunlikely to be able to produce ecosystems that aremore efficient at oxygen production. Plants pro-duce oxygen as a direct by-product of theirgrowth. Over millions of years there has beenevolutionary selection for those plants andecosystems that can grow and reproduce more ef-ficiently. It seems likely that net production islimited by factors other than genetics.

However, genetic engineering may help in pre-venting the recycling of organic matter. It hasbeen suggested that the development of lignin inplants during the Carboniferous age resulted inthe production of a form of organic matter forwhich there was no metabolic pathway for degra-dation. This in turn resulted in the efficient bur-ial of organic material and a sharp rise in O2(Berner et al., 2000). Genetically engineeredplants on Mars may be needed in order to pro-duce organic material that does not easily de-compose, consuming the O2 produced. However,we note that genetic engineering does notpromise to lift the environmental limits of life. Or-ganisms can be altered to produce desired com-pounds (e.g., PFCs) or for enhanced environ-mental protection, but they do so at a metaboliccost that will influence their survival in a naturalecosystem. The efficacy of genetic engineering ina community subject to natural selection may be

vastly different than in the controlled conditionsof the laboratory or bioindustrial reactors.

There may be nonbiological ways to producean oxygen-rich atmosphere, but none is withinthe reach or even scope of current technology.Possibly in the long term we can hypothesize theexistence of some rooftop fabric covering 1% ofthe martian surface that captures the incidentsunlight and affects the chemical reduction of car-bon dioxide to oxygen with 10% efficiency; eventhis approach would only reduce the time re-quired to 10,000 years.

WARMING AND OUTGASSING

The warming of Mars is the essential ingredi-ent to making the planet habitable. In a final state,for both a CO2- and a breathable O2-rich atmos-phere, the planet must be capable of remainingat a stable warm temperature for a significantamount of time without continued technical in-tervention.

As discussed above, the basic scenario pro-posed for warming Mars consists of adding su-per greenhouse gases to the atmosphere, therebywarming the surface. This in turn will cause CO2to be released from the South Polar Cap and theregolith. This released CO2 will augment the su-per greenhouse gases and cause further warm-ing, causing more CO2 release. The dynamics ofthis process for both a polar cap and a regolithreservoir have been analyzed by McKay et al.(1991) and Zubrin and McKay (1994).

The sublimation of the South Polar Cap in re-sponse to heating provides an example of themechanism. Figure 3 shows the vapor pressureof a CO2 ice cap compared with the climatic re-sponse of the polar region to atmospheric CO2.As long as a cap of CO2 exists, both curves mustbe satisfied [i.e., the lines must cross (points Aand B)]. Another possible equilibrium is when allthe CO2 has evaporated into the atmosphere(point C).

Currently Mars is at a stable point, with a lowpressure and a low temperature (point A in Fig.3). A slight increase in pressure will generate aslightly warmer temperature, but that increasedtemperature is lower than the temperature forwhich the caps would be in equilibrium, and soCO2 would condense on the caps, lowering thepressure and lowering the temperature. Simi-larly, a small decrease in pressure results in a tem-


perature warmer than the cap equilibrium, caus-ing evaporation and an increase in temperature.Hence, the system returns to state A for any smallperturbation about A. This behavior is shown bythe arrows. Unlike point A, point B is an unsta-ble equilibrium point. Small perturbations willgrow moving the system away from point B.Hence, if the temperature and pressure are forcedto rise from point A to point B, a runaway green-house effect will take place, and the planet’s en-tire exchangeable CO2 inventory (frozen on thepolar caps and absorbed in the regolith) will beoutgassed into the atmosphere, reaching point C.

It is important to note that once started, therunaway greenhouse effect will continue until thesystem reaches its own end with all the CO2 inthe atmosphere. This implies that there are twoways in which the runaway greenhouse effect onMars might be problematic. The first problem re-sults from there being too much CO2, and theother from there possibly being too little. In gen-eral, point C can represent an excessively thickand warm atmosphere, giving Venus-like condi-tions. If there were unexpectedly large polar capson Mars, the final climate state might be toowarm. The more likely problem is the opposite—too little CO2. If there is not enough CO2 to pushthe system beyond point B it will relativelyquickly return to its present state (point A). Cur-rently we do not know how much CO2 is avail-able on Mars. The range of estimates are from 10to .100 kPa. Even this lower value is probablyenough to move the climate state up above pointB with a little assistance from other greenhousegases, and enough for habitability with major as-sistance from other greenhouse gases.

Despite the various techniques available forwarming Mars, the planet’s CO2 inventory is theonly viable solution for creating the required$60°C warming. Artificial methods are intendedfor use in pushing the planet’s balance past pointB, past which the planet can itself take over thewarming process.

In addition to the super greenhouse gases dis-cussed above, various other methods have beensuggested for the warming of Mars. These in-clude placing dark material over the poles to de-crease their albedo and release CO2, and placinglarge mirrors in orbit to increase the sunlight ab-sorbed by Mars, perhaps targeted at key locationslike the polar caps. However, mirrors the size ofTexas (1012 m2) would be required to increase thetotal solar energy incident on Mars by 2%.

Fogg (1992) pointed out that any specific green-house gas is most efficient at warming Mars inlow concentrations (see also Marinova et al.,2000). This also applies to other methods ofwarming Mars. Thus the most efficient way towarm Mars may be a synergistic combination oftechniques (Fogg, 1992). As pointed out byZubrin and McKay (1997) the introduction ofgreenhouse gases other than CO2 or the addi-tional absorption of solar energy raises the cli-mate curve shown in Fig. 3, effectively movingpoint B closer to point A and making the systemmore susceptible to runaway outgassing.

SUPER GREENHOUSE GASES

As discussed above, the most promising tech-nique for warming Mars using current technol-ogy is the use of the super greenhouse gases, suchas CFCs and PFCs. As seen on the Earth, supergreenhouse gases are a demonstrated method forwarming a planet, and the gases can be readilyproduced at rates that cause a noticeable warm-ing in a short time frame. Super greenhouse gasesare very potent—over an order of magnitudestronger than CO2—and therefore are required invery small amounts, part per million level, to pro-duce the warming required to make Mars habit-able.

Greenhouse gases are transparent to visible ra-diation but are very effective at absorbing the in-frared rays radiated from a planetary surface.Thereby, the gases form a blanket that traps heatnear the planet’s surface, rather than letting it es-cape into space.

A variety of criteria must be used when se-lecting the appropriate gases. The gases to beused must have a strong greenhouse effect, havea long lifetime in the martian atmosphere, nothave any negative effects to a biosphere, be read-ily synthesized from materials available on Mars,and ultimately should be capable of being pro-duced by microorganisms if super greenhousegases become part of long-term warming for athin CO2 atmosphere or an N2/O2 atmosphere.In order to block the maximal amount of infraredradiation from leaving the planet and thereforecause maximal warming, a mixture of gases mustbe used, which, taken together, effectively coverthe IR spectrum. As shown in Fig. 4, there are twoadjacent regions that require the most attention.The gases used must be strong absorbers in the


200–800 cm21 (50–12.5 mm) range, where Mars iscurrently radiating most of its thermal energy(Fig. 4). As the planet warms up and the peakshifts to higher wavenumbers, it will be impor-tant to use gases that cover the 800–1,200 cm21

(12.5–8 mm) region, where CO2 and water vapordo not absorb any thermal radiation.

While the choice of greenhouse gases to beused is strongly based on their greenhouse po-tential, there are a number of other properties thatmust be considered. To simplify production, thegreenhouse gases used must have very long life-times in the martian atmosphere. This is espe-cially important since Mars’s thin ozone layerdoes not significantly block UV light, and there-fore very-short-wavelength UV radiation pene-trates all the way to the surface. Because of this,gases that typically have longer lifetimes on Earthwill have shorter lifetimes on Mars. As an exam-ple, on Mars, NH3 would have a short lifetime,thereby making it an impractical greenhouse gasdespite its great greenhouse potential (Kuhn andAtreya, 1979). Because the reason for warmingMars is to make it habitable, the greenhouse gaseschosen should not in any way be harmful to thebiosphere. This means that gases with chlorineand bromine, which chemically destroy ozone,would not be used. Another crucial characteris-tic when choosing a greenhouse gas is the viabil-ity of producing it on Mars. While super green-

house gases are very effective, it is not viable tobring them to Mars even to create a part per bil-lion concentration because of the prohibitivelylarge mass that would have to be transported.Therefore, all the constituents of the proposedgases must be readily available in the martian at-mosphere and regolith, and the synthesis processmust require low energies. Recent work (e.g.Marinova et al., 2000; Sturges et al., 2000; Gerstellet al., 2001) has shown a large number and vari-ety of gases that fit the above criteria, such asPFCs, SF6, and other large molecules containingonly C, F, H, S, and O. Industrial processes cur-rently produce most of these molecules. Similarprocesses could be used on Mars for their pro-duction, without the need for inventing new tech-nologies or processes. The locations and strengthsof absorption bands of selected PFCs and SF6 areshown in Fig. 5. Figure 6 shows the spectrum ofone gas, C3F8, at three different concentrations.

The warming potentials of some PFCs and SF6are shown in Fig. 7 and are calculated based onthe method of Marinova et al. (2000). Of these,the most effective gas with respect to a Mars tem-perature profile appears to be C3F8, which pro-duces a warming of , 7°C for 0.1 Pa of the gas inthe martian atmosphere.

To warm Mars, a suite of efficient greenhousegases is needed that can effectively absorb acrossthe entire IR spectrum. McKay et al. (1991) have


FIG. 3. Equilibrium constraints for polar cap CO2 on Mars (adapted from McKay et al., 1991). Dotted line showsthe phase equilibrium of CO2 solid and gas. Solid line shows the climatically determined temperature, including ad-vection and greenhouse effect. Possible equilibriums are shown as points A (stable cold state with most CO2 frozen),B (unstable equilibrium), and C (stable warm state with all CO2 in atmosphere). To transition Mars from state A tostate C it must be moved beyond point B.

FIG. 4. Thermal balance of warm Mars. The two curves show radiative flux (Planck curve) leaving the top of themartian atmosphere (260°C) and the radiative flux leaving the surface of a warmed Mars (15°C) as a function ofwavelength. The difference must be compensated for by greenhouse forcing. The greenhouse effect for CO2 and H2Ois shown schematically. The “window” region of 8–12 mm (1,200–800 cm21) must be filled by super greenhouse gasesin order to warm Mars.

FIG. 5. Schematic diagram of the location of absorption bands for PFCs and SF6. The absorption strength of thebands is shown by the thickness of the line, where thick lines represent strong absorption bands and thin lines areweak absorption bands. Note the gases coverage of the 8–12 mm (1,200–800 cm21) region.

FIG. 6. Absorption spectra for C3F8 at mixing rations of 10 2 7, 10 2 3, and 100. C3F8 appears to be the PFC with thestrongest greenhouse potential.

FIG. 7. The warming produced by PFCs and SF6 as a function of their partial pressure in the atmosphere. Allvalues are for current Mars; the calculations do not take into account overlap of the super greenhouse gas bands withabsorption bands of CO2.

FIG. 8. Proposed basis vectors of environmental ethics (adapted from McKay, 1990). These are not categories butrather components. Systems of environmental ethics are composed of various mixtures of these three components.Only anti-humanism necessarily rejects making Mars habitable.

FIG. 3.

FIG. 4.

FIG. 5.

FIG. 6.

FIG. 7.

FIG. 8.











shown that just blocking the window region of8–12 mm (800–1,200 cm21) is not sufficient. Theshort list of gases in Fig. 5 do not form an ade-quate mixture; however, these gases are thosethat only occur naturally. It is important that webegin to design gases that have a large green-house potential. Recent work by Gerstell et al.(2001) evaluated theoretical molecules that had asignificant greenhouse potential. Further work inthis area may discover molecules that are a fac-tor or two more effective than C3F8, which wouldfurther simplify the process of warming Mars.One possible drawback to these newly discov-ered molecules is their size. Because of the largemolecular mass, they may be more difficult tomanufacture, requiring more materials and en-ergy for their fabrication. Super greenhouse gaseshave an important role to play in terms of warm-ing Mars to create a thick CO2 atmosphere andkeeping a breathable O2/N2 atmosphere warm.

There have not been any detailed studies look-ing at how PFCs and other super greenhousegases might be produced on the present Mars.However, it is possible to use basic energy con-siderations to get a sense of the magnitude of theenergy required for the task. If we assume thatwith a suitable mixture of super greenhouse gasesthe total concentration required to warm Mars is1 Pa, and if we assume that the energy of forma-tion of these gases is on average , 107 J/kg, thenthe total energy required is 4 3 1020 J—equiva-lent to 5 hours of martian sunlight. Consider forillustrative purposes that this power was pro-vided by generating facilities of size 500 MW (thesize of a small nuclear reactor); if we take 100years as the production time, then 245 such facil-ities would be required.

ECOLOGICAL SUCCESSION ON MARS

As discussed above, the inventory of gases andthe thermodynamics of changing the martian at-mosphere will determine the environmental con-ditions in the initial stages of ecosynthesis. Withinthis framework the lifeforms that can be im-planted on Mars can be considered. J. Graham(paper in preparation) has likened the ecologicalchanges over time on Mars to the ecologicalchanges moving down a mountain: from barrenfrozen rock at the mountain top through alpinetundra and arctic and alpine grasses and eventu-ally to trees and forests.

Currently Mars is too cold and dry to allow forthe survival of any terrestrial organism. There-fore, purely engineering mechanisms, such aschemically produced PFCs, would have to beused to warm Mars to a minimal level before anylife from Earth can survive there. The ecosystemon Earth that thrives in the coldest driest, mostMars-like environment are the cryptoendolithicmicrobial ecosystems found in the dry valleys ofAntarctica (Friedmann, 1982). The Dry Valleysare so cold and dry that they appear lifeless.However, lichens, algae, and bacteria live a fewmillimeters below the surface of sandstone rocks,where they find a warmer, wetter environmentthan on the rock’s surface. Enough sunlight pen-etrates through the rock at these depths to allowfor photosynthesis (Nienow et al., 1988). Anaer-obic counterparts of these Antarctic microbialecosystems could be the first photosynthetic life-forms living naturally on the surface of Mars(Friedmann and Ocampo-Friedmann, 1993).Studies of the physical ecology of the Antarcticcryptoendolithic microbial ecosystems show thatthey survive with a growing season of only 100h/yr (Friedmann et al., 1987; McKay et al., 1993).These organisms grow in locations where the airtemperature virtually never rises above freezing.By analogy, microorganisms in a rock habitatcould survive on Mars when the air temperaturesreach values of 210°C during the daytime for afew weeks during the warmest part of the year—presumably at the subsolar point in southernsummer when Mars is at perihelion. Sunlightwarming the rock would raise the temperatureabove freezing. Light levels in the Dry Valleys ofAntarctica during summer are 600 W/m2 at noon(Dana et al., 1998). By comparison, the subsolarpoint on Mars at perihelion has a sunlight levelof 580 W/m2, assuming a typical clear-sky at-mospheric transparency of 0.8.

As Mars continues to warm, Arctic and alpinetundra ecosystems could survive. These systemsrequire air temperatures above freezing duringthe growing season along with low levels of rainor meltwater runoff (Chapin et al., 1997). In theseareas precipitation ranges between 5 and 50 cm(Chapin et al., 1997). With further warming andextension of the growing season, alpine plantsmight survive and cover vast regions of equato-rial Mars.

A major event in the developing ecology ofMars would occur when the treeline first reachesthe lower parts of the equatorial canyons on Mars.


In the temperate zone on Earth, the treeline is de-termined by the summer temperature. However,the equatorial regions on Mars will have only aslight seasonality because of the eccentricity ofMars’s orbit. Thus tropical treelines provide amore direct example of how the first trees willgrow on Mars. The highest treeline on the Earth(4,000 m) is found on Pico de Orizaba at 19°N inMexico (Lauer, 1978; Körner, 1998) and corre-sponds to a ground temperature of between 3°Cand 7°C (Lauer, 1978). Thus we suppose thatwhen the equatorial regions of Mars reach a com-parable temperature, trees will be possible.

On Mars, the first introduction of photosyn-thetic microbial ecosystems and Arctic and alpinetundra will be of biological interest. However,only with the development of ecosystems basedon higher plants will the ecological developmentof Mars become significant in terms of the pro-duction of organic matter and O2. Plant-basedecosystems (e.g., forests and grasslands) could beeffective O2 producers because they have veryhigh biomass density, can produce organic ma-terial—like wood—that does not lock up phos-phorus and nitrogen nutrients, and can possiblybe engineered to produce lignin-like materialsthat are resistant to microbial decomposition.

As discussed above, a thick CO2 atmosphereon Mars may have levels of O2 sufficient for plantgrowth. Such levels would provide important UVshielding due both to the O2 (Rosenqvist andChassefière, 1995) and O3 produced (Hiscox andLinder, 1997) and to scattering of UV by CO2 (e.g.,Cockell et al., 2000). In addition, the PFCs mayprovide direct UV absorption. Yung and DeMore(1999) suggested that one solution to the creationof a UV shield may be a polysulfur layer in theupper atmosphere like that in the atmosphere ofVenus. To create the shield, carbonyl sulfide(COS) would need to be injected into the atmo-sphere at a rate of 107 tons/yr. Finally, we notethat many microbial communities can survivehigh UV rates with strategies that include rapidrepair, protective pigments, and UV-absorbinglayers of dead cells or inorganic material (e.g.,Averner and MacElroy, 1976; Cockell, 2000). Inaddition, it is not known how life on early Earthsurvived without an effective ozone shield (e.g.,Cockell, 2000; Cockell et al., 2000), but life did sur-vive, and perhaps similar solutions might oper-ate on a future Mars.

While plants will be the dominant agents ofecological change on Mars—as they are on

Earth—small animals could play a key role. In-sects and soil invertebrates, such as earthworms,would be beneficial additions to the developingmartian ecosystems. In particular, pollination byflying insects would greatly increase the diver-sity of plants grown on Mars at every stage of theprocess (J. Graham, paper in preparation). Un-fortunately, the minimum O2 requirements andmaximum CO2 and UV tolerance of flying insectsat 0.38 g remain unexplored.

While lifeforms from Earth might be introducedto Mars in a controlled and prescribed way, thisdoes not imply that the resulting ecological sys-tems will develop as predicted. In fact, they areunlikely to do so. As life on Mars interacts withitself and the changing martian environment, itwill follow an independent evolutionary trajec-tory that will be difficult or impossible to control.This is not a problem and, in fact, should be con-sidered an asset. The resulting biological systemis more likely to be stable and globally adapted tothe altered martian environment than any pre-conceived ecosystems. If the Gaia hypothesis iscorrect, and life works to stabilize the biosphere,then this will be how it does so (McKay, 1982;Lovelock and Allaby, 1984).

ENVIRONMENTAL ETHICS

It is important to consider not just the physicsand biology of making Mars habitable, but also theenvironmental ethics. This poses a challenge to thefield of environmental ethics, which is a disciplinethat has never before looked beyond the Earth. OnEarth there is no distinction between life and na-ture, terms often used synonymously. However,when we venture beyond the Earth, we find worldsapparently devoid of life. A distinction between lifeand nature, never needed before, becomes neces-sary in considering life on Mars. Consider, for ex-ample, Leopold’s (1966) egalitarian ecosystemethic, arguably the founding principle of modernenvironmental ethics: “A thing is right when ittends to preserve the integrity, stability, and beautyof the biotic community. It is wrong if it tends todo otherwise.” It is obvious how to apply this onEarth, but not clear at all on Mars where there isno “biotic community.” Do we “preserve” what isthere, or do we introduce a “biotic community”?To apply environmental ethics to the question ofmaking Mars habitable, we need to generalize thetopic.


McKay (1990) proposes that any system of en-vironmental ethics is based on three basic nor-mative axioms—each system of environmentalethics is composed of varying mixes of these threebasic axioms. The basic axioms are: anti-human-ism or preservationism, which holds that humansshould not exercise their technological capabili-ties to alter the Earth, and by extension the cos-mos; wise stewardship or utilitarianism, whichholds that humans can use, control, and changenatural systems but must do so wisely and tomaximize long-term human benefit; and intrinsicworth, which holds that human use is not the ul-timate value and that living systems have intrin-sic worth independent of human utility.

These three axioms are not categories of sys-tems of environmental ethics, but rather the com-ponents of these systems. All systems have someof each of these axioms but to varying extent. Inthat sense the three axioms are like the basis vec-tors of Cartesian coordinates. This is illustratedin Fig. 8, where the three normative axioms of en-vironmental ethics are shown as a basis vectorsystem. If systems of environmental ethics arecomposed of different amounts of these three ba-sic principles, and if we examine the implicationsof each of these basic axioms for introducing lifeto Mars, we can then understand how systems ofenvironmental ethics would address the issue.

Anti-humanism would argue against alteringthe martian environment either to assist any mar-tian life or to allow for life from Earth. Wise stew-ardship is certainly the major part of most sys-tems of environmental ethics, especially in theWestern Tradition. Of the three axioms definedin Fig. 8 it is the only one with universal appeal.McKay (1990) argues that restoring Mars to a biological state would be consistent with wisestewardship in that such a project would help usdevelop a scientific understanding of how bios-pheres work as well as providing aesthetic andeducational advantages.

The principle of intrinsic worth provides a newperspective on terraforming Mars. The notion of in-trinsic worth as a principle in environmental ethicstraces back to a seminal paper by Naess (1973, 1984)in which he elaborates on what has become knownas “Deep Ecology.” The first two tenets of DeepEcology are particularly relevant to the question ofbringing Mars back to life. They are:

1. The well-being of nonhuman life on Earth hasvalue in itself. This value is independent of

any instrumental usefulness for limited hu-man purposes.

2. Richness and diversity in lifeforms contributeto this value and are a further value in them-selves.

The implications of this are that a biologicallyrich and diverse Mars is of more value than thebeautiful, but virtually dead, world we exploretoday. A further conclusion from these tenets ofdeep ecology is that indigenous martian life, dif-ferent from Earth life, should be the first aim ofrestoration ecology on Mars, thereby increasingthe diversity of life. Even though it is unlikely, itmay be possible to find viable martian life or re-vive dead but frozen martian microbes. If life onMars did experience its own separate genesis,then restoring that life to global diversity wouldhave to be the best possible option for Mars. Iflife on Mars is genetically related to life on Earth(a possibility due to exchange between the plan-ets) or if there is no life to revive on Mars, then aMars teaming with Earth life is the next best op-tion.

FUTURE WORK NEEDED

Our knowledge of Mars is incomplete, as is ourunderstanding of many of the functions and in-teractions of life on Earth. It is no surprise there-fore that there is a long list of future researchneeds dealing with the question of making Marshabitable. We suggest the following list as in-dicative and by no means inclusive:

1. Are there carbonate deposits on Mars, andwhat is their total mass?

2. What is the total amount of CO2 in the SouthPolar Cap on Mars?

3. What is the total amount of CO2 absorbedinto the regolith on Mars?

4. What are the location and state of subsurfaceice and water on Mars?

5. Are there nitrate deposits on Mars?6. Are there subsurface organics on Mars, and

are these associated with the nitrate deposits?7. What is the expected photochemical concen-

tration of O2 in a thick ($100 kPa) CO2 at-mosphere on Mars?

8. What is the minimum O2 for plant growth,and how will different plants respond to veryhigh CO2 levels?


9. What is the minimum level of N2 at which bi-ological nitrogen fixation can occur?

10. Could microorganisms be genetically engi-neered to produce PFCs?

11. What are the lower limits of O2 and upperlimits of CO2 for insects and soil inverte-brates?

12. What are the buffer gas requirements for hu-mans and other animals?

13. Would biogenic CH4 produced in a CO2 at-mosphere make a significant contribution tothe greenhouse warming on Mars as it isthought to have on early Earth?

14. Are there biological methods for recyclingcarbonates on Mars, and if not, what is theexpected lifetime of the atmosphere?

15. What would be the rate of warming on Marsas PFCs are produced?

16. What would the global climate and weatheron Mars be when it has an extensive body ofwater?

17. Can genetic engineering produce plants thatproduce nondecomposable organic matter,UV-resistant lifeforms, and invertebrate ani-mals with high tolerance for CO2 and low de-mand for O2?

Astrobiology is a newly formed discipline. Ofthe topics examining the question of life, the ex-pansion of life beyond its planet of origin is thearea with the least heritage of previous research.Much remains to be done, but a great ecologicallesson is before us.

ABBREVIATIONS

CFC, chlorofluorocarbon; PFC, perfluorocar-bon.

REFERENCES

Averner, M.M. and MacElroy, R.D. eds. (1976) On the Hab-itability of Mars. NASA Special Publication, NASAAmes Research Center, Moffett Field, California.

Bandfield, J.L., Hamilton, V.E., and Christensen, P.R.(2000) A global view of Martian surface compositionsfrom MGS-TES. Science 287, 1626.

Banin, A., Han, F.X., Kan, I., and Cicelsky, A. (1997) Acidvolatiles and the Mars soil. J. Geophys. Res. 102, 13341–13356.

Bazzaz, F.A. (1990) The response of natural ecosystems tothe rising global CO2 levels. Annu. Rev. Ecol. Syst. 21,167–196.

Berner, R.A., Petsch, S.T., Lake, J.A., Beerling, D.J., Popp,B.N., Lane, R.S., Laws, E.A., Westley, M.B., Cassar, N.,Woodward, F.I., and Quick, W.P. (2000) Isotope frac-tionation and atmospheric oxygen: implications forPhanerozoic O2 evolution. Science 287, 1630–1633.

Böhlke, J.K., Ericksen, G.E., and Revesz, K. (1997) Stableisotopic evidence for an atmospheric origin of desertnitrate deposits in northern Chile and southern Cali-fornia, USA. Chem. Geol. 136, 135–152.

Booth, M.C. and Kieffer, H.H. (1978) Carbonate formationin Mars-like environments. J. Geophys. Res. 83, 1809–1815.

Burns, J.A. and Harwit, M. (1973) Towards a more habit-able Mars—or—the coming Martian Spring. Icarus 19,126–130.

Carr, M.H. (1986) Mars: A water-rich Planet. Icarus 68,187–216.

Carr, M.H. (1996) Water on Mars.Oxford University Press,Oxford.

Carr, M.H. and Malin, M.C. (2000) Meter-scale character-istics of Martian channels and valleys. Icarus 146,366–386.

Clark, B.C. (1998) Surviving the limits to life at the sur-face of Mars. J. Geophys. Res. 103, 28545–28556.

Clifford, S.M. (1993) A model for the hydrologic and cli-matic behavior of water on Mars. J. Geophys. Res. 98,10973–11016.

Chapin, F.S., Jeffries, R.L., and Reynolds, J.F. (1997) Arc-tic Ecosystems in a Changing Climate: An EcophysiologicalPerspective, Academic Press, San Diego.

Cockell, C.S. (2000) The ultraviolet history of the terres-trial planets—implications for biological evolution.Planet. Space Sci. 48, 203–214.

Cockell, C., Catling, D., and Waites, H. (1999) Insects atlow pressure: applications to artificial ecosystems andimplications for global windborne distribution. LifeSupport Biosph. Sci. 6, 161–167.

Cockell, C.S., Catling, D.C., Davis, W.L., Snook, K., Kep-ner, R.L., Lee, P., and McKay, C.P. (2000) The ultravio-let environment of Mars: biological implications past,present, and future. Icarus 146, 343–359.

Dana, G.L., Wharton, R.A., Jr., and Dubayah, R. (1998) So-lar radiation in the McMurdo Dry Valleys, Antarctica.In Antarctic Research Series, Vol. 72: Ecosystem Dynamicsin a Polar Desert: The McMurdo Dry Valleys, Antarctica,edited by J.C. Priscu, American Geophysical Union,Washington, D.C., pp. 39–64.

Davies, R.E. and Koch, R.H. (1991) All the observed uni-verse has contributed to life. Philos. Trans. R. Soc. Lond.B Biol. Sci. 334, 391–403.

DeLucia, E.H., Hamilton, J.G., Naidu, S.L., Thomas, R.B.,Andrews, J.A., Finzi, A., Lavine, M., Matamala, R., Mo-han, J.E., Hendry, G.R., and Schlesinger, W.H. (1999)Net primary productivity of a forest ecosystem with ex-perimental CO2 enrichment. Science 284, 1177–1179.

Donahue, T.M. (1995) Evolution of water reservoirs onMars from D/H ratios in the atmosphere and crust. Na-ture 374, 432–434.

Dole, S.H. (1970) Habitable Planets for Man, 2nd edit., El-sevier, New York.


Fanale, F.P. (1976) Martian volatiles, their degassing his-tory and geochemical fate. Icarus 28, 179–202.

Fanale, F.P. and Cannon, W.A. (1974) Exchange of ad-sorbed H2O and CO2 between the regolith and atmo-sphere of Mars caused by changes in surface insolation.J. Geophys. Res. 79, 3397–3402.

Fogg, M.J. (1989) The creation of an artificial dense Mar-tian atmosphere: a major obstacle to the terraformingof Mars. J. Br. Interplanet. Soc. 42, 577–582.

Fogg, M.J. (1992) A synergic approach to terraformingMars. J. Br. Interplanet. Soc. 45, 315–329.

Fogg, M.J. (1995a) Terraforming: Engineering Planetary En-vironments, SAE, Warrendale, PA.

Fogg, M.J. (1995b) Terraforming Mars: conceptual solu-tions to the problem of plant growth in low concentra-tions of oxygen. J. Br. Interplant. Soc. 48, 427–434.

Fogg, M.J. (1998) Terraforming Mars: a review of currentresearch. Adv. Space Res. 22, 415–420.

Fogg, M.J. (2000) The ethical dimensions of space settle-ment. Space Policy 16, 205–211.

Friedmann, E.I. (1982) Endolithic microorganisms in theAntarctic cold desert. Science 215, 1045–1053.

Friedmann, E.I. and Ocampo-Friedmann, R. (1995) Aprimitive cyanobacterium as pioneer microorganismfor terraforming Mars. Adv. Space Res. 15(3), 243–246.

Friedmann, E.I., McKay, C.P., and Nienow, J.A. (1987) Thecryptoendolithic microbial environment in the RossDesert of Antarctica: nanoclimate data, 1984 to 1986. Po-lar Biol. 7, 273–287.

Friedmann, E.I., Hua, M., and Ocampo-Friedmann, R.(1993) Terraforming Mars: dissolution of carbonaterocks by cyanobacteria. J. Br. Interplanet. Soc. 46,291–292.

Gerstell, M.F., Francisco, J.S., Yung, Y.L., Boxe, C., andAaltonee, E.T. (2001) Keeping Mars warm with new su-per greenhouse gases. Proc. Natl. Acad. Sci. USA 98,2154–2157.

Haberle, R.M., Tyler, D., McKay, C.P., and Davis, W.L.(1994) A model for the evolution of CO2. Icarus 109,102–120.

Hart, S.D., Currier, P.A., and Thomas, D.J. (2000) Deni-trification by Pseudomonas aeruginosa under simulatedengineered Martian conditions. J. Br. Interplanet. Soc. 53,357–359.

Hartman, H., and McKay, C.P. (1995) Oxygenic photo-synthesis and the oxidation state of Mars. Planet. SpaceSci. 43, 123–128.

Haynes, R.H. (1990) Ecce ecopoiesis: playing God onMars. In Moral Expertise, edited by D. MacNiven, Rout-ledge, London, pp. 161–183.

Head, J.W. and Solomon, S.C. (1981) Tectonic evolutionof the terrestrial planets. Science 213, 62–76.

Head, J.W. III, Hiesinger, H., Ivanov, M.A., Kreslavsky,M.A., Pratt, S., and Thomson, B.J. (1999) Possible oceanson Mars: evidence from Mars Orbiter Laser Altimeterdata. Science 286, 2134–2137.

Hiscox, J.A. and Linder, B.L. (1997) Ozone and the habit-ability of Mars. J. Br. Interplanet. Soc. 50, 109–114.

Jakosky, B.M. and Jones, J.H. (1997) The history of mar-tian volatiles. Rev. Geophys. 35, 1–16.

Jakosky, B.M., Pepin, R.O., Johnson, R.E., and Fox, J.L.(1994). Mars atmospheric loss and isotopic fractionationby solar-wind-induced sputtering and photochemicalescape. Icarus 111, 271–288.

Jones, T.H., Thompson, L.J., Lawton, J.H., Bezemer, T.M.,Bardgett, R.D., Blackburn, T.M., Bruce, K.D., Cannon,P.F., Hall, G.S., Hartley, S.E., Howson, G., Jones, C.G.,Kapichler, C., Kandeler, E., and Ritchie, D.A. (1998) Im-pacts of rising atmospheric carbon dioxide on modelterrestrial ecosystems. Science 280, 441–443.

Kahn, R. (1985) The evolution of CO2 on Mars. Icarus 62,175–190.

Kasting, J.F. (1993) Earth’s early atmosphere. Science 259,920–926.

Kieffer, H.H. and Zent, A.P. (1992) Quasi-periodic climatechange on Mars. In Mars, edited by H.H. Kieffer, B.M.Jakosky, C.W. Snyder, and M.S. Matthews, Universityof Arizona Press, Tucson, pp. 1234–1245.

Klingler, J.M., Mancinelli, R.L., and White, M.R. (1989).Biological nitrogen fixation under primordial martianpartial pressures of dinitrogen. Adv. Space Res. 9(6),173–176.

Körner, C. (1998) A re-assessment of high elevation tree-line positions and their explanation. Oecologia 115,445–459.

Krebs, C.J. (1985). Ecology, 3rd edition, Harper and Row,New York.

Kuhn, W.R. and Atreya, S.W. (1979) Ammonia photoly-sis and the greenhouse effect in the primordial atmos-phere of the Earth. Icarus 37, 207–213.

Lauer, W. (1978) Timberline studies in central Mexico.Arctic Alpine Res. 10, 383–396.

Leopold, A. (1966). A Sand County Almanac, BallantineBooks, New York.

Liu, S. and Donahue, T.M. (1976) The regulation of hy-drogen and oxygen escape from Mars. Icarus 28,231–246.

Lovelock, J.E. and Allaby, M. (1984) The Greening of Mars,Warner Books, Inc., New York.

Malin, M.C. and Edgett, K.S. (2000) Evidence for recentgroundwater seepage and surface runoff on Mars. Sci-ence 288, 2330–2335.

Marinova, M.M., McKay, C.P., and Hashimoto, H. (2000)Warming Mars using artificial super-greenhouse gases.J. Br. Interplanet. Soc. 53, 235–240.

McElroy, M.B., Kong, T.Y., and Yung, Y.L. (1977) Photo-chemistry and evolution of Mars’ atmosphere: a Vikingperspective. J. Geophys. Res. 82, 4379–4388.

McKay, C.P. (1982) Terraforming Mars. J. Br. Interplanet.Soc. 35, 427–433.

McKay, C.P. (1990) Does Mars have rights? An approachto the environmental ethics of planetary engineering.In Moral Expertise, edited by D. MacNiven, Routledge,London, pp. 184–197.

McKay, C.P. (1997) The search for life on Mars. OriginsLife Evolution Biosphere 27, 263–289.

McKay, C.P. and Davis, W.L. (1991) Duration of liquidwater habitats on early Mars. Icarus 90, 214–221.

McKay, C.P. and Nedell, S.S. (1988) Are there carbonatedeposits in Valles Marineris, Mars? Icarus 73, 142–148.


McKay, C.P. and Stoker, C.R. (1989) The early environ-ment and its evolution on Mars: implications for life.Rev. Geophys. 27, 189–214.

McKay, C.P., Toon, O.B., and Kasting, J.F. (1991) MakingMars habitable. Nature 352, 489–496.

McKay, C.P., Nienow, J.A., Meyer, M.A., and Friedmann,E.I. (1993) Continuous nanoclimate data (1985–1988)from the Ross Desert (McMurdo Dry Valleys) cryp-toendolithic microbial ecosystem. In Antarctic ResearchSeries, Vol. 61: Antarctic Meteorology and Climatology:Studies Based on Automatic Weather Stations, edited byD.H. Bromwich and C.R. Stearns, American Geophys-ical Union, Washington, DC, pp. 201–207.

McSween, H.Y. Jr., Grove, T.L., Lentz, R.C.F., Dann, J.C.,Holzheid, A.H., Riciputi, L.R., and Ryan, J.G. (2001)Geochemical evidence for magmatic water within Marsfrom pryoxenes in the Shergotty meteorite. Nature 409,487–490.

Mellon, M.T. (1996) Limits on the CO2 content of the mar-tian polar deposits. Icarus 124, 268–279.

Melosh, H.J. and Vickery, A.M. (1989) Impact erosion ofthe primordial atmosphere of Mars. Nature 338,487–489.

Mukhin, A.P., Koscheev, Yu, Dikov, P., Huth, J., andWanke, H. (1996) Experimental simulations of the de-composition of carbonates and sulphates on Mars. Na-ture 379, 141–143.

Naess, A. (1973) The shallow and the deep, long-rangeecology movements: a summary. Inquiry 16, 95–100.

Naess, A. (1984) A defense of the Deep Ecology move-ment. Environ. Ethics 6, 265–270.

Nienow, J.A., McKay, C.P., and Friedmann, E.I. (1988) Thecryptoendolithic microbial environment in the RossDesert of Antarctica: light in photosynthetically activeregion. Microb. Ecol. 16, 271–289.

Owen, T., Biemann, K., Rushnek, D.R., Biller, J.E.,Howarth, D.W., and Lafleur, A.L. (1977) The composi-tion of the atmosphere at the surface of Mars. J. Geo-phys. Res. 82, 4635–4639.

Pavlov, A.A., Kasting, J.F., Brown, L.L., Rages, K.A., andFreedman, R. (2000) Greenhouse warming by CH4 inthe atmospheres of early Earth and Mars. J. Geophys.Res. 105, 11981–11990.

Pollack, J.B., Kasting, J.F., Richardson, S.M., and Poliakoff,K. (1987) The case for a wet, warm climate on earlyMars. Icarus 71, 203–224.

Rampino, M.R. and Caldeira, K. (1994) The Goldilocksproblem: climatic evolution and long-term habitabilityof terrestrial planets. Annu. Rev. Astron. Astrophys. 32,83–114.

Robinson, J.M. (1989) Phanerozoic O2 variation, fire, andterrestrial ecology. Palaeogeogr. Palaeoclimatol. Palaeoecol.75, 223–240.

Rosenqvist, J. and Chassefière, E. (1995) Inorganic chem-istry of O2 in a dense primitive atmosphere. Planet.Space Sci. 43, 3–10.

Sagan, C. (1961) The planet Venus. Science 133, 849–858.Sagan, C. (1971) The long winter model of Martian biol-ogy: a speculation. Icarus 15, 511–514.

Sagan, C. (1973) Planetary engineering on Mars. Icarus 20,513–514.

Salisbury, F.B. and Ross, C.W. (1985) Plant Physiology,Wadsworth, Belmont, CA.

Schaefer, M.W. (1993) Aqueous geochemistry on earlyMars. Geochim. Cosmochim. Acta 57, 4619–4625.

Seckbach, J., Baker, F.A., and Shugarman, P.M. (1970) Al-gae thrive under pure CO2. Nature 227, 744–745.

Sleep, N.H. (1994) Martian plate tectonics. J. Geophys. Res.99, 5639–5655.

Solomon, S.C. (1978) On volcanism and thermal tectonicson one-plate planets. Geophys. Res. Lett. 5, 461–464.

Squyres, S.W. and Carr, M.H. (1986) Geomorphic evi-dence for the distribution of ground ice on Mars. Sci-ence 231, 249–252.

Stoker, C.R., Gooding, J.L., Roush, T., Banin, A., Burt, D.,Clark, B.C., Flynn, G., and Gwynne, O. (1993) The phys-ical and chemical properties and resource potential ofmartian surface soils. In Resources of Near-Earth Space,edited by J. Lewis, M.S. Matthews, and M.L. Guerrieri,University of Arizona Press, Tucson, pp. 659–707.

Sturges, W.T., Wallington, T.J., Hurley, M.D., Shine, K.P.,Sihra, K., Engel, A., Oram, D.E., Penkett, S.A., Mul-vaney, R., and Brenninkmeijer, C.A.M. (2000) A potentgreenhouse gas identified in the atmosphere: SF5CF3.Science 289, 611–613.

Walker, J.C.G., Hays, P.B., and Kasting, J.F. (1981) A neg-ative feedback mechanism for the long-term stabiliza-tion of Earth’s surface temperature. J. Geophys. Res. 86,9776–9782.

Watson, A., Lovelock, J.E., and Margulis, L. (1978)Methanogenesis, fires and the regulation of atmo-spheric oxygen. Biosystems 10, 293–298.

Yung, Y.L. and DeMore, W.B. (1999) Photochemistry ofPlanetary Atmospheres, Oxford University Press, Oxford.

Yung, Y.L., Wen, J.-S., Pinto, J.P., Pierce, K.K., and Allen,M. (1988) HDO in the Martian atmosphere—implica-tions for the abundance of crustal water. Icarus 73,516–526.

Zent, A.P. and Quinn, R.C. (1995) Simultaneous adsorp-tion of CO2 and H2O under Mars-like conditions andapplication to the evolution of the martian climate. J.Geophys. Res. 100, 5341–5349.

Zent, A.P., Fanale, F.P., and Postawko, S.E. (1987) Carbondioxide: absorption on palagonite and partitioning inthe martian regolith. Icarus 71, 241–249.

Zubrin, R.M. and McKay, C.P. (1997) Technological re-quirements for terraforming Mars. J. Br. Interplanet. Soc.50, 83–92.

Address reprint requests to:Dr. Christopher P. McKay

Space Science DivisionNASA Ames Research Center

Moffett Field, CA 94035

E-mail: [email protected]


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