Viking Orbiter Views of Mars

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VIKING ORBITERVIEWS OF MARS

BY THE VIKING ORBITER IMAGING TEAM

M . H. CarrW . A. BaumK. R. BlasiusG. A. BriggsJ. A. CuttsT. C. DuxburyR. GreeleyJ. Guest

H . MasurskyB. A. SmithL. A. SoderbiomJ. VeverkaJ. B. Weilman

Cary R . Spitzer, Editor

Scientific and Technical Information Branch 980I \ J I \ S ANational Aeronautics and Space Adm inistrationWashington, DC



FOREWORD

T

HE VIKING plan to explore Mars was not simple. Two spacecraft were

dispatched at different times in the launch window, and put intodifferent orbits about the planet. After they scouted landing sites, some ofwhich were rejected as too risky, two soft-landers were detached andboth—incredibly—managed safe entry and set about making detailed surface

measurements and returning dramatic, close-in photographs of anotherworld. Meanwhile the two Orbiters continued to encircle Mars, sendingphotographic coverage of almost the entire planet. In concept, Viking was

ambitious to the edge of audacity.It was amazingly successful. The Landers revealed a desolate, rocky

world, provided fascinating chemical information about the surface, and

served as weather stations emplaced tens of millions of miles from Earth.The Orbiters have given us more than 46 000 images so far of the planetary

surface, in all seasons, lighting, and weather. In a way Viking still continues

even though the major mission has ended, for one of the radioisotope-powered Landers still checks in, and at this writing one Orbiter still hasenough attitude control gas on board to continue working.

This volume presents a selection from the orbital images provided by one

of the longest-running successes in the history of space exploration. Theyshow Mars as an extremely diverse planet. As you study them, it is difficult

to avoid the conclusion that, though Viking contributed immeasurably tobreaking the code of the Martian enigma, we do not yet confidently

understand its dramatic and turbulent past.

May 9, 1980 Thomas A. Mutch, Associate Administrator

Office of Space Science



ACKNOWLEDGMENTS

T

HE ENORMOUS SUCCESS of the Viking Project was the result of the

combined efforts of more than 10 000 people. To name even thatgroup of people who contributed directly to the success of the VikingOrbiter Imaging Experiment would be an almost impossible task. Threenames, however, stand out. In the early days of the Viking Project, theexistence of the Orbiter Imaging Experiment was in constant jeopardy.James S. Martin Jr., Project Manager, A. Thomas Young, Primary Mission

Director, and Di. Conway W. Snyder, Orbiter Scientist, were unswerving in

their support of a high quality orbiter camera. The pictures in this book are,in large part, the result of that loyal support.

Special mention should also be made of the Space Photography Section

of the Jet Propulsion Laboratory, which was responsible for overseeing the

design and fabrication of the cameras, and the Mission Test Imaging System

personnel of the Jet Propulsion Laboratory, who did most of the picturereconstruction and processing.



CONTENTS

Foreword.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. V

Acknowledgments..........................................vi Introduction .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 1The Vking Mssion .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .3

Earth and Mars: A Comparison ...............................11The Great Equatorial Canyons ................................17Channels... .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 31Volcanic Features ... .. .. .. .. ... .. .. .. .. .. .. ... .. .. .. .. .. ... 47

Del ormational Features....................................... 63Craters.... ... ... ... ... .. ... ... ... .. ... ... ... .. ... ... ... .. .73Variable Features .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 85

Martia n Moons .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 95

Surface Processes...........................................107Polar Regions .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .125

The Atmosphere ...........................................139The Viking Landing Sites ....................................161Gobal Color .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 169

Appendix I.Glossary .....................................173Appendix II. The Viking Orbiter Imaging System ................177Appendix III. Other Sources of Viking Data .....................180

Appendix IV. Project Viking Management Personnel ..............182

For the stereo images, a collapsible viewer is included on the inside back cover of this book.

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INTRODUCTION



MARS has long had a special fascination for man. As early as the 17thCentury, it was recognized that Mars had polar caps and rotated once

every 24 hours, much like the Earth. Perception of the planet as Earth-like

was subsequently reinforced by observations of white patches that wereinterpreted as clouds. Late in the 19th Century interest was enormouslyheightened by reports of canals on the surface. This led to speculation that

life might thrive there under climatic conditions similar to those on theEarth and that the canals might be the product of an advanced civilization.

Fact has proved to be nearly as bizarre as fiction. Although the canalswere an illusion, and the possibility of life now seems less likely, the planet

retains it fascination.

Mars is a geologist's paradise. Many features familiar on Earth aredisplayed on a vast scale made more awesome by the planet's modest size.

Great canyons are incised into the surface, huge dry river beds attest to past

floods, volcanoes tower to heights almost three times that of Mt. Everest,and vast seas of sand surround the poles. Global dust storms regularly cover

the entire planet. At the poles, caps of carbon dioxide advance and retreatwith the seasons.

This book incorporates images acquired by the Viking orbiters, beginning

in 1976. The pictures here represent only a small fraction of the manythousands taken, and were chosen to illustrate the diverse geolog y of Mars,

and its atmospheric phenomena. We hope they will also arouse the samewonder and excitement that we experienced on first seeing them.



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T H EVIKING

MISSION



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VIKING was begun by the National Aeronautics and Space

Administration on November 15, 1968. The main objectives of theproject were to achieve a soft landing on the surface of Mars and to relay

scientific data back to Earth. Scientific goals for the mission were established

in response to recommendations of the Space Science Board, a NASAadvisory panel. Foremost among their recommendations was that the firstMars lander mission should emphasize life detection experiments to answer

the question about the possibility of life on the planet. In consideration ofthese goals and the everpresent constraints of funds and spacecraft

capability, the following investigations and associated instruments were

established:

I n v e s t i g a t i o n s I n s tru m e n t sOrbiter imaging Two vidicon cameras

Water vapor mapping Infrared spectrometer

Thermal mapping Infrared radiometer

Entry science

Ionospheric properties Retarding potential analyzer

Atmospheric composition Mass spectrometer

Atmospheric structure Pressure, temperature and accelera-

tion sensors

Lander imaging Two facsimile cameras

Biological analyses Three separate experiments, gas ex-

Metabolism change, labelled release, and pyro-Growth lytic release, were included to test

Photosynthesis different biologic models.

Molecular analysis Gas chromatograph mass spectrom-

Organic compounds eter

Atmospheric composition Mass spectrometer

Inorganic analysis X-ray fluorescence spectrometer

Meteorology Pressure, emperature, nd ind

velocity sensors

Seismology Three-axis seismometer

Magnetic properties Magnets on sampler and a camera

test chart, observed by cameras

Physical properties Various engineering sensors

Radio science

Celestial mechanics Orbiter and lander radio and radar

Atmospheric properties systems

Test of general relativity

3



The main functions of the orbiter cameras, whose pictures are displayedin this book, were to aid in the selection of safe landing sites, to establish the

geologic and dynamic environments in s r hich the lander experiments wereperformed, and to add to our knowledge of the evolution of the Martiansurf ace and the dynamics of its atmosphere.

Mariner 4, launched in 1964, weighed only 262 kg and carried only a

small science payload. As it flew by Mars at a minimum range of slightl y lesthan 10 000 km, it returned 21 pictures of the planet. An airbrush mosaic of

two of the better Mariner 4 frames is shown here. The resolution is

approximately one kilometer, about 150 times better than Earth-basedphotographs. The pictures show a cratered surface that superficially

resembles the Moon, a result that was somewhat disappointing in view ofsome of the more extreme expectations. Certainly, no canals were observed.

In 1969, twin spacecraft, Mariner 6 and Mariner 7, were sent to examineMars. Like Mariner 4, these spacecraft were flybys, and between them they

acquired 201 images that were a blend of low-resolution, wide-area frames

and nested, high-resolution frames. A typical image pair is shown in theframes taken south of Sinus Sabaeus. The highest resolution achieved was

approximately 500 meters. Again, most of the area photographed resembled

the Moon, and craters dominated the landscape. Only a few picturesdisplayed nonlunar-like features, such as chaotic and featureless terrains, that

might have hinted at what remained to be discovered. In fact, the twospacecraft had passed over only those parts of the planet that retain anancient cratered surface, and they had missed the parts of the planet thathave younger and more diverse geological features.

Mariner 9, the final predecessor to Viking, eventually revealed theextraordinary diversity of the planet's surface. Arriving at Mars in November

1971, Mariner 9 was the first spacecraft to go into orbit about another planet.

It took more than 7300 images of Mars, covered the entire surface at a

resolution ranging from 1 to 3 km, and covered selected areas at resolutionsdown to 100 meters. The typical Mariner 9 image included here was taken in

the same area as the Mariner 4 images. Mariner 9 operated for almost a

year, nearly four times the minimum mission requirement, before runningout of attitude control gas. Along with the images, extensive data wereobtained on the atmosphere, surface temperature, and global weatherpatterns.

The images from Mariner 9 were the most exciting ever obtained inplanetary exploration, revealing giant canyons and volcanoes, large channels

(possibly cut by liquid water), and puzzling features that defied all geological

explanations. The stage was set for the Viking missions.

Viking 1 was launched from Kennedy Space Center at Cape Canaveral onAugust 20, 1975, and arrived at Mars on June 19, 1976. Initially, thespacecraft was put into a Mars-synchronous elliptical orbit with a period of

24.66 hours, an apoapsis of 33 000 km, and a periapsis of 1513 km. During

the first month, Viking 1 was used exclusively to search for and certify a safe

landing site for Viking Lander 1. After the lander touched down on Mars on

4



July 20, 1976, (the seventh anniversary of the first manned lunar landing)the orbiter began systematically imaging the surface. Its highly elliptical or-

bit was particularly suited for studying the surface because it allowed a mix

of close-up, detailed views at periapsis and long-range, synoptic views nearor at apoapsis.

Table 1 is a chronology of the orbit of Viking Orbiter 1. Two events

merit further description. On February 12, 1977, the orbit was changed topermit a flyby of Phobos, the larger, inner Martian moon. At closest

approach, the orbiter flew within 90 km of the surface of Phobos. On March

11, 1977, the periapsis of Viking Orbiter 1 was lowered to 300 km from theMartian surface. At this low periapsis, surface features as small as 20 meters

across could be identified, while the ability to acquire lower resolution and

greater area! coverage away from periapsis was retained. At the beginning of

this decade, Viking Orbiter 1 had taken more than 30 000 pictures of theplanet and was still operational.

TABLE 1.—Viking Orbiter 1 Chronology

Date Revolution Event

June 19, 1976 0 Mars orbit insertion

June 21, 1976 2 Trim to planned site-certification orbit

July 9, 1976 19 Orbit trim to move westward

July 14, 1976 24 Synchronous orbit over landing site

July 20, 1976 30 VL-1 landing at 1153:06 UTC

Aug. 3, 1976 43 Minor orbit trim to maintain synchronization over VL-1

Sept. 3, 1976 75 VL-2 landing

Sept. 11 , 1976 82 Decrease of orbit period to begin eastward wa lkSept. 20, 1976 9 2 Orbit trim to permit s ynchronization over VL-2

Sept. 24, 1976 9 6 Synchronous orbit over VL.2

Jan. 22, 1977 213 Period change to approach Phobos

Feb. 4, 1977 227 Orbit synchronization with Phobos period

Feb. 12, 1977 23 5 Precise correction to Phobos synchron ization

March 11, 1977 26 3 Reduction of periapsis to 300 km

March 24, 1977 278 Adjustment of orbit period to 23.5 hours

May 15, 1977 33 1 Small Phobos-avoidance maneuver

July 1, 1977 37 9 Adjustment of orbit period to 24 .0 hours

Dec. 2, 1978 89 8 Adjustment of orbit period to 24.85 hours; beginning slowwalk around planet

May 19, 1978 1061 Adjustment of orbit period to 25.0 hours; acceleration ofwalk rate

July 20, 1979 1120 Raising of periapsis to 357 km; adjustment of orbit periodto 24.8 hours; and slowing of w alk rate



Viking 2 was launched September 9, 1975, and arrived at Mars onAugust 7, 1976. Like its predecessor, Viking 2 spent nearly a month after

arrival finding and certifying a landing site for its lander. Table 2 is achronology of Viking Orbiter 2. A major difference in the orbit of this space-

craft compared to that of Viking Orbiter 1 is its high inclination, which al-lowed Viking Orbiter 2 to observe the complex, enigmatic polar regions at

relatively close range. Later in its mission, Viking Orbiter 2 flew by Deimos,the smaller of the two Martian moons, at a distance of only 22 km. Spec-tacular pictures showing features as small as a compact car were taken.Viking Orbiter 2 returned nearly 16 000 pictures of Mars and its satellites

before it was powered down on July 25, 1978.

TABLE 2.—Viking Orbiter 2 Chronology

Date Revolution Event

Aug. 7, 1976 0 Mars orbit insertion

Aug. 9, 1976 2 Period and altitude adjustment; beginning of westward

Aug. 14, 1976 6 Increase of period to increase walk rate

Aug. 25, 1976 16 Decrease of walk rate to proceed to landing site

Aug. 27, 1976 18 Synchronous orbit over landing site

Sept. 3, 1976 25 VL-2 landing at 2237:50 UTC

Sept. 30, 1976 51 Change of orbit plane to750 inclination and b eginnin

westward walk

Dec. 20, 1976 12 3 Lowering f eriapsis o 00 m nd increasing of

inclination to 80°March 2, 1977 18 9 Synchronous orbit over VL-2

April 18, 1977 235 Period change: 13 revolutions equals 12 M ars days

Sept. 25, 1977 404 Change of orbit period to approach Deimos

Oct. 9, 1977 41 8 Orbit synchronization with D eimos

Oct. 23, 1977 43 2 Change of orbit period to 24.0 hours and lowering ofperiapsis to 300 km

July 25, 1978 706 Powered down

6



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Mariner 4 Mosaic of Southern Hemisphere Ancient Cratered Terrain. This mosaic of twoMariner 4 frames suggests a lunar-like surface and shows no traces of the canals andoases of early observers nor the volcanoes, canyons, and channels revealed b y latermissions. The major surprises to come from Mars remained hidden. [Mariner 4 frames15-1ÔA; 470 S. 141° W']



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Mariner 6 Frames South of Sinus Sabaeus. (a) A low resolution frame from \lariner 6shows the large craters \ islicenus (left) and Flaguergues (right). (b) This is ahigh resolution frame of a small portion of the northeast rim of Flaguergues and theadjoining terrain. Fortuitous placing of all the Mariner 6 and 7 frames within the oldestterrain of Mars was such that none of the more spectacular y ounger features wasobserved. [\lariner 6 frames (a) 6\21. (li) 6\22: 130 4_0 j

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9



EARTH AND MARS:

A COMPARISON



M

ARS, the outermost of the four terrestrial planets (Mercury, Venus,Earth, and Mars), is the second closest to Earth after Venus. It is

slightly more than half the size of Earth and almost twice the size of theMoon. The atmosphere is very thin, less than one one-hundredth that ofEarth, and composed primarily of carbon dioxide.

Temperatures are cold, the mean annual surface temperature beingapproximately -50°C at the equator and close to -130°C at the poles. Be-cause of the thin atmosphere, the diurnal temperature range is large, greater

than 100°C at the equator. Summer temperatures rise above 0°C at middaydespite the low diurnal mean.

The rotational axis is inclined to the ecliptic, like Earth's, so that

Mars experiences distinct seasonal weather patterns. A particularly striking'seasonal event is the annual dust storm. During summer in the southernhemisphere, large dust storms develop and obscure much of the planet's

surface from view. Such storms, long known from telescopic observations,

were observed from orbit by Mariner 9 and both Viking orbiters. During the

height of the 1977 dust storm season, wind speeds up to 25 meters/sec were

recorded by the Viking landers, although they were far from the center ofdust storm activity.

Another regular seasonal event is the formation of clouds of carbondioxide ice particles in the polar regions during the fall as gas starts to con-

dense out of the atmosphere onto the growing cap. So much of the atmos-

phere condenses out in this process that atmospheric pressure decreasesmore than 30 percent from fall to winter. The pressure decrease is smallerin northern winter because of the smaller northern cap.

Other cloud activity is related to water in the atmosphere. Although very

small amounts of water are present, the atmosphere is close to saturationmuch of the time, and a wide variety of water ice clouds have been observed.

The Martian surface has some characteristics of Earth's surface, some ofthe Moon's, and some unique features. The planet is very asymmetric in

appearance. Most of the southern hemisphere is densely cratered andsuperficially resembles the lunar highlands. In contrast, the northernhemisphere is relatively sparsely cratered and has many large volcanoes thathave no lunar counterparts.

The most prominent volcanic region is Tharsis, where there are severalvery large volcanoes that resemble terrestrial shield volcanoes, such as those

in Hawaii, except that those on Mars are many times larger. The different

features of the volcanoes, such as calderas, lava flows, and lava channels, are

also many times larger than their terrestrial counterparts.

11



Tharsis is close to the center of a 6000-km diameter, 7-km-high bulge in

the Martian crust. Numerous fractures radiate from the center of the bulge

and extend out as far as 4000 km. The fractures are arrayed unevenlyand are concentrated in intensely fractured zones called fossae. The

radial fractures are so extensive that they are the dominant structural ele-

ment over half the planet's surface.Close to the equator, east of Tharsis, a series of vast interconnected

canyons constitutes the Valles Marineris. These canyons also are enormousby terrestrial standards, being almost 4000 km long, up to 250 km across,

and up to 9 km deep. The canyons are aligned radial to the center of theTharsis bulge, and appear related in some way to the radial fractures.

To the east, the canyons become shallower and merge into a type ofterrain peci.iliar to Mars: Large areas of the surface apparently collapsed to

form arrays of jostled blocks that are at a lower elevation than the

surrounding terrain. Because of the jumbled nature of the surface, thisterrain has been termed chaotic.

From many of the regions of chaotic terrain, large, dry river beds

emerge. The channels generally start full size and extend down the regionalslope for several hundred kilometers. Most large channels emerge from the

chaotic terrain just east of Valles Marineris and flow into the Chryse basin tothe north, but several occur elsewhere. In addition to these very largechannels, numerous smaller tributary systems and dendritic drainagenetworks are present throughout the equatorial regions.

The origin of the channels is not known, and they have been the subject

of a lively debate since their discovery in 1972. The main issue is whether or

not they were formed by running water. If they were, then different climatic

conditions in the past may be implied. There are also some intriguingbiological implications if "wet" periods have occurred in the planet's history.

The effects of wind are evident over almost all the Martian surface. Many

of the classic dark markings apparently are associated with wind activity.

They can commonly be resolved into arrays of streaks that start at craters

and are aligned parallel to the predominant winds. The streaks may be lighter

or darker than the surroundings.Wind action is also evident from the streamlined form of many features.

High resolution pictures show that the north pole is almost entirely sur-rounded by dune fields that form a dark collar around the pole. Dunes also

occur elsewhere, such as in the canyons and within craters, especially inhigh southern latitudes. Thick sequences of layered deposits of unknownorigin are found at both poles. They lie unconformably on the terrain and

appear to be very young compared with most other features on the

planet. The deposits possibly are accumulations of windblown debris mixed

with condensed volatiles like \%rater

The geologic histories of Mars and Earth are quite different, partlybecause of the internal dynamics of the planets and partly because of thediffering effects of the atmospheres and oceans. Earth's geology is

dominated by the effects of plate tectonics. The rigid outer shell of the

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Earth (the lithosphere) is divided into plates that move laterally with re-

spect to one another. Where plates diverge, as at midoceanic ridges, newcrust forms; where they converge, one plate generally rides under the other

to form a subduction zone. Thick sediments may accumulate in a subduc-tion zone; these may ultimately be compressed, partially melted, and up-lifted to form linear mountain chains of folded and partly metamorphosed

rocks, such as the Andes and the Himalayas. Melting of the subductedplate as it moves down into the mantle may also given rise to volcanism in

the subduction zone. Where plates move laterally with respect to oneanother, they form transcurrent faults such as the San Andreas. The pres-ent configuration of the Earth's surface is thus a partial record of the mo-tion of different plates with respect to one another.

Mars displays little, if any, evidence of plate motion. The crust appearsvery stable. Long linear mountain chains and subduction zones are absent,and transcurrent faults and compressional features of any kind are rare. Itsgeologic history is thus very different from that of the Earth.

Greater stability on Mars results in the preservation of much older

features. On Earth, surface materials are recycled at a relatively rapid rate byerosional processes and subduction. The two processes are commonlyinterdependent; for example, erosion is greatly increased in mountainous

regions along subduction zones. On Mars, however, recycling of crustalmaterials is extremely slow, as evidenced by the preservation of large areas of

old, densely cratered terrain that probably dates back approximately fourbillion years.

Crustal stability may also be the cause of the large size of the Martianvolcanoes. On Earth, volcanoes are limited in size because plate motionusually carries them away from the magma source. On Mars, however, a

volcano remains over its source and can continue to grow as long as magma isavailable.

The preservation of features billions of years old on the Martian surface

indicates extremely low erosion rates. On Earth, most erosion results from

running water. Small channels in the old cratered terrain of Mars areevidence of an early period of fluvial action, but survival of the old craters

indicates that the period was short. For most of the planet's history, windhas probably been the main erosive agent. Despite giant dust storms,however, the wind clearly has not been very efficient in eroding the surface,

because so much old terrain survives. Most of the wind's action probablyinvolves reworking previously eroded debris.

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COMPARISON OF EARTH TO MARS

Earth Mars

12 756 km Diameter 6787 km

5.98 X 0 24 kg Mass 0.646 X l024kg

9.75 rn/s 2 Gravitational acceleration 3.71 rn/s2

149.5 X 0 m (average) Distance from Sun 227.8 x 06 km (average)

839 cal/em 2 /dav Sunlight intensit y 371 cal/cm 2 isol

23° 27" Inclination 23° 591

24 h O O m Length of da y 24h40m (1 sol)

365 (la y s Length of y ear 686 da y s (668 sols)

60 000y Magnetic field 50- 1 O0-y

101 3 mh (average) Atmospheric pressure 7 mb (average)

Known satellites 2

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PA G E M IS SIN G F R O M A V A I L A B L E V E R S IO N



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THE

GREATEQUATORIAL

CANYONS



VALLES MARINERIS is composed of steep-walled canyons, individuallymeasuring up to 9 km deep, 250 km wide, and 1000 long. They

were named for Mariner 9, the Mars-orbiting spacecraft that took the firstpictures of the canyons in 1971. The entire Valles Marineris system extends

over 4000 km from west to east near the Martian equator, and its size dwarfs

all similar terrestrial features except, perhaps, the 5000-km-long midoceanrift system.

Pictures taken by the Viking orbiters show large areas of Vailes Marineris

at a resolution far better than that achieved by Mariner 9. The new featuresobserved indicate that, although erosional landforms (such as landslidescars and deposits) and tributary canyons are common, faulting apparent-ly has been the dominant factor in canyon development.

Since discovery of Valles Marineris, the method of their formation hasbeen a nagging puzzle. The canyons do not form a well-integrated drainage

system; some are completely closed depressions, and lateral transport bywind or water would be considerably impeded. Now, however, the newevidence of faulting suggests that most negative relief results from

subsidence. Low, straight sc.arps, which apparently indicate downward

subsidence of canyon floors along faults, cut across erosional features onmany canyon walls. Similar scale faulting occurs on Earth: in East Africa the

continental crust is in tension across large rift valleys. Erosion of the Valles

Marineris walls apparently continued into the recent past, so the crustaltension causing the faulting within the canyons may also have been a

relatively recent phenomenon.Another exciting discovery resulting from Viking images is the presence

of thick layered deposits on the floors of several canyons. Layered rock isalso visible in the canyon walls, and thus is part of the Martian crustpredating the canyons. Some materials on the canyon floors are distinctive

for the fine scale and regularity of their layering. Only climatic modulation

of a sedimentary process seems adequate to explain them. Possibly, regular

changes in the Martian climate, governed by known orbital variations, have

controlled the level of dust storm activity and the rate of deposition ofsediment from the atmosphere. Another theory is that some sections ofValles Marineris were sites of lakes in which layered sediments %rere

deposited. Before Viking, regularly layered deposits were known only in thepolar regions of Mars, and their creation may also be associated with cyclicalclimatic change.

Impact craters, which are so numerous on other Martian terrains, are scarce

within Valles Marineris. They appear most frequently on smooth areas of

canyon floor, and are possibly the tops of blocks down-faulted from the

17



upland plain. Shallow pits, possibly eroded impact craters, are abundant in

other places. Impact craters are probably scarce in the canyons becauseerosion and deposition by landslides and wind have been actively renewing

interior surfaces. No evidence of flow of water has been found within Valles

Marineris, although some channels on the adjacent upland are abruptly

truncated by steep canyon walls.

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Global View ,of the Valles Marineris. As Viking 1 ap-proached Mars in June, 1976, it recorded this color pictureshowing Valles Marineris stretching more than 4000 kmacross the face of Mars. North is toward the upper right,and it is the winter season in the southern hemisphere. The

annual southern ice cap extends . up to450

S latitude,

blanketing the Argyre basin, an impact crater 800 kmacross. [IPL, ID I 2038MBV1, VERS2; 30° S, 70° WI

18



Western Valles Marineris. Because these canyons are poorly linked with one another, their floors n ot a regularly graded slope, they cou ld no t have forme d as water draifeatures. The straight alignments of many canyon walls, and the faulting in sevdirections associated with Noctis Labyrinthus, combine to suggest that the VaMalineris system is composed of great rift valleys formed on the surface of a domethe summ it of the dome, near the labyrin th, the crust was stretched in all directionform a netwo rk of fault-bou nded valleys. On the f lanks of the dome, the greatest stwere concentric abou t the sum mit, giving rise to a set of radial rift valleys. The shows names of individual canyons (chasrna).[40A37-52;13' S, 8 7 OW ]



Eastern Valles Marineris. The broadest valleys merge with patches of cliaotic terrain,apparently the product of collapse and erosion of ancient cratered terrain. Channelshegin~ling t the rnar~rinsof chaotic terrain extertd northcastward onto Chryse Plani~ia,the region in which Viking Lander1 s located. Th e relationship between Valles I\IIaiiuerisand the chaos is not well u~lderst ood . rregular collapse, to form the chaos, may reflect-crustal stresses similar to those forming the rift valleys, but differing in orientation andcomplexity. It is also possible tha t the chaos formed during the catastrophic release ofliquid w ater derived fro m artesian reservoirs o r the melting of ground ice.[32A11-15;9" s, 53" W ]



View of C oprates Chasma Looking East-Southeast. Interplay of faulting and ero

N within Valles Marineris is appa ren t here .A low, steep, and generally straight scarp occat the foot of the nortll canyon wall. The scarp apparently results from downward oof thr: flat canyon floor relative to the wall. Gullies on the canyon wall are truncatethe scarp and thus predate it . In con trast, large-scale lanclsliding from tw o great cawall alcoves postdates the latest down-faulting; landslide debris has buried the scarpdistance of about40 km . [5811\89-91;15O S , 60' R' ]



Evolu&on of Canyons by Z,andslidin$;,Th1. I I I O ~ ~ I Cf 1 ~ L i n g ~ / ) I ~ I ~ Ij )~ct~iz'5 3s t c ~ l , t a ~ i

loolii~lgsouthr\rard t o w x c l tile lunction ot (;dr ig~ s dirt1Cc1pil Cl~d-,~rr~l( 'liiggt~(l < Y I ~ ~ * I

cons isting of num ero us ti1tc.d bloclts occur s ~vltliirri lcovc~i n t l l ~all\ otr 5% 111. I,\tencllnc.lroln t l r c alcol tb s ate tlnrt b l d i ~ l ~ t ~ .1 ~i~dt ( , t ld ll t l t JII-1d.r d t l t l ~ ~ i b1 \ I I L / e i t t ldt~oll*Where tw o pat tcrns interscrt,on ? tllin lobr clrarl! ovr~r1ic.s h r oth ria n d ot t ~ t sstriations. T liese icature s a rc k ~ an t an;lqlitlr clt~posits that lo ~n le tlnl lci i s t~ct iorr~fcanyon rirn collapsed.The broad, thin lobes of m aterial app are ~ltlv lo.rterl at highvelocity from the bases of the collapsing masses.T h r mechariism by \c4iicli the surfacestriations formed is not well understoo d, although similar features liave heeil ob sc r~~ r~c lnterrestrial landslides. Gro ups of hills, similar to cha otic terra in, and sand dunes contr ibuteto a variedthe south (

canyon floc:anyon wall

lands4thin

;cape. Laythe lands1

,erillide



Noctis Laby~inthus.The origin of the canyons by fatilting is most apparent in NoctisLabyrinthus at the western end of Valles Marineris. Many canyons have a classic gral-~rnform, with the upland plain surface preserved on the valley floor. Other canyons are moreirregular in form and have rough floor terrains; evidentlythe co1ist;cIucrlce of ial~dslitii~igand the puzzling process of pit forma tion. I11 places it appears tha t surface materials haves i f t ed do~vn~vardnto a gaping hole in the subsurface. The inset shows a slope coveredwith light albed o dunes an d several small landslide lobes.[46A13-28, 41A17-28,

48A21-28,49A22-28, 50A14-28,Inset 62A64; 7 S, 100' W ]Q

Stereogram of Central Tithonium Chasma. This section of Tithonium Chasma is about6 km deep. Overlapping landslide lobes cover th e cany on floor and scarps that bo und arift valley within the canyon. On the south canyon wall, distinct bright and darkhorizontal stripes are probably outcrops of layered rocks. Parallel chains of pits andgraben mark th e upland surface to the south . [L eft57A45, Right 64A22; 5' S, 85' W ]



Large Albedo Contrasts and Relief in Tithonium Chasma. Large contrasts in tbrightness of surface materials can so confuse perception of depth in single frames sterc~oscopic maging is necessary to interp ret surface features. T o the left is a lnarroxr rift valley ab out 5 km deep. Nortli of this valley the canyon floor is1 o 2 kmhigher arid irregularly mottled. A mountain with finely gullied flanks rises about 2 from the canyon floo r. This mounta in is representative of many plateaus, ridges, atld on the floors of broader canyons. They differ drastically from canyon xvall materialtheir pattcrns of erosion. RIany are composed of materials with a distinct horizolayering. [Left 44 A27, Right 63A63; 5'S, 65' W ]

West Candor Chasma. Here the canyon floor is entirely covercd with eroded, layematerials. Layering is most prominent at A, B, andC. A n offset spur (D) arid a low , stescarp (EF) along thr .tvestern ~vallsof the canyon may have been for~n erl y fautrending nor th to so ut l~ cross the m ain Valles WIarineris. The tributary canyon aseems to have developed by two processes; subsidence of a block of crust (graben), a

irrrgular collapse into a string of pits. [65A2 5, 5 7; 66A17 -27; 7'S, 75' W ]



Candor and Ophir Chasmae. Large plateaus(A , B), formed at least in part of regularlylayertd materials (as at A) , rise from the floor of Candor and extend across the gapbetween Candor and Ophir Chasmae. At C, plateau niaterials apparently were depositedupon an erocled spur of the canyon wall and are now themselves being eroded away.Streamlinrd ridges and grooves in Ophir Chasma are probably wind sculpted.[66A23-30:5" S, 3" W ]



Plateaus betw eerl Oph ir Chasrna an d Caridor Chasm a. T1it:se eno rm ous, streaml inedplateaus bridge the ga p between tht: two canyons, possibly indicating wind erosion ona

very large scale. Alternately, it has been suggested thata lake (or sea) once existed in t henortlt section of Ophir Cliasina until t h e canyon wall was breached soutliward to unleaslran en ormo us flood. The dark material onthe canyon floors is probably a wind deposit,asdune-like forms are visible in other images. [IPLID, IV2515CCrX2, 12515DGX2;5" S,73" W ]

a

Layered Material in Juventae Chasma. A ridge of very uniformly layered light and darkmaterials rises from the floor of Juventae Chasma. Cyclical changes in sedimentation,perhaps modulated by climate, seem the most probable explanation for their origin.[81A15-17; 5" S, 62 " W ]



CHANNELS



C HANNELS are among th e m ore pttzzling and intriguing features of theMartian surface. Th e most controve rsial aspect of the channels is

whether they were form ed by running water. Present climatic conditions onMars prevent the existence of l iquid water a t the surface, so a water-wornorigin implies tha t very d ifferen t climatic conditi ons prevailed in th e past. Adenser atmosphere and higher tem peratu re are bo th required. Because ofthe difficulty in explai~lil lg ow climatic conditions could have changed sodrastically, alternative met hods of erosion, such as by w ind and lava, havebeen suggested.

Three m ain type s of channels have been recognized:(1) unoff channelsappear as dendritic networks, or arrays of relatively small channels or valleyslocated mainly in th e o ld, densely cratered terrain;(2 ) outf low channels ap-

pear as large scale tributaries; and(3 ) fretted channels appear as long, rela-tively wide, flat-floored valleys that possess tributaries and increase in sizedownstream.

Much of t he old, cratered terrain, particularly in th e equa torial regions, isdissected by channels of some type, the most co mm on of which is th e simplegully, typically a fe w tens of kilo me ters long. The gullies generally have fewtributaries w hich, if present, have small junction angles. The nume rouswell-integrated tributary networks provide the strongest evidence for watererosion at some period in Martian history, because they are unlikely t o formby wind action or lava erosion. The runoff channels are largely restricted tothe oldest terrain and are themselves commonly degraded. Most of thesechannels therefo re appear t o have form ed early in the planet 's history.

Most outflow channels occur around the Chryse basin. They commonlye me rg e f ~ d lize from chaotic terrain th at has seemingly collapsed to fo rmareas of jostled blocks as many as 3 km below the surroundings . Thechannels extend from the chaotic terrain downslope several hundredkilometers in to t he plain of C hryse Planitia. Th ey may be tens of kilometerswide and more tha n a kilometer dee p, a size indicating erosion on anenormous scale .

Within the channels are many features, such as teardrop-shaped islallds,longitudinal grooves, terraced margins, and inner channel cataracts, th at arealso found in regions on Earth affected by large floods. The dimensions ofthe Martian channels suggest peak flood discharges ofl o 7 t o l o 9 m3/sec. Bycomparison, the average discharge of the Amazon is10' m3/sec , and thelargest k now n terrestrial flood , the Lake R/lissoula flood t ha t occ urred ineastern Washington in th e late Pleistocene, had a peak discharge of



l o7 m3 /sc'c. Tllus t hc C hry se o ~ ~ t f l o whannels, and similar ones else\vheprovide evidence of eno rm ous floo ds on Mars-far greatcr thana ny knowon Ear th .

The time period in which th e climatic conditions permitted liquid wato exist is uncertain because of the difficulty of precisely dating channels. Most of the evidence, however, suggests that the more clern

coilditions prevailed very early, perhaps during Mars' first billion years, that this period was followed by general global cooling. The present hconditions have probably existed for mo st of the planet 's history.

Fretted channels occur mostly within the old, densely cratered terrespecially at i ts boun daries with y ounger units. Th ey lack features indicof catastrophic flooding. The presence of tributaries and a decreasechannel size upstream also argue against form ation b y floods. T he origithe f re t ted channels is not know n, bu t nu merous features on the f lsuggest that masswasting may have p layed a significant role.



Dendritie Channels in the Southern Highlands. These channels are deeply incised withrelatively wide, u~ld issecte d nterfluvcs. Most terminate abruptly at their lower ends.[P-1811 5; 25" S, 10"W ]

Q

Finely Channeled Old Cratered Terrain. The channels are concentrated on crater rims andtend to b e app roxima tely parallel, a few tens of k ilometers long, with few tributaries.Such channels are ty pical of much of the heavily craterecl terrain of M ars, bu t are rare inthe sparsely crater ed areas. [84A 16-2 2; 23" S , 0"W ]



Dense Drainage Net.worlr in the Southern Highlands. Dendriticpatterns like these suggesta fluvial origin and argue against alternatives, such as erosion by w ind or lava.[ 6 3 A 0 9 ;48" S, 98" W ]

Outflo w Chruulel Emerging fro m C haotic Terrain. O blique view, looking so uth, of source region of an apparent flood. The channel starts full scale in a region of cherlclosed by cliffs. Possible mechanisms for producing such a relation are rapid releaswater fro m buried aquifers or the melting of ground ice by volcanism.[P-16983; o

43" W ]



Channels and Chaotic Terrain at the Source of Tiu Vallis.A 50-ltm-wide channel emergesfrom chaotic terrain. I t extends off the picture to th e no rth, down the regional slope toChryse Planitia 100 0 km away. [P1913 1;5" S, 29" W ]

Closeup of Part of Preceding Image. This frame sh o~ vs iu Vallis (left)extending north\vest from Hydaspis Chaos (right). Hyriaspis Chaos is anelongated area of collapsed terrain almost 1 00 km wide, fromwhichemerge th e lineated bedform s and teardrop-shaped islands of Tiu Vallis.[83A37; 3" S, 27" W ]



Part of Ares Vallis. (a) This mosaic sho ~v s ar t of

Ares Vallis incised in the heavily cratere d upla nd.Tllc channel is 2.5 km ~ v i d c n d a b o u t 1 km deep.Several layers, probablj lava flows, are exposed inthe walls of the cliannel. IIany craters arc present int h e upland surface, But craters are few on thechannel floor. (b) 4 stereogram sl~ ow s he originof Arcs Vallis in chaotic terrain. Channels from tw olarge arras of chaos have merged into a singlechannel. Where flow from the lower chaos hasmerged into thc channc4, many streamlined fo rms arevisible. [(a) 211-5238 ; 10" W, 4" W , (b ) 451A03-l o ; 2" N, 19 " R1]



"Islands" near Chryse Planitia. Teardrop -shape d "islands" are shown at the mou th ofAres Vallis near th e s outhern boundary of Chryse Planitia. Flow was from the south andapparently diverged arou nd obstacles such as craters and low hills to form a sharp prowupstream and an elongate tail downstream.A shal lo~vmoat surrounds the entire island.Similar patterns o n Earth have been formed by catastrophic floods, wind erosion, anclglacial action. Fro m to p to b otto m, the three large craters are named Lod , Bok, and Gold.[211-4987; 21" N, 31" W ]



Gllalmels behvwrk Lunae Pianurnand Chryse Plariil-ia. (d) C,h'irt~tt~lst,~vc.br>etl cu t dcros iold cratered terraill l~t\twrrrllir Idva plains o t I,ur~atsPlant~rii r1 t l r t h left dtirl t l ~ r )ldiiis fChryse Planitid to th e right. Tllrre separate cll,~rlllel> terr~s rc visible, starting ir orr ~ herioltli Vrdra Vdli?, \ l d u i n ( ~ Vdllis. arid \Idj,r \'allis. I 2 \ o \ i dotkg l l i t a ed~tt'kil t*dge ofL u r ~ a cPlatlu rr~ converg etl t o c u t Alalaja Vallis. N uri it~ rou s edrdrop-s1lapr.d islarlds o cc urupstream of the nlairi cliarinc~l.Belo\+ t li r c l~anr ie l o the eas t(okf the riglit side), the tlowdiverges across Chryse Phi t ia . (b ) This stereogralrl sllows Vedrd and Maurnee Valles

bet~veel lLun ae Plarlrlr~l and Chryse Planitia. N ote th at a branch of Vcdra Vallis passesthro ugl i Bat111 Crate r. [(a) 21 1-5 19 0,(L ) 211-5419; 18' N, 5' W ]

Upper Reaches of Maja Vallis. Th e surface of Lunae Planum is extremely scoured, withlong linear grooves and te ardro p islands. Flo w ap paren tly collverged on i\iIaja Vallis fro m awide area of Lunae Planum.[44A44;17 ' N, 57' R i ]

39



/r Western Chryse Planitia. The west side of Chryse Planitia has been extensively sculpteflow fr om i\iIaja Vallis, which is situa ted just t o th e lef t of this mosiac. Flow div

Nacross the gently sloping plain of Chryse Planitia t o fo rm the sculptecl features seen imosiac. Ridges, similar to th ose o n the lu nar maria, app ear to have partly dammediverted flow to for m a variety of scour patterns.[211-5015; 21' N, 49' W 1



Flow around Dromore Crater in Chryse Planitia. Fl o~ v was frorn thp left and apparentlj

por~ded west of th r mare ridge. I t then cut gaps as it flo~vecl ver low points in th e ridge.- -Sirnilar relations occur in t he channeled scabla ~lds f Washington stat e. Aftcr cros sit~ g he

\

ridge, the flo~v u t grooves in th e Chryse Planitia floor as it flolted arouncl Dromore, anolder impact crater. [20 462 ; 20' N, $9' R 7 ]



Flat-Floored Valley Northeast of Hellas. This valley is several kilometers wide and is cutinto layered deposits that are clearly exposed in the valley walls. In some places, a

AN

channel is visible in the valley floor. Extensive debris fans surround many hills in th e area

and are probably formed by creeping of near-surface materials, perhaps aided byinterstitial ice. [97A60-68;43' S, 253' W ]

4

Section of Valles Marineris. Each tribu tary on the sou ther n wall of the can yon heads in acirque-like feature and lacks a fine-scale drainage net~vork.The morphology suggestsformation by ground water sapping rathe r than by surface run-off. Ground ice isapossible source for the water. [211-5158; 80° S, 5' W ]



V OLCANIC ACTIVITY 011 Earth can be divided into two basic types:eruptions that occur repeatedly from the same conduit and slolvly

build rougldy circular mountains, and eruptions from any widely spacedvents, usually fiss~~res, ha t create extensive lava plains. Both types are fo ~ tndon Mars. Volcanic rocks are of particular interest to the geologist becausethey originate deep within the planet and provide a means of assessing th econditions and processes that operate there. Although we are unable toexamine the rocks on Mars directly, the volcanic features give an indicationof rock composit ion. For example, silica-rich lavas tend to have higherviscosities and yield strengths than silica-poor lavas and so form differentlyshaped flows; volatile-rich, viscous lavas tend to produce abundant ashduring eruptions, so ash deposits rather than lava flows are the predominantlandform. The volcanoes are also interesting in that their shapes and sizesprovide information on thermal conditions in the interior of the planet. Thevolcano height gives a means of estimating th e depth of melting, and th edegree of sagging of the crust under the weight of the ~~olcanoermits theviscosities of the crustal materials and hence the temperature profile to becalculated.

Martian volcanoes are most common in the region of Tharsis, wherethree large volcanoes (Ascreus Mons, Pavonis Mons, and Arsia Mons) form anortheast- southwest line. Another large volcano, Olyinpus Mons, is locatedabout 150 0 km northwest of the line. All fou r are enormous by terrestrialstandards. Olympus Mons is more than 600 km across and towersapproximately 2 7 km above the mean surface level. Alba Patera, just t o thenorth of Tharsis, although only a few kilometers high, is 1700 km indiameter. The Hawaiian volcanoes, which are among the largest on Earth,are generally less than 12 0 kin in diameter and 9 km above the ocean floor.Surrounding the massive Martian volcanoes are extensive lava plains andmany smaller volcanoes such as Biblis Patera and Tharsis Tholus. Vol-canoes occur in regions of the planet other than Tharsis, but tend to besmaller and older.

Each of the three Tharsis shield volcanoes has a caldera complex at it ssummit, apparently formed by repeated collapses following eruptions. 011

the flank of each edifice is a faint radial texture formed by numerous long,thin flows, some with central channels. The general morphology of the

flows is similar to those on the flanks of the Hawaiian shield volcanoes andsuggests f luid flo w. Various concentr ic features such as terraces , breaks inslope, and lines of rimless depressions are superposed on the radial texture.On th e northeast and southwest sides of each volcano, numerous pits in th eshield coalesce to form alcoves that evidently were sources of enormous



\ ~ l u m e s f lava. Flows spread from thesf, alcove s ovc.r th c adja cen t p la i i~covering th e l o ~v er lanlis of the vo lca~lo es lld cx ttnd ing several llrirldrliilotnetcrs from th r sotlrcc. Tluts, c u ~ ~ p t i o n srom the T h a r ~ i s o l c n n olorm ed b oth the volcanic td ific ts and tl-tc surrou nding plains.

Th e main edifice of Olym pus Rlons resembles th e Tharsis shields excethat i t is surrounded by a cliff t ha t, at som e points, reaches6 1tm in heigh111 several places, lava has flowed over the cliff and across the surrou~lplains, exten ding th e volcanic edifice bey ond t he scarp. All aroun d Olym pMons, b locks of strongly ridged terrain extend as far as 10 00 kin from tscarp and colzstitute the so-called aureole. The origin of the aureole unclear, b ut sugg estio~ls re tlzat it is th e reinlzant of a pre-Olym pus volcanth at i t coizsists of erod ed ash-flow tuffs, or vast th rust sheets.

Alba Patera, just to the north of Tlzarsis, differs from the volcanoalready described. Although it is mo re than 17 00 km across, i tis only abou2 kin high. Many flo w fe ature s are visible on it s flanks. These featu res aoften as m any as 1 0 times larger th an their terrestrial counterparts, botherwise show great similarity. The nature of Alba Patera's flow featuraga in s~ ~g ges t sluid lavas.

Relatively featureless plains cover much of th e planet's surface. Torigins of most of the plains are not known. Altlzo~~ghollie ma y be largelaeolian and fluvial, evidence indicates that most are volcanic. The plaiar o~ tn d he larger volcalloes have num erous flow featu res and are almcertainly volcanic. Other plains have ridges and rille-like features thresemble those o n th e Moolz and so are su spected of being volcanic like tlunar maria. Where visible in section, the plains are layered, perhaindicating interb edd ed materials of differe nt origins.

DOlympus Mons. (a) This volcano, the largest on M ars, measures over 600 km across at base, and is about 27 li ~ n igh. It is surromlded by a well-defined scarp that is up t o6 kmhigh. Flows drape over the scarp and extend on to the surrounding plains. In many pl

the scarp is associated with small block faults, indicating tha t faulting may have playepart in its development. Parts of the plains surrolinding the volcano are characterizedridged and grooved terrain that is fanlted in places. The origin of this terrain is known. (b ) The stereogram permits a greater appreciationof the structure of OlympNIons, especially the caldera and th e scarp. [(a) 211 -536 0, (b) Left 211-5 345, Ri211-5360; 18" N, 33 " W ]



Summ it Caldera of Olymp us Mons. Th is mosaic consists of several frames that sh

'

eatures on the surface as small as18 meters across. The circular caldera on the le ftSalmost 3 km d eep and 25 km across, and has wall slopes of abo ut 32'. It probably form

I 5 k m / as a result of recurren t collapse following drainage of magma o ut of th e central con duithe volcano during flank eruptions. The floor of the deepest calderais featureless at thiresolution, but the floor materials of other parts of the caldera complex are markedfault patte rns and ridges similar t o mare ridges o n the hloon . Fluting of the caldera wsuggests landslide activity. [21 1-56 01;18' Dj, 133' W ]

Terraces on Upper Slopes of O lympus Mons. The origin of the lava terraces is no t knoIn some respects, they are analogous to terraced featu res seen on pahoehoe flowsIlo un t Etn a, Sicily, where they formed as a result of emban kments developing at fronts of lava flows and the accumu lation of lava lakes behind the embank ments. Somthe small craters appear to be rimless volcanic pits. [46BL2;17' 3, 32 ' W ]



Lava Flow Drapes over Olympus Mons Scarp.(a) Lava channels and partially collapsed lavatubes are visible along the crests of ridge-likeflows. The surface features on these flows aresimilar to tho se d eveloped o n basaltic flows onEarth. Clearly, the scarp in this area is olderthan the flows, indicating that a t least theyounges t flows on the lnou n t a i~ ~ccurred afterscarp formation. In this region, the OlympuslMons flows make up the plains surface at thefoo t of the scarp. However, in oth er areas.O l y ~ n p u sRilons flows h ave bee11 ove rlain by t h esmo oth-su rfaced material of the plains.(b) Thestereographic pair graphically portrays theruggedness of thc scarp. [(a) 47B25, (b) Left46B34, Right 45B45; 21"N, 130" hi ]



Arsia Mons. The summit is at about the same elevation as that of Olympus Mons, ri1 16 km above the Tharsis Ridge-itself abo ut11 km high. The caldera is less comp lex ththat o f Olympus Mons, being a single, large circular structure abou t 1 40 km in diamSurrounding the caldera are concentric graben; the main northeast-southwest trendfracture zone underlyillg the volcano is indicated by numerous collapse pits seen herethe u pper side of the caldera. This mosaic shows an enormous flow-like feature extends from the volcano flanks onto the adjacent plains, and which consistshummocky terrain with faint concentric features. The flow terminates in fine scale ridparallel to the flow's front edge. The origin of this feature is not clear, but it may major landslide that developed high on the flanlrs of the volcano a t a time ~ vh evolcano slopes were u nstable. Tlle concentric ridges in the d istal parts appear to through all the topographic features without substarltially m odifying them , and maypressure ridges that developed in the underlying terrain at the foot of the u1211-5317;9" S, 123" W ]



Possible Landslide Deposit on Arsia Mons. t iummockyterrain rnakes u p rnost of this flo~cl, nd grades into th efinely ridged, concentric flow front. These features may bepressure ridges a t the f ront of the flow o r, in some places,deceleratior~ idges formed as the flow cam eto a standstill.Small lava flow fronts are visible on the smoodl plains infront of the main i lov . (49B89;3' D;, 117' W ]



Extensive Lava Flows fro m A rsia Mons. Th e florvs th at erupte d from Arsia Nlons ex

\ some 150 0 hm away from the sum mit and bury the older cratered terrain of the southemisphere. Flow fronts are visible within the large crater Pickering (120-km-diamwhere they have been diverted aroun d high groun d associated with the central peak ocrater. Flows of this ty pe associated with the big volcanoes may have lengths in exce100 0 k m, and may resemble the large flow s fou nd in Mare Imbrium on th e RiIoondiscovery of these flows on the ou ter flanks of the m ajor volcanoes on Mars has shthat the basal diameter of many of these volcanoes is considerably larger than subpectrd from nlariner9 data . [56A12;34' S, 33 ' w]



Arsia Mons Summ it. Part of the caldera is visible at the upper left of the picture. Thesumm it of the volcano is cut by lines of pits marking th e fracture zone running throughthe volcano. Most of the lava at the middle and bot tom right of the picture appears tohave originated fr om the fracture zone, and postdates the summit cone of the volcano.Awell-defined channe l/tub e system is visible toward the lower right of the pictu re; smallpits at the head of this channel system represent the vent area.[52A04; 12"S, 120"W ]

Sum mit of Alba Patera. This volcano is only a few kilometers above the surro unding plainwhich, coupled with its large diameter of some1700 km, gives it a much lower profilethan th e Tharsis volcanoes. The rim of a11 old caldera nea r the summ it, partly buried byyounger lava flows, is visible a t the bottom left ; at th e bottom right a younger caldera isat the top of the youngest summit cone. Lava flows are well preserved, and flows can beseen extending from near the lower right of the picture toward the upper left.[7B94;41" N, 09 " W ]



\ Lava-Covered Upper Flanlrs of A lba Patera. Dif fere nt kinds of flows are visible. La

Nrelatively flat-top ped flows with w ell-defined flow fro nts occur in th e middle ofha me . A t th e lower le ft are long flow--ridges, some of which ex tend fo r several hunkilometers. T he flat-topped flows are generally considered to have been fed by lava tuOne flow has a si~ luou s hannel-tube rurulirlg along the crest of the ridge. Superpimpact craters on A lba Patera are more n umerous tha n o n Olympus I\iIons and iAIons, suggestirlg an olde r age for man y of these flows.[7B24; 48' N, 115' W ]



Ridge-Like Lava F lo ~v s n Alba Patera. This part of the flanks of Alba Patera hasrid ge -lik e la va f l o ~ ~ ~ sit11 com plicated d endritic pa ttern s developecl on the m. Som e ofthese channels may be directly associated with the form ation of the lava flows, bu t somemay have resulted from fluvial modificatio n of the volcano flanks. Cutting the lava flowsin this area is a well-defined graben, within which are numerous collapse pits.[7B53;46" N, 119" W ]

Biblis Patera. This volcano, situated bet~vcenArsia VIons and Olympus Afons, is mlichsmaller than those so far described. Fl o~ v eatures on the flanks of the volcano aretruncated by the surrouncling plains, indicating partial burial by later deposits. Theexposed part of the volcano has a basal diameter of about100 km. Its original size mayhave been larger, although, from the small size of the caldera, it is unlikely-even

considering the buried ba se-that it was ever as large as th e giant Tharsis volcanoes. Thesumm it caldcra is surrounded b y almost circular fa~ llts , vhich erms characteristic ofRflartian volca noes . [44B50; 3' N, 24" W ]



Ulysses Patera. This volcano lies just to the east of Biblis Patera in the nortllwestof Tharsis. It is similar in size to Biblis Patera, is surrou nded by youn ger flows, antwo superposed craters, probably of im pact origin. These craters are older thansurrounding plains, and they have intersected the caldera walls and pushed m ateriathe floor of the caldera.(49B85;3' N, 121" W ]

IrN

I 4 0 k m I

Tharsis Tholus. This 17 0-km diameter volcano differs in ffrom th e volcanoes previously illustrated. T he caldera has a bench around one side. This bench may represent an early lake level before further collapse occurredin the middle of caldera. Scarps intersecting the caldera appear to be normal frather than graben. The base of t he volcano is covered by youmaterials so its original size cannot be determined. [225A13 " N , 92 " W ]



Tyrrhena Patera. T he flanks of this ancient, south ern hemisphere volcano have beenstrongly modified and embayed. At the summit is an irregular depression that iscontinuous with a valley, extending down the outer flanks. Concentric gaben surroundthe summit. The volcano is so degraded that there are no well-defined primary volcanicdepositional features to provide clues regarding the nature of the erupted materials.However, th e low profile of th e volcano, and the way in which outliersof the volcanoform mesa-like bodies, suggest ash flow deposits rather than lavas.1211-5730; 20" S,252" W ]



I fadr iaca Patera , This volcano's caldcra 15 much b e t t r r t l e f i n e d t h a n t h a t o f T ~ r r

,Pa t t m , bu t i t s f la ii h s arc, a t r o ~ l~ lvcgraclrd B y r ad ia l va ll rr a. T h r i o l c a ~ ~ os youngtxr h

I\

m an y o f t h e s ~ ~ r r o u n d i n praters, b u t s t il l ~n uc l i lder than the Tl iars is volcanoesir1dicatc.d 1)) t l ~ r u rnbc l i of s u p t ~ r ~ t o s e dmpac t c r a t e rs . [99A42.30° S, 70' 111

I 20 Iktn I



DEFORMATFEATURES



A THOUGH DEFORMATIONAL FEATURES are common on theMartian surface, the t ype of deformation differs from that on Earth.

Deformation of Earth's surface is control led largely by plate motion. Whereplates converge, intense folding, overthrusting, and transcurrent faultingresult, and mountain chains may form. Where plates diverge, as atmidoceanic ridges, tensional features develop, but they are commonlymasked by volcanic deposits. Apparently no plate motion occurred on Mars,and the deformational features associated with plate motion on Earth areabsent. The dominant type of deformation on Mars is normal faulting;compressional and transcurrent features, although present, are rare.

Most faults are associated with the Tharsis uplift, a 6000-ltm diameter,7-km high bulge in the Martian crust roughly centered on Tharsis. Around

the periphery of the bulge, and aligned approximately radial to it , arenumerous fractures, some of which extend as far as 4000 kin from thecenter. So extensive are the radial fractures that they are the dominantstructural fea ture of the enti re hemisphere. Fracturing seems to vary greatlyin intensity and age. In some places, such as in the Ceraunius and TantalusFossae (north of Tharsis) and the Claritas Fossae (south of Tharsis),fracturing is extremely intense; other areas are completely free of fractures.The number of craters siiperposed on the fractures is a measure of theirrelative ages and indicates a wide range of ages. Thus, fracturing associatedwith the Tharsis bulge evidently has continued for much of the planet'shistory.

Although fractures around Tharsis include the most prominent tectonicfeatures on the planet, several fracture systems seem unrelated to Tharsis.Some fractures occur around old impact basins and are generally concentricto them. Especially prominent are the Nilae Fossae around the Isidis basin,but less distinct concentric graben and scarps are visible around the Argyreand Hellas basins. Dominantly northeast-southwest and northwest-southeastlineaments are detectable throughout much of the old cratered terrain asescarpments or linear sections of crater walls. Where the old cratered terrainis eroded, as in the frett ed terrain, the erosion occurred preferentially alongthese directions.

Mare ridges are other possible examples of deformational features. Suchridges are common on the sparsely cratered plains of Lunae Planum, SyriaPlanum, Hesperia Planum, and around the site of Viking Lander 1 n ChrysePlanitia. In fact, Viking Lander 1 is believed to have landed 011 a ridge crest.Similar ridges have been studied intensively on the Moon and are consideredto be the surface expression of reverse or thrust faults, which formed eithercontemporaneously with deposition of the lunar mare rocks, or some time



aftt'r, as a rrli~llt of accommoclatlon of thtl Moon's 5111Eacc o the ladrposition. Tlle ridgcs on Alars have a strilting rc.sr~mhlancc~ o those on IlRIIoorl and are probablv of si~ nil ar rigin. Terrestr ial cui~alogs lavc not bcetlf ou nd , h o ~ v e ~r, and their origm rc~rlailzs tnccrtain.

i\llost of the crat crrd plains in the n orthe rn lat itl ~de s f Mars cxhibit apolygolial pattern of fractttres for which there is 110 terrestrial analog

Individual polygorls average approximately 10 liln across and extenduniformly in all directions. Ice wrdging and co~ ~t ra ct io n y cooli ~lg avbeen suggested as possibilities, I ~ u t 10 complrtrly satisfactory ex~>lanatiohas yet l~e en o ~ ~ i l d .



Chains of Rir l~less i t s wi th in Graber l of C erar i r~ i r~sossae . Ri~ r i l r i s r~prr i s ior i s or l lmor( o i u i r 1x1 t l lc g~abt . i i1 L / I I ~ rea. ~ I I Citr (lo 1101i w i 11 to br- rotlkcei oi tlica tx\tei~Ilveav

\ t lo\ \s viiiblc 111 th r p ic ture , l>ut ~ ts tc .adc t ~ icross f l o ~a n d W I ~ I P ractt~rr's.T l i ~iliesp t i a rcb 1hua111 loca ted \ \ i th i l l grahrn an d n ot or1 the i l l tervc ,n ir ig ~ h i r ~ s .224A13, 32" \102" n' ]



Fracture d Terrain of the Thautnasia Region. An escarpmentin tllc cente r of this picture is

N at the south extension of the end of Claritas Fossae. Tlle fractures are roughly radial tothe Tharsis bulge and cut mostlyold cratered terrain. Crater counts indicate that most of

50 km the fractures are older than the correspo~ltlir~gractures north of tlie l'harsis bulge.[[57/\04-13; 37' S, 03' F ]



Fractured Terra in North of Olympus Mons.

(a) This f rac ture i s par t of an o ld arcua

s t ruc t r~ r e ha t i s pa r tl y bu r i ed t o t he souby lavas from Olyrnpus , \Ions. (b ) Thstereogram shows the area out1inc.d in (aand gives a greater sense of the stefpness t l le walls in the fractures.[211-5528;38' N131" W ]



Grooved Terrain arou nd Olympus Mons. Grooved terrain, the tcrm applied to the

fracirtred surface in the bottom left quadrant of this picture, occurs in a discontinuousring arou nd O lympus \Ions. Th e origin of the terrain is unk now n, but i t has beensuggested that i t occurs at th e surface of vast thrust she ets caused by the loading of thecrust by Ol y~ np us \Ions. An other suggestion is that th e terrain is formed by erosion ofas11 flow tuffs tha t o~ igin ate d rom O lympu s AIons. To tlir n orthpast, young lava flowstransect the grooved terrain and an older fractured surface.[48E43-47 ; 32' PI, 32 ' W ]



Fractured Plains. Large areas of plains i11 the40' t o 50' n' atitude belt have a fractupattern similar to that show n here. Th e pattern resembles those that fo rm on som elahcs as a result of cooling. It also resembles pa ttern ed ground form ed by ice-wedginperiglacial regions. Tlie polygons 011 Mars are, however, approximately 10 0 times thsize of tlicl suggested terrestrial analogs, and their origin is unknown.[32A18; 44' N18' \\'I

Fractured Plains. Tlie fracture pattern here is coarser tl ian that slio~rnn the previophotog raph. hlost of tlie crevasses have flat floo rs. The low hills at the bott om ofpicture ma!- be erosional rem nants of old cratered terrain, the main body of which ocjust to tlie south. [35A64; 40' N, 14' D']



Nilae Fossae. The fractures seen here are co~lce ntr ic o what is probably an old impactbasin centered on Isidis Planitia. The fractures appear to be very old, as indicated by thesuperpositiori of large impact craters.1211-5657; 25' N, 282' W ]



I IPACT CRATE RS, which occur allnost everywhere on th e Jlartiansurface, are significant because the number of irnpact craters per unit areaarea gives an indicatio n of t he relative ages of diff erent parts of the surface.They also provide clues t o the p roperties of th e near-surface materials andrecord the effects of various processes, sucll as wind actio n, th at m odify th esurface. The density of impact craters varies from the heavily cratcreclsouthern hemisphere to very sparsely cratered regions l ike the polar clunefields anel la~n iilat ederrain.

Most surfaces photographed by the Viking orbiters have crater densitiesin excess of typical lunar mare surfaces. This condition suggests that most ofth e Martian surface is prob ably billions of years old. Th eimplication i s thatres~ lrfac ing , uch as by volcallic processes or wind act ion , is extrem ely slow

in a no st places comp ared w ith Ea rth. C rater densities low enough to suggestsignificantly higher rates of resurfacing are found only in the polar regions,around some volcanoes, and very locally in other areas. In the40' t o 60'

lati tude belts, particularly, sigll ificant surface modification has occurredsince the presently observed crater population form ed. Craters in these areashave apparently been modified by repeateel burial and stripping of debrislayers. The era in which this activity occurred, ancl ~vhetheror not i t i scont inuing, i s unknow n.

The Viking orbiter pictures revealed some unique characteristics ofMartian impa ct craters. The ejecta pattern arotund most fresh lilartian im pactcraters is distillctively different from that around lutlar and rnercuriancraters. On the Moon and on Rilercury, the ejecta typically have a coarse,disordered texture c lose to th e r im. Far ther ou t , the texture becomes f inerand, with increasing radial distance, grades imperceptibly in to dense fields ofsecondary crate rs a nd rays. Most Ailartian craters have a c~u ite ifferent ejec tapattern . Th e ejecta co m ~n on ly ppear to consist of several layers, with theouter edge of each marlied by a low ridge or escarpment. Features on theejecta st ~rfa ces nclude closely spaced radial striae ancl con centric grooves,ridges, and scarps, especially toward the outer margin. These unic[ue IIartianfeatures wcre seen vaguely in the R'Iariner 9 pictures and tentativelyat t r ibuted to wind act ion.

Vili ing pictures sl~owha t m any of th e peculiar cllaracteristics of Nlartiancraters are pr imary ernp lace~nent eatures not due t o wind. The f resher thecrater a ppears, the bet ter preserved are the stria e, ram parts, ancl conc entricfeatures. Very small secondary craters indicate that the crater has iundergonevery li t t le modification since i ts formation. R'lartian craters look differentfrom those on the R/loon and Mercury because the process of ejectaernplacement is different. Th e final stage of e mplacem ent of ejecta on Mars



i s tl louglrt to be an ot ~ t~ va rdnoviiig debris flow instcad of the simpl~allistic deposition tha t occurs o n the Moo11 and Mcrcurv. Why templacement process is so different on Mars is not understood; possic n c o t ~ n t t ~ r i n gater at de pth or th e melting of large amo unts of ground by the impact results in fluid, mud-like ejecta. The preserlce of atmosphere may also have an effect 011 the Inalllier in which the ejecta a

emplaced. These charactel-istics of the craters indicate that the propertiesnear-surface materials on hlars are quite different from those or1 the Moand Mercury.

Although some features of M artian craters formerly attribute d to waction are now believed to be p rimary, mo dification by wind is ssignificant in many areas. I11 t h e $0' t o 60° N la t i tude band, numeroucraters that have been termed pedestal craters occur in the center orough ly circular p latform Inany crater-diam eters across. Th e pedestals hbeen a t t r ibuted to the par tia l st r ipping of a formerly co ~i t in uo ~l sebriblanket by the wind. The general surface layer of debris has been removeverywhere except around craters where ejecta have armored the surfagainst t he wind, wit11 the result tha t mo st craters are surrounded by

platform composed of remn ants of th e debris blanket. This featureespecially striking where fields of secondary craters occur, and resultsarrays of low hills with cen tral craters tha t sim ulate a field of volcanic conIn other areas, repeated burial and stripping have led to bizarre formatiosuch as ejecta surfaces at a lower elevation tha n t he surrounding terrain.



Ram part Crater in Chryse Planitia. The outer edge of the inner ejecta layer of Belz Crateris demarc ated by a low ridge or rampa rt. On th e surface of the ejecta layer are faint radialstriations and co ncentric ridges and grooves. Outside the ramp art, the topograph y issimilar t o the distal parts of lunar and mercuriarl ejecta , with numerou s isolatedhumm oclts and indistinct radial ridges. [10A 54, 5 6; 22"N, 43 " W ]

Layered Ejecta aroun d the Crater Tarsus. Each ejecta layer seen here has an outer ridge orescarpment. Th e upp er layer appears to have flowed over and transected the o uter m argillof th e lower layers. At the arrow, ejecta have flowed around a low obstacle. [10 A6 6,6 8,70, 92-98; 23" N, 40" W ]



Yuty Crater. This cra ter lras several ejecta layers, each comple xly lobed and each witha

N outer rampart. Although buried by 17uty ejecta, a pre-l 'uty crater closeto the rim clearly visible, indicating th at th e ejecta de posit is thin.[3A07; 22' N , 34' W ]



Arar idas Cra ter. The outer layer o f i :jt :cta has I ' lo~+retl ver the s tt rr ot ~n rl i~ ~grach~rcc lplait ls . The two arrows indicate~ v h e r e n inner c jcc ta layer has f lowed a round pre-exis t ingcraters. Numt:rous low ridges occur on thr: inner ejecta laycr closr. to a nd parallcl to i tso l ~ t c r a rg i n . [32A20-31,91\42: 43O N, 14 " W ]

10 krn 1



A Detail of Arandas Crate r Ejecta. T he fractures of th e underlying plains at AranU location arc clear11 visible through th e ejecta, even close t o th e rim, showing th at

rject a are ver) tlrin. On the ejecta surface isa fine radial patt ern. [32 A2 8; 43ON , 14' W



Poolla Crater. Tllis crat(lr is closcb to 1List.i Vallis, t l ~ v dg e o f ~ 1 1 1 1 ~ 1 1 3 ~ n a r l ~ e t l ) anN

escarpment in the northwt.st corner o f the image. The ejecta have. a niarked radial patter11

and n o outer rampart. 122A.54; 24' N, 5 2 OW ]



Pedestal Craters. Alm ost all the c raters in this area are situated within a pedestaplatform that stand s above the surrounrling plains. The diameter of the platfdecreases ir ~ize as the crater diameter decreases, so tha t small craters may occur ato pdomes. Th e mode of form ation for the pedestal craters is poorly understood. observed configuration may be partly primary and partly the result of selective stripof a forme r layer of debris that covered th e surface, with the laycr now remaining aroun d craters. [4+3A 04; 6' N, 353' W ]

Pedestal Craters. Th e ejecta deposit arou nd th e crater in the left half of the frameretains its multilayered character anrl faint radial surface texture. Other craters well-developecl pedestals, b ut primary texture s are less well preserved.A cluster of crata t the to p of the f rame s t ronglyresembles a group of volcarlic cones bu t is probably pof the impact cra ter cont inuum. [60A 53:48' N, 349' W ]



Craters ~ v i t h rregular Ejecta Blanket. Som e craters in this picture have well-de-veloprd prclcstals; others are snnoundccl by bright ejecta hut no\veil-defined outcr

1U

escarpment. T he large crater in the bo tto m half of the frame has a highly irregularejrcta pattern with elongate lobes extending out over the su~~oundingvacturetlp l a i ~ ~ s .lOR52 ; 45' U, 59' W ]



Hamelin Crater. Hamelin Crater, only 9 0 km northea st of th e original landing site selectedfor Viking Lander 1 in Chryse Planitia, shows the raised edge or rampart around theejecta blanket that is characteristic of many cratersin the area . [77/08/10/180701;20" N, 33" W ]



FEATURES



Mixed-Tone St reaks in Memnonia . These d is t inc t ive mixed - tone s t reaks appear to consis tof a centr al, tapere d light strea k bordercrl by t w o dark side-lobes. Similar streaks havebeen produ ced in wind t tl ri riel s imula tions . If t l ie wind t u ~ ~ n e lesults are valid, th e clarklobes represe i i t a reas of w i~ id :ros io ii , and the br ight cent ra l por t ion is a r e ~ i o n fdcposiitiori wh ere br ight finr: particles acc umu late . iVutnerous ragged dark strea ks are alsovisible, atid ar e in terp rete d as zoricas of erosion associated with topogr aphic obstacles-in

this case in clu di~ ig rater rampa rts, 1:jecta blank ets, and possible lava flow fronts. [4 1B 51 ;13" S, 3 9" h' ]

Mesogaea Area. Only subtle albed o changes have occurre d in this comp lex mixedstreak sirtce t l ie Rlariner 9 coverage. The com plex str eak se ems to be the resudef la tion of low albedo mater ia l f rom the cra ter a t upper r ight . The numerous bs t reaks outs ide the large mixed- tone s t reak are in terpre ted as accumulat ions of d us t sfallout. C,ratt,rs a nd isolated hills seem t o prod uce similar bright streaks. Bright wi th in tht , main dark s t reaks co ~l ld e e i ther drposi ts of dus t s torm fa llout or shazones b t ,hind topograph ic obs tac les whcre dark mater ia l f rom the upwind cra ter wacleposited. Thu s, the co nspiclions bright, hil l-associated streak within t he m ain clark scould be a normal bright streak ora "shadow" s t reak. [88A81 -88;8" N, 192" W ]



Frost Streaks in th e Annual S outh Polar Cap. These bright, streamlined albedo featuresare associated ~vitll raters near the retreating margin of the annual frost cap. The featuresbecoinc more prominent (relative to the background) as the cap edge approaches theirlocation, and disappear shortly after the margin passes. Tlle bright streaks are interpreted

as accum ulations of carbon d ioxide frost in the lee of craters, and suggest tha t winds maybe effective in redistributing frost in th e polar regions of Mars. The wind directionindicated by the streaks suggests that the streaks are laid dolvn during southern winter.[161B26;61" S, 71" W ]

Contrast Reversal in the Cerberus Region. These frames sh o~ v he contrast reversacrater-and-hill associated strealts. Through a red filter (left, about 0.58 pm), the strhave a "normal" appearance and are brighter tha n tl~ eir urround ings; through a vfilter (right, abou t 0 .45 pm ), they appear darlter. Laboratory measurements indicatesuch contrast reversal is a common property of well-sorted, very fine samples of oxide materials. h o t all bright streaks on Mars show such contr ast rrversal in the vi[Left 53B56, Right 53B57; 12'N, 202" 1'1



b Aeolian Activity on Pavonis Mons. Persistent aeolian activity was observed on the flanks

N of the three large Tharsis volcanoes. Early Viking imaging revealed that since the lastMariner 9 co verage in 1 97 2, each of tlle three large Tharsis volcanoes (Ascraeus, Pavonis,

N

A

and Arsia), had developed a more or less complete dark albedo ring on its flanks. The1 40 km Idark albedo ring was especially well-developed on Pavonis Mons, as seen here, where 1 lo o m Ii t was 20 km wide and s i tuated a t a l t itudes between 2 0 and 2 5 km.

(a) The boundaries of the dark albedo ring are ragged, and the upper boundarj iscompo sed partly of coalescing, ragged d ark streaks trending downhill . (b) A nother viewshows the same area taken after tl le 1 97 7 global dust storm. The observed changes arebest explained by the erosion by do~vnslopewinds of brigllt albedo material-probablystorm fallout. Aeolian activity has been observed up to the summits of the Tharsisvolcanoes, proving that Martian winds are strong enough to transport fine particles evenat th e very low pressures a t the to ps of th e Tharsis volcanoes. [L eft 52A 15, Right416A4 5; 0' h, 13" N ]



Spectacular Albedo Changes in Syria Planum. These four views of Syria Plarium show

both long-term and short-term changes in the surface markings. (a)A M a r i n 9 view ofthe area in southern summer is shown. (b)A Viking Orbiter 1 view of the same areaalmost three RIIartiari years later was taken shortly after the start of the global-scalestorms. (c) and (d) These were taken in m id to late southern summ er. Th e changesobserved in the bright streaks in the last three frames are attributed to strong~iortli-to-southwinds during the global dust storms. [(a) Mariner9 DAS 08585544,(b )294A69, (c) 416A49,(d ) 439A48; 12" S, 110" W ]



Changes in Wind Streaks on the Slopes of Arsia Mons. The changes in both dark and lightstreaks shown occurred after the global dust storms. In the lower image, winds blowingdown the long slopes of t he volcano redistributed some of t he light coating of dust de-posited du ring the global dust storms, fonning both dark streaks (erosion of dust) andbright streaks (deposition of additional dust). [574A46,648A03;9 S, 124' W ]



Erosion of Dust from Large Areas FoUo~ ving he GlobalDust Storms. (a) Shortly afterthe global du st storms, wind erosion downwind of craters produced dark streaks; acombination of global circulation a nd local winds blowing down slopes forme d streakspointing in tw o directions from the same craters. (b) Later winds, also blowingdo.rvnslope, stripped dust from much wider areas to form the large dark markings.[603A08 ,639A67; 31" S ,117" nil



MARTMOONS



T WO MOONS orb it Mars: Phobos (mean diameter, 22.0 km) and Deimos(mean diameter, 14.0 km). As part of the centennial celebration

commemorating Asaph Hall's discovery of Phobos and Deimos in 1877, anextensive exploration of the two Martian moons was conducted with theViking orbiters. The spectacular high-resolution imaging data obtained haverivaled in resolution any previous flyby or orbiter imaging data 011 any bodyin our solar system.

These data provided much more knowledge of the moons' surfacemorphology and their physical and dynamical properties. Phobos wasobserved to be somewhat smaller than determined by Mariner 9 (2.5200 k m3rather than 5700 k m3 ), and Deimos was somewhat larger (1.1200 km 3rather than 100 0 km3) . Both satellites are locked into a stable, synchronous

rotation about Mars, with their longest axes pointing toward Mars and theirshortest axes normal to their orbi t planes (which are within a few degrees ofMars' equator). Both satellites have topographic variations as large as 20percent of local mean radii, and Deimos has a few large fla t areas.

Viking found Phobos and Deimos to be within 1 0 to 1 5 1tm of theirpredicted positions based on Mariner 9 images. Precessing ellipses accuratelymodel the orbits of t he two moons, with short-period Mars gravityperturbations having displacement amplitudes of less than a few kilometerson Phobos' orbi t, and solar perturbations having displacement amplitudes ofless than 5 km for Deimos (except for one 110 -km , 54-year periodic.longitude perturbation) .

Phobos, one of the three satellites in our solar system whose period(711391n) is less than th e rotational period of the primary planet (24h 37m forMars), is losing orbita l energy to surface tides it raises on Mars. As the orbitof Phobos decays and gets closer to Mars, Phobos may eventually be tornapar t when t he t idal forces of Mars overcome the cohesive bond between itsparticles. Phobos, already inside the "Roche Limit" where internal gravityalone is t oo weak to hold i t together, could collceivably become a ring planeabout Mars within the next 50 million years.

Phobos and Deimos are both uniformly gray. Albedos of-0.06 putboth in a class with the darkest objects in our solar system. These darksurfaces appear t o be layers of regolith with depths of a few hundred metersfor Phobos and at least 5 to 10 meters for Deimos. Cratering of the surface

of Phobos continued during and after the formation of the regolith, and theregolith is saturated with craters. However, 011 Deimos it appears that theregolith continued to develop after the cratering subsided, and the smallercraters (< lo 0 meters) are partially filled or covered. This obscuration of thesmaller craters gives Deimos a much smoother appearance than Phobos when



vic~ved at ranges of more than a few 1111ndrrrl kilometers, because t l ~ t illecraters arc near or below the resolvirlg power of th e cameras and tllercforcarc not visible.

111 cont rast t o the smootl l appearance of Deimos, the surface of Phobosis dominated by sharp, fresh-looking craters of all sizes and a vast network oflinear features resembling crater chains. These linear grooves, up to tens of

kilometers long and hundreds of meters across, appear to be surface fracturesassociated wit11 the formation of Stickney, the largest crater o n Phobos.Crater densities on both satellites are comparable to densities on the lunaruplands, a fact that suggests ages of up to a few billion years. However,impact fluxes may have been significantly higher for Phobos and Deilnosbecause of ejecta being tl~rown nto orbit about Mars and then recollected asth e satellites swept it u p it1 their orbits.

Similar networks of striations have not been identified on Deimos;however, they may have been covered by regolith, and picture resol~ttionmay not have been s~~fficient o identify such features. For example, a largedepression 1 0 km across at the south pole of Deimos may have been causedby a single impact or may have been the result of fragmentation if Deimoswas once part of a larger body. Linear features radiating from the center ofthis depression are s~~ggested y the data, but low picture resoltztion haslimited any interpretation of these features or determination of the origin ofthe large depression.

The close encomlters with Phobos and Deimos have yielded preliminarymass determinations of approximately 1 X 1016 and 2 X 1015 kg, respectively.Using the volumes mentioned earlier, mean densities of about 1900 kg/m3 foPhobos and 1400 kg/m3 for Deimos are obtained. These low densities, aswell as their colors and albedos, make Phobos and Deimos compositionallysimilar to Type-1 carbonaceous chondrites found in the asteroid belt. Thesedata strongly suggest caphtre as the origin of the two asteroid-like moons of

Mars.Viking also obtained pictures of Phobos and Deimos, or their shadows,against Mars. The transit pictures were used in refining knowledge of theshapes of the satellites, and the shadow pictures helped locate Viking Lander1. The satellite and shadow images were used to improve map coordinates offeatures on Mars surrounding the images.

D

Phobos Closeup. The photom osaic on t op was taken a t a range of 30 0 Itm as VikOrbiter 1 was approaching Phob os. The areas covered by three pictures taken a t a raof 11 0 to 13 0 km are outlined on the photomosaic. The upper right area of

photomosaic sh o~ vs region dominated by grooves. The grooves are probably fracturthe surface of Phobos from a large impact. Two large craters with dark material on floors are seen near the bottom of the photomosaic. These flat-bottomed craters evidence that Phobos is covered by a regolith of up to a few hundred meters thick. three pictures shorv the heavily cratered surface; crateis as small as 10 to 15 metersvisible. [To p 211-5356 , Left 244A03, Center 244A04, Right 244A061



Phobos from 480 Kilometers. Viking Orbitewithin 48 0 km of M ars' inner satellite, Pobtain the pictures in this mosaic of the astmo on. As seen here, P hobos is nearly75% lluand is about 2 1 km across and 19 km frobottom. Some features as small as 20 metecan be seen. Surface features include grosembling linear chains of craters and small h.tvhich appear to be resting on the surfregolith-covered surface is saturated withHummocks, mostly seen near the terminatoare about 50 meters in size and may be surfafrom impacts. [211-53531



High Resolution View of Grooves on PThis picture shows a northern arva011 Pwhich is dominated by grooves. A n arethr terminator (7 5 X9.0 km) is s w nvisible features as small a5 20 meters. Crall sizes abound, with a significant formed later than the grooves. The groo

iate from the antipodal point of Sticltnare probably surface fractures caused impact that formed this large crater. Poutgassing of volatiles during formatiohave caused the raised rims along the fby ejecting regolith. [246A06]

Stereoscopic Views of Phobos. The upper pair shows the side facing away from M ars at arange of 50 0 km from t he orbiter. The large craters near the limb are about4 km acrossand a few hundred meters deep. The lower pair shows the side facing Mars at a range of300 km. The grooves are radiating from Stickney and are tens of kilometers long, hu n-dreds of meters wide, and can be tens of meters deep. [Upper left 246A76, Upper right246A66, Lower left 343-408, Lower right 343A251



Phobos O verflying Ascraeus Mons. This spectacular pic ture , taken byVikiilg Orbiter 2, is the first picture ever taken showing such detail onboth a satellite and primary planet. Viking Orbiter 2 was about13 000

krn above the surface of Mars and about8000 km above Phobos, ~vhic hincreases the apparent size of Phobos relative to features on R'Iars.Phobos is about 22 km across, and Ascraeus Mons is over300 km acrossat its base. The complete outline of Phobos is seen from direct andreflected sunlight. Transit pictures such as this are used to determinethe size and shape of the satellite as well as improve the mapcoordinates of features on Mars registered near the satellite's image. Aunique tie between Mars surface (map) coordinates and inertial spacecan be made when the inertial positions of the satellite and spacecraftare known accurately. [304B88]

Phobos Overflying the Mouth of Ares Vallis. These mosaics of pictures fro m Viking Orbiter1 show Phobospassing beneath th e spacec raft with th e surface of Mars in the background. These mosaics, take n about aminute apart, show an apparent motion of Phobos across the surface of Mars of abou t50 km. Orbiter 1 wa s13 700 km above the Margaritifier Sinus region of Rilars, and Phob os was6700 km from the spacecraft.Phobos, f our tim es darke r than Mars, appears black against Mars in these unenha nced pictures. This regionof Mars contains chaotic terrain along the equa tor; it is near the head of Ares Vallis, a major channel leadingto Chryse basin ~vh ere iking Lander1 is located. [451A03-10; o N, 19q W ]



Phobos Shadow Transit over the Viking Lander 1 Site. Thepassage of the Phobos shadow over Viking Lander1 was imagedsimultaneously from Viking Orbiter1 and Viking Lander 1. T he

time of shadow passage, as observed by the lander, was used tolocate tlle position of the shadow (and therefore the position ofthe lander) in the orbiter pictures. This picture shows tlie shadowof Phobos (-60 X 120 km across) in the Chryse Planitia region afew kilometers directly no rth of Viking Lander 1. T o the left ofbottom center is Maumee Vallis, approximately 420 km south-west of the lander's location.[463A21]

Deimos from Near and Far. A two-picture photomosaic sho~vinghe co mp lete side of Deirnos visible fromViking O rbiter 2 is on the left, and a high reso lutio~l hree-picture mosaic of a small area near the termina-tor is on the right. The two-picture photomosaic, taken at 50 0 km , shows a smootll surface with limitedcratering and a few large flat areas. No linear grooves are seen; howev er, bright patches of rnaterial nearthe intersection o f th e large flat areas are visible. The three-picture pho ton~ osa ic aken at ab out 50 krn givesa co mpletely different view of Deimos than does the two-picture (lower resolution) photornosaic.A surfacesaturate d with craters and strewn wit11 boulders is revealed by the factor-o f-10 increase in resolutio n.Craters have been partially filled or covered by rego lith, which gives a smooth appearance to the surfaceat lower resolution (a range of 500 km or more).A "wind streaking" e ffect from upper right to lower leftp robab ly r e sul ted f rom a base su rge p l~ e~ lo rn en o ~~hen ejecta rnaterial was trarlsported and depositeddowntrack by the impact of an incoming meteoroid.A few dark-rimmed craters are seen. [Left 42 8B10-11 ,Right 423B61-631



Deirnos from 30 ICilometers. Dcimos was observer1 on October 15 , 197 7 when VOrb iter 2 passed w ithin 3 0 krn of the surface. This is one of the highest resolpictures ever taken of any body in our solar system by an o rbiting or flyby spaceTh e plchlrc covers an area of 1 .2X 1.5 km, and shows features as small as3 meteT'iking Orb iter 2 would liavr been visible from the surface of Deirnos duringc\ceptionally close flyby. The iurface of Deimos is saturated with craters.A layer of dapptxars to co ver cr aters smaller thanSO meters, making Deimos look smoother t

Pllol-tos. Roilld rrs as large as hou ses (10 to 30 meters across) are strewn over snrtacr-probably blocks q ecte d from nearby craters. Long shadow s are seer1 cathese ho ~rld ers stlnlight is coming fromth e left). The image was taken when the was on11 10' ahov e the ho nzo n. [423 B03]



Phobos and Deimos-Similar bu t No t Identical. In the upper images, the surfaces ofPhobos and Deimos are compared at a range of 14 00 km. Features as small as 10 0 metersare d etectable. Ph obos is viewed at a 90' phase angle and Deim os at a 60' phase angle.Grooves and craters dominate the uniformly dark surface of Phobos at this resolution.Deimos, however, appears to be very smooth, w ith few craters and.~.vith reas of brightalbedo. The grooves on Phobos radiate from the large crater Stickney (10 km across)at the left. The bright patches on Deimos are near the intersection of large flat areas.Higher resolution imaging in th e bott om images dispels th e initial impression of a smo othsurface for Deimos, by showing a surface saturated with craters that have been obscuredby regolith. [Upper left 357A 64, Upper right 413B 83, Lower left 244A05, Lower right423A611



Phobos and Deimos in Color. Color pictures of the two Martian moons have confirmedEarth-based spectra by also showing both satellites t o be gray. The Viking imaging datashowed the surfaces to be uniformly gray over the complete surface to a resolution of afew hundred meters. No significant color differences were seen on either surface,including areas around craters and those within the bright albedo features on Deimos. Thecolor indicates composition is of a carbonaceous chondritic material. Phobos (a) is at arange of 4200 km, and Deimos (b) is at a range of 2100 km. In these pictures, colordifferences have been exaggerated. Most of the color differences are due to noise or areartifacts of the processing, especially around craters and the limb. [Left 357A03-07,Right 355B01-091



SURFACEPROCESSES



T HE MARTIAN SURFACE has been subjected to a wide variety ofprocesses, collectively termed gradation, throughout its geological

history. The net ef fect of gradation is to bring planetary surfaces to acommon level by eroding topographically high areas and filling in low areasby deposition. Tlz~ls, radation involves the weathering, erosion, transporta-tion, and deposition of surface materials by wind, water (frozen or liquid),and gravity.

Even before spacecraft were sent t o Mars, telescopic observations showedthat dust storms are common, and it was speculated that these storms couldalter the surface. When hlariner 9 arrived at Mars, a major dust storm hadobscured the surface of the planet. After the dust storm cleared, theMariner cameras revealed a wide variety of landforms related to aeolian

(wind-related) processes, including dune fields, yardangs, and shifting albedopatterns consisting of light and dark streaks. The Viking orbiters and landershave provided much additional information on both aeolian processes andlandforms.

In the t en t~ ou s tmosphere of Mars, much stronger winds than those onEarth are required t o pick up particles and set them into motion. Winds ofsome 150 kph are estimated as minimum for initiation of particle movement.Viking orbiter pictures show several areas in which storms seem to originate;these areas include Daedalia, Hellas, and Syrtis Major, svhich also displaynumerous "streaks" associated with craters. Streaks appear t o be zones inwhich fine-grained particles are redistributed in response to wind patternsgenerated around craters and other landforms.

Some areas on Mars appear to be zones of deposition for windblownparticles, as evidenced by enormous dune fields. These areas include thenorth polar region, the floor of the large impact basin, Hellas, and the floorsof other smaller impact craters. The most spectacular of the dune fields,those at the n ort h pole, are discussed in the section Polar Regions.

Wind-eroded features include yardangs and grooves etched in someplains. Because the atmosphere is very thin the wind speeds needed to moveparticles are much higher on hlars than on Earth, so that the grains travelmuch faster once set into motion. Consequently, .rvhen they strike otherparticles and bedrock surfaces, they have a greater erosion capability thanthey would have on Earth.

Mass wasting is the dow~lslope movement of materials, primarily causedby gravity, and is seen as landslides, avalanches, and soil creep. Itseffectiveness is controlled by factors like cohesion of the material, steepnessof slope, gravity, and the presence of lubricants w~ch s liquids and volatiles.Mariner 9 and Viking pictures show many features that can be attributed to



mass ~vastin g.Alass wasting along the walls ofV alles IIa ri~r eris as proclucedsom e of tllc srlost spectactrlar lalldslicles observed an y~ vh er e.

Surface and near-surface processes that occur in the vicinity of forllislid existing ice regions are referred to as periglacial processes. Althougperiglacial features and related p heno men a have no t been positiveidentified on Mars, i t is reasonable to expect them in view of the lo

temperatures and the probable existence of subsurface ice in some region"Etch " pits, polygonal groun d, and rock "glaciers" are alllong the featurobserved from o rbi t tha t rnay be related to periglacial processes on Mars.



Details of Valles Marineris Sand Dunes. An e nlargem ent of the dune field in the precedpic h~ re s presented here t o S ~ I O \ V nrlividual dunes about 500 meters across. The windppears t o l~ av e een blowing from the west and leading dunes to t he east appear to clthe canyon nall. lP16950; 7 S, 5' W ]

1,andslide in Nortis Labyrinthns. Tliis landslide mass completely fills the floor of canyon. The canyons in this area appear to hegraben that resulted from crustal extensiowith subsecluent .rvidening and modification hy iandslides.[46A 19-22 ; 0' S, 96' W ]



Small Dune Field in Kaiser Cra ter. Cra ters and o th er topographic c l tpress ions are ~ la t ura lt raps for windblowl i sediments .Tlie crater show11 here is typical of many that have beellpho to g raph ed f r o m o r l i t . [94A42; 46" S, 39" \ I T ]



Part of the Dune Field in the North Polar Region. T he du ne field covers an area of a t le3500 krn2 and is com posed of barchan (crescent-shaped) dunes. In the area shown hethe dunes are aligned in ridges tha t appe ar to be transverse t o tlie prevailing wind. Frthe relation of the dune field to the crater at the bo ttom of th e picture, the prevailwinds seem to be f rom th e,~ ves tleft side of picture). [59B65 ; 76" N, 88' W ]

Barehan Dunes at Edge of North PolarCap. This figure sl io~v s he well-defined lihof individual barcha n d unes. Th e wind directio n is from left to right.[58B22;75' N53" W ]



"Etched" Terrain in Southern Chryse Planitia. This etched terrain shows light-toned,angular depressions in southern Chryse Planitia in the area where Tiu Vallis empties into

Athe C h~ ys e asin. T he etching process th at removed the dark plains material may be the

N

result of cavitation or plucking during active channel form ation or wind deflation. Manysmall, volcano-like features occur in this region. The arrow points to one of thesefeatures, a low m ound with a su mm it crater. This feature (also discussed in theVolcanoessection) lies on a sinu ous line of unkno wn origin; th e line may be the trace of a fractureor possibly a dike. [2 11 49 90 ; 19' N, 35' W ]



/r Northern Con tact of C hryse Planitia. Chryse Planitia "plateau," the mottled light surfat the botto m, is show n at its contact with the darker plains. Irregular pits on the plat

N (lower right) suggest forma tion by co llapse; th e scalloped scarp of the plateau seemsresult from scarp retreat and the connection of the irregular pits. The morphology of pits arld scarp resembles thermokarst features on Earth that result from the meltingground ice and the s ubsr cpe nt settling of the ground.[211-4994; 23' N, 6' W ]



Concentric Flow F eatures at th e F oot of Olympus Mons Scarp. These flow features aremore like those typically developecl on avalanchras and landslides. The un it on which the)occur is probably material for me d by landsliding on the scarp fro nt. This procrs s may

\k

have played a major part in developing tht. scarp arol ~rld he volcano.[48BO4: 2 3 O 1,138" Ili1



Mosaic of the Nilosyrtis Region. This is a transitional zone between an ancient cratere

h terrain to the south (bottom ) and sparsely cratered terrain to the north.In many of thelow-lying areas th ere are sub-parallel ridges and grooves that suggest creep of near-surfamaterials. They resemble terrestrial features where near-surface materials flowen rnasse

1 ery slorrly, aided by the freeze and thaw of interstitial ice-rrater frozen betrveen layeof grou nd materials. This is additiona l evidence suggesting the presence of ground ice the near-surface materials of Mars.[P-18086; 34' N , 290' W ]



Flow Structures in Ancient Cratered Terrain East of Hellas. Mass-wasting struc ture saround posi t ive features extend up to 20 km from the source. The aprons are notcomposed of discrete lobate flows, as ~v oul dbe expected if they were formed bylandslides, nor are they talus deposits close to tlie angle of repose; surface slopes areprobably less than 10". Instead, these fe ature s may be t he result of slow creep of debriscontaining interstitial ice. [97A62;41" S, 257" W ]

Chaotic Terrain North of Elysium. The plains of the so uth(lower half of this mosaic) appear to have partly collapsedand then eroded so that only isolated remnants remain.

Collapse may have occurred as a result of removal ofsubsurface ice. A process of planation app ears to haveremoved materials down to a specific depth and created anew planar surface a t tha t depth . It is unclear what theerosive mechanism was or where the material went.[211-5274; 33" N, 213" W ]



Contrasting Terrain l e s t of Deutemnilus Mensae. (a) Th c smooth areas shorvn mayN either clsl~rismantles or remnants of older terrain. I n the textured areas, the line

markings niay mark the positiori of former escarpments-the outline of smooth are(52A31-44;44" K, 52 " D i ]



Contrasting Terrain West of Deuteronilus Mensae. (b) h view is shown of part of theCyctonia region of hlars, a 65-km -long rem nan t of t he same plateau unitshown in (a).1 2 6 4 7 2 :45" N, '7" W ]



Striped Groun d. (a) G eometric m arkings resembling conto ur plowing in the Cydonia regionare seen, and consist of low ridges and valleys abo ut1 km from crest to crest . The-features may mark successive positions of the retreat of an escarpme nt during removal ofa plateau or m antling unit.(b ) In this high resolution image of striped ground similar tothat in (a), the parallel markings are caused by low ridges and, less commonly, shallowdepressions. [(a) P-17599;4 6 ON, 350 ' W , ( b) l l B 0 1 ; 5 0 °N , 289' W ]



Highly Textured Eroded Surface. The upp er half of this image shows a layer of relative

'

crodable material tha t is being sculpted and swept away by the wi nd 111 the lower lefNmore rrsistant older surface has been exposed which is dominated by small hills asinuous, narrow ridges. The hill at the bo ttom may be of volcanic origin. The narroridges are especially puzzling. It has been suggested that they maybe dikes but theiextensive continuity and ridge-like surface forms argue against this.An alternative, buweaker, hypothesis is tha t they may he eskers.[724A22; 2 S, 10' W ]



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POLAR



T E APPEARANCE of the polar regions contras ts sharply with the restof the planet, partly because of varying arlzo~ults of frost cover andpartly because of some highly distinctive terrain not found elsewhere. Bothpoles have a cap of f rozen carbon dioxide tha t advances and recedes with theseasons. In the north a small permanent residual cap left in midsummer iscomposed of water ice. The composition of the small residual cap left at thesouth pole is no t known. The residual l~ or thern ap is substantially largerthan the residual southern cap, so much so that the unique polar terrains ofthe rlortlz are rarely seen without some frost cover. The polar scenes are allfrom Viking Orbiter 2, which was placed in a high-inclination orbitspecifically to view the poles. Because its periapsis was in the high northernlatitudes, the highest resolution photographs are of t he no rth.

The most distinctive geologic features of the polar regions are thick,layered deposits that cover much of the surface poleward from 80'. Thelayering is best seen where the frost has been preferentially removed such ason terraces and on walls of valleys within the deposits. The layers, whichrange in thickness from several tens of meters down t o the resolution limit ofthe available photography , can be traced laterally for considerable distances.Unconformities occur but are relatively rare. In the nor th, the layers rest onsparsely cratered plains; in the south they rest on old cratered terrain. Thelayered terrain is almost completely devoid of impact craters. Eitherresurfacing by erosion or deposition is at a rate that is high compared withthe impact ra te, or the impact craters "heal" relatively quickly by flow orinfilling.

The layered deposits are believed to be accumulations of volatiles andwind-blown debris, with the layering caused by variations in the proport ionand absolute amounts of these two components. If thi s interpretation is true,then the layered deposits preserve a partial record of the history ofatmospheric activity, and hence climate, in the recent geologic past.

A vast belt of dunes, several hundred kilometers across, surrounds thelayered terrain in the north. In some areas, the dunes form a nearlycontinuous sheet that almost completely masks the underlying topography.In other areas, particularly around large topographic features, the sheet isdiscontinuous and breaks up in to strings of cresceiltic dunes or isolatedforms. Dune fields of comparable continuity do not occur around the southpole, although numerous dark splotches on the surface in the high southernlatitudes are probably local dune fields.



Remnant North Polar Cap Detail. This high-resolution, closeup view was made bycombining three black-and-white images obtained through color filters. Above center inthe picture i s a giant cliff about 500 meters high. Layers averaging 50 meters in thicknessare seen in the cliff face and surrounding areas, which are highlighted by occasional whitepatches of frost. The regularity of the layering suggests tha t it comes from periodicchanges in th e orbi t of Mars-a relationship that, on Earth, may be at least partiallyresponsible for ice ages. These orbit changes may affect the frequency and intensity ofglobal dust storms, in turn varying the amount of material available to form layeredterrain. The cliff is apparently an erosional feature; the variety of scarps shows thecomplexity of erosion in the polar regions. Dune-like features (dark areas with a rippledtexture), possibly formed from material eroded from the layered terrain, can be seen atthe center and a t the right of the picture. Just above the scarp, the polar ice layer is verythin and patchy; in other places it appears to be considerably thicker. The maximumthickness of the polar cap has not been determined. [75B52, 75B56, 75B58 (P-1M59);84+' N, 237" W ]



A D une Field in Borealis G hasmn Dark dun e-form ing rnatt:rials appear to have htransported away from the pole in a curving stream extencling from the top of this fraThey are accumulated in an approximately tria~lgula r une mass that occupies tlie ceof the mos aic, The sinuous ridgesin the clunr: rnass rot atc ir t a clockwise dirc:ctiort tllrougan angle of approximately 45O frorn the northern to the southern margin. Tdiscontinuous dark texture on the right side arises from partial dune cover. Perennial is visible near th e top of the frame and associated with the crater near thebottom of frame. The bright patch near cellter right may bea cloud. [58B21-34; 48O N, 52' W



Sand Dunes a t the Rim of the North Polar Cap. Th e dunes form a sharp-edged, dark bandnear the bottom of this image. Martian sand is dark, unlike Earth sands which ar e usuallylight colored. This shows th e minerals in Martian rocks most resistant to erosion are th edark ones. The center of th e image shows a flat desert region. At th e upper right are aregion of mottled terrain of unknown origin, a strip of layered terrain (its layering clearlyvisible in this view), and a pinkish-white region of polar frost. [IPL, ID:I2398AX;81 ° N,83 " W ]



Widespread Nod11 Polar Edge Sand Du ne Fields. (a) Dunes here have a co ~lsi strrll rend(approximately north-soutli) with minor sinuosity, branching and inergkg. V a p e circularforms are probably buried craters, and bright spots within the ridges are ice deposits. (b)Dunes wit11 much more va riation in direction also occu r; a shorter wavelengtll and greatersinuosity appear in this du ne field which adjoins-and in places appears to be mantledby-frost deposits. Vague circular forrns again are probably buried craters. The brightpatches of ice near the upper left are associated with a distinct change in the dunepattern, possibly indicating that the deposits of ice preceded the development of the ppresent du ne pattern. (c) T ransition from a transverse ridge structure to isolated linearand equant dunes. [(a) 59832; 81' N, 141° W , (b ) 58801; 80" N , 120 O W, (c) 58828;

km ,8' N, 50' W ]



Phot omoa ic of South Pole. 4 t tllr l t f t of this photomosaic is the rrmtlallt south polarcap of Ala~r. nt i l ~ecmtly, vidt,nccx suggc,strcl tliat iti composition $3 as ~vdtc~r ccl l ih r th

remnant north polar cap. Nc~v rmp rrat ure measurements, ho~vevcr, uggest tliat it may~ I L arl~oll dlo11dr tcib. Exit>ndlng rom Lt.nrat11 thc polar cap t o t l ~ e ottom 01 th e trarn

art: large, lobate espanses of glacioaeolian deposits with wind-sconrcd surfaces. A t the

north ern margins, these deposits overlap and partially fill a number of crate rs. They also

rnarltle the entire southern ~vall of a huge impact basin, 800 km in diameter, which is

approximately at th e center of th e photomosaic. The unburied p art of th e basin rim, or.rampart, forms a mountainous, semicircular arc, with plains in the interior and a rugged

landscape of large craters stretching to the nor th. At t he smallest scales, the polar terrains

exhibit mysterious patterns and te xtures which can possibly be attribut ed to volcanic and

wind action, arid to cyclical climate change. [383B0 4-75, 211-5541 ]



Layered Materials Resting Unconformably on Cratered Terrain near the South Pole. \

Layered material with a smooth, uncratered surface partly covers a 40-km diameter craterN

in the upper half of the picture. Strings of secondary craters around the larger crater arealso transected by the layered deposits. [383B50; 81' S, 71' W ]



Secondary Craters in Layered Materials Close t o th e Sou th Pole. Layered deposits1 slro\vn in th e lower half of the picture bu t have been eroded away in the upp er halN

form a low scarp to the north which is il luminated by the Sun. Numerous secondcraters occur in the layered deposits aroun d a partly e roded crater. The relations sug

1 20 km 1 that the crater formed after the layered deposits but before the erosional episode formed the north-facing scarp. Part of the remnant cap is visible in the lower l[421B79;85" S, 352" W 1.

Sinuous Ridges on the South Polar Plains. The origin of these ridges is unclear. Tbranch and rejoin like river channels, and somewhat resemble terrestrial eskers (rid/sforme d by deposits fr om subglacial rivers), but a volcanic or tect onic origin is mo re lihere. Similar features occur elsewhere on the planet, such as on the floor of Arg120

kill1

[421B33; 78" S, 40"W ]



Pitted Terrain near the South Pole. Some a reas peripheral t o th e layered deposits at thesouth pole appear to be deeply etched with nume rous irregularly shaped depressions inset

"1

into a formerly planar surface. The depressions may form by collapse after melting ofground ice or, alternately, the y may be simply deflation hollow s form ed by removal ofmaterial by th e wind. Similar features do no t occur at the no rth pole.[390B90;77' S,74O W ]



THEATMOSPHERE



T HE MARTIAN ATMOSPHERE is persistently hazy. T he haziness is dueto the scattering of l ight by suspended dust and condensate particles.

This haze causes the M artian sky t o be gray to yellow instead of blue as onEarth; the blueness of Earth's sky is due to the scattering of l ight by airmolecules. Superim posed o n the Martian haze are various types of localcondensate c lo t~ ds nd fogs. At t imes, d ust storm s raise great yellowishclouds that s tand o ut against the haze and ul t imately contr ibute t o i t .

Because the axis of Mars is t i l ted with respect to i ts orbit plane, theMartian atm osphere undergoes seasonal changes analogous t o those on E arth.Viking spacecraft arrived just before northern summer solstice. Approachimages show a relatively dense haze covering th e northern hemisphere and amuch clearer a tmosphere in the south . With the beginning of s o ~ ~ th e rnspring, an even denser haze blanket formed over the southern hemisphere,largely obscuring the surface even from vertical view. Later this southernhaze th inned bu t , as southern sutnmer approached, d t ~ st torms againobscu red large areas.

Northern lati tude s were obscured by condensate clou ds and hazes duringfall and winter in th at hemisphere. North of a bo ut 60' lati tude , this "polarhood " was diffuse and featureless an d, because of th e very low atmospherictemperatures in these regions, is believed t o be a t least partly carbon dioxideice particles. The zone between 40' and 60' N was swept by f ronts thatmoved sou th ou t of th e polar regions; cloudiness was associated w ith theseweather systems.

Images of the Martian limb repilarly show a high, layered haze structureextending to more than 35 kin above the surface, rvith individual layerstypically extending over large areas. The vertical distribution of light-scat-tering particles is no t directly proportional t o th e brightness profile in thelimb image. This condition is because lower layers are seen along paths ofvarying length throug h upp er layers. Th e true distribution of scatterers wascalculated, and results revealed the existence of clear layers between thecloudy ones.

The diffuse haze blanket i tself is not without structure.111 ome regionsits features include broad longitudinal streaks, cellular lumpiness, and wavetrains. Cells, which range in size from abou t1 o 1 0 km, indicate convect ionwithin the haz e blanket. Wave trains up t o several hund red kilom eters long

are visible in a large percentage of high-altitude frames near the morningterm inato r. These waves are visible l>ecause of th e alter nate co nden sationand evaporation of ice crystals in the troughs and crests of a pressure wavetraveling through an atmosphere of high static stability.

One prominen t typ e of condensate c loud on Mars forms around the giantvolcanic mo untain s of Tharsis and Olympus. These clouds, evidently form ed



by orographic uplift , Corm in late ~nornillgand ohsctlre th e flanks of thvolcanoes up to an elevation of about 20 krn, leaving the suminiu n o h s c ~ ~ r e d .n Ea rth-ba sed observations, thesr cloud s have beet1 linownfodecades as the "W clouds" because of their repeating config uratio n, Ottypes of condensate clouds occur over less than1 percent of the Martisurface at any particular time. These include convective-like-formatio

cirrus-like wisps, and low -lying canyon clouds.Observers using telescopes have ltnown for many years that global-s

dust storm s are com m on when Mars is closest t o t he Sun in i ts relativelliptical orbit . Su ch a storm enveloped the planet w hen Mariner9 arrived Mars. Two smaller global dust sto rms were observed b y Viking orbitduring the exten ded mission. T he first occurred early in the southe rn sprand the other shor t ly af ter southern summer sols t ice . Both s torms probastarted in th e Thaum asia-Solis Planum region, an d rapidly engulfed m osthe p lanet. The y greatly affected m eteorology at th e landing sites, and eprevented the acquisition of clear images of th e Martian surface for2-months. Several dozen localized du st storm s were also observed by Viking spacec raft . Most of these occurred near t he retreating so uth polar or in the region to t he south of t he canyons on th e southeastern s lopeTharsis.

Water-Ice Cloud on Flanks of Ascraeus Mons. This southern view of the dawn side NIais was taken during Au gust 1 97 6 by V iltirlg Orbi ter2 as it approached the planBecause it was winter in the southern hemisphere at th at time, the south pole is indark. Part of the adjacent seasonal frost cap is visible at the bottom center. The gequatorial canyon system, Valles Marineris, is faintly visible at center light; but hatmosphere obscures surface features northof that e scept for the protrudi~lg ummitthe giant volcano, Ascraeus \!Ions. The w hite feature on its western flank is thou ght ta type of water-ice cloud frequently observed in that region. [P-190091



Early Morning Clouds in th e Tharsis Montes and Valles Marineris Region. Ascraeus Monsand Pavonis \Ions are prominently displa3ed in this mosaic, and dense cloud blanketscling to their north ern slopes. High cirrus cloud s lie to the west of Tharsis, and waves arevisible in the clouds surrounding the peaks. Bands of clouds appearing to have a cellular

structure extend north from the canyon, and the areas within and immediatelysurrounding the chasm exhibit \+ater-ice ogs.[211-5049; 5' S, 05' W ]



Wave and Dust Clouds in Arcadia Planitia. T his mosaic of V ilting Orbiter 2 frames showsan area nort h of Olym pus Mons. Surface detail nort11 of 45' is obscured by the polarhood. Well-developed wave clouds, seen at the upper right, are produced by strongwesterly w inds pert urbe d by the large cra ter , I\llilankovic (55ON, 147' W ). Thewavelength (distance between crests) of these clouds is about60 Itm; their persistencethrough more than 50 0 km implies stability in the atmosphere ~v l~ ic hrevents thedissipation of th e waves by turbulenc e. T he d ust clouds a t the lower left are probablyassociated with passage of a cold front moving out of the polar hood region. [211-5378;43O N , 124O W ]



A

Condensate Clouds over the Viking Lander 1 Site. During the-summ er, the northe rn hemisphere of Mars is generally qu itehazy-as shown in th e Orbiter views taken in red, green, andviolet l ight (left to right) from a distance of 32000 km. Becauseall co lors show some ob scuration, the haziness is probably causedby b oth dust an d condensates. T he large diffuse cloud near thetop center, however, is brighter in violet l ight than in red,suggesting tha t i t is largely composed of condensates. I t appearedover the Viking Lander 1 site in the Chryse basin just a few daysbefore landing. [211-51 43; 25' N, 45 ' W ]

Changes in Atmospheric Clarity. These two views in violet lightillustrate th e dramatic change in th e clarity of the atmosphere inthe region east and northe ast of th e Argyre basin during winter inthe souther l l hemisphere . (a) RiIost of the s~~o~vcoveredrgyrebasin is shown. This was taken just after the winter solstice whensolar heating was minimal. (b) This view was taken in late winterwhen the area had started to warm. The cold southern regionsmay trap water vapor from the much warmer northern hemi-sphere to form these clouds, or water vapor may be released fro mthe seasonal polar cap as it retreats. [(a) 34A 13, (b) 8 1B 21;47 " S, 22 " W ]



Wave cloi form in a stable bd atmowhen winds pass over topographic features such as cratThe distance between crests (wavelengths) depends dimensions of the perturbing feature and on the speed tical profile of th e wind. (a) These 20-km-wavelengthseem to be formed by westerly winds perturbed by thridge to the west of the clouds. (b ) This complex pattern has ~vavelengths etween 2 km and 15 km, and may be cowith the south polar crater field seen through the haze or

with instability induced by wind shear. The air is quite dthe picture, which was taken in red light soon after the the second global d ust storm . (c) This view shows ~vavwest of Arglrre which are associated with a weather systemalso produced the Argyre dust storm.[(a) 4OA21; 30" S, (b) 287B43; 60" S, 154"W , (c) 13 1B64; 55" S, 5" W]



G ro -C um ul us and S trato-Cum ulus Clouds. Clouds with cellular structure resembterrestrial cirro-cum ulus and strato-c umulu s clouds are quite com mo n on Mars, espein the p olar-hoo d region. Small convective cells, created when t he base of the cloud is heated by g round rad iation, are respollsible for th e stru cture . (a) Cellular cloud larc seen at the edge of the polar hoo d, viewed from a distance of 1 5 00 0 km. Note thwaves produced by the crate r. (b) View, taken from a distance of 1 40 0 km , of celclouds in the n ort h polar hoo d, showing the alignment of th e cells into "streets." Tfeatures can be produced by vertical wind shear. [(a) 470A07; 40' N , 210' W, (138B53; 73' N, 318" W ] .

Limb Pictures. Limb pictures (those that include the edge of the planet's disk) showcondensates, and perhaps dust, exist in layers in th e atmosphere up to 40 km abovplanet's surface. The lim b struc ture in the southern hemisphere is shown in (a) durinearly winter and in @) during the late winter. View (c) depicts the north polar limb (d) the south polar limb. Both polar views were obtained during the late summereach hemisphere. [(a) 5 3A6 5; 40° S, 40" W, (h) 79 B0 6; 48" S, 253" W, (c) 78B80" N, 346" W, (d) 3938 01; 78' S, 84" W]



Clouds Surrounding Olympus Mons.In this mosaic, Olympus-. - . A

hiIons, wreathed in c loud s at midmorning, was viewed obliquely(at an angle of 70" fro m vertical) from a range of 800 0 kmthrough a violet fil ter. The season is early summer w hen Olym pusRIIons receives close to its maximum solar flux. The top of thecloud blank et is ab out 1 9 km above the mean ground level and8 km below the summit. Water-ice, which condenses as upslopeair currents cool, is thought to form these clouds. Parts of thecloud cover have a cellular appearance, indicating convection

within the clouds. A well-developed wave cloud several hundredkilometers long is visible towar d the limb. [P-174 44;18' N,133" W ]

Clouds arou nd Pavonis Mons. Early mo rning views, taken 3 weeksapart, sllow Pavonis AiIons, the central volcano of the TharsisMons receives close to its maximum solar flux. The top of thecloud blanket is abou t 1 9 km above the mean ground level and8 km below t he sum mit. Water-ice, which condenses as upslopeair currents cool, is thou ght t o form these clouds. Parts of thecloud cover have a cellular appearance, indicating convectionwithin the clouds.A well-developed wave cloud several hundred

kilometers long is visible toward the limb. (a) 40A95, (b)62A 18; 0" N, 113" W]



Discrete Clouds on Volcano Slopes. Discrete clouds are fre-quently seen above th e slopes of th e large volcanoes. (a) Theunusual plume cloud was repeatedly seer1 over AscraeusMans inthe early mon ling during the summer. (b) Th e cloud show11 islocated over the northwest slopes of Ascraeus R'Ions; the picturewas taken when the local season was early autumn and the timeabo ut 2 :00 p.m. Picture (c) sho ~v s n unusual combination ofcirrus-like clouds, thin wave clouds, and a prominent discretecloud (which may be a turbu len t roto r) over Arsia R'Ions. [(a)58A12; 11" N, 105" W, (b) 225A05; 12"N , 104" W, (c) 344B8 8;9" s,120" W ]



Cirrus Clouds. Clouds resembling terrestrial cirrus clouds are often seen in the Maatmosphere. That these clouds are condensate phenomena is well illusbated by the grcontrast in (a), taken through a violet filter, than in (b), taken through a red filter asame time. W ithout shadows to determine altitudes and a knowledge of temperaturthe proper heights, it is difficult to distinguish water and carbon dioxide ices. not i~np roba ble hat b oth types of cirrus clouds exist. The group of cirrus clouview (c ) occurred t o the n orth of the Valles &Iarineris canyon system ; the varying oritions of the clouds may indicate differences in wind direction a t the altitudes at wparticular clouds occur. Picture (d ) sho~vs bright winter cloud as it appeared overElectris region. I t was observed to recur at the same place on several days during season. Bjerknes Crater is a t the lolver left. [(a) 10 1A 10 ; 6'N, 244O W , (b) 101A6" N, 244' W , (c) 58,402; 6" S, 76 " W, (d) 88A 03; 42" S, 192" W ]



Cloud Shadows. Shadows of clouds may be used to determine the altitudes of clouds.This information , coupled with h eight profiles of temperature an d pressure, can lead t o adeterm ination of th e com position of a cloud. In mosaic (a), high-altitude clouds are seenover ancient, cratered terrain t o the east of the Hellas basin. Arrows con nect three smallcondensate clouds to their shadows, which appear to be abou t 200 km away. Usingsimple geometric relationships involving the cloud, its shadow, and the sun elevationangle, one finds the clo uds to be at approximately 5 0 km altitud e. View (b) shows alarger (10 0 km long) cloud south of Valles Marineris, abou t 5 0 km above the surface. Atthis altitude, where temperatur es and pressures are low, carbon dioxide is the probablecomposit ion. [ (a) 97A75, 77 ,7 9 ,8 1;50' S, 246' W, (b) 31 8A 24 ; 20' S, 44' W]



Enigmatic Uou ds. These four frames show Martian atmospheric phenome na tha t do no tfit into any of th e preceding categories. Picture (a) is an unu sual polar ho od cloudform ation associated with t he large crater, klie. Superpo sition of lee waves fro m parts ofthe tcrrairi aroun d Rlie could produce such a forma tion under appropriate atmosphericconditions. In (b), a cloud in the so uthern he~nispllere an be seen; to catch th e Sun's raysthe cloud must be high in the atmosphere. Linear, optically thin streaks are seen in theT h a u m ~ i aegion in (c). Streaky, condensate hazes that have developed near the dawnterminator during the on set of autum n in the southern hemisphere are seen in (d). [(a)470i105: 48" N, 220" W, (b) 211B60; 55" S, 234" W, (c) 67A06; 39" S, 85" W , (d )431B03; 26" S, 280" W]

Early Morning Surface Fog. Th e presence of morning fogs in some crater and chamlnelbottoms is a 1-iking discovery with possible implications for the future biologicalexplo ration of Rlars. These early m orning views of the Mem nonia region were takenone-half hour apart using a violet fil ter to enhance th e co ntrast of the condensates. Theareas marked by arrows are noticeably brighter in th e later picture. T he fogs indicatespecific spots where water is exchanged, probably on a daily cycle, between the surfaceand tlie atmosphere. The surface a id lower air layers in this region become unusually coldat nigh t because of the thermal properties of the surface. When the surface warms in themorning, it seems that a small amo unt of water vapor-estimated to be abou tone-millionth of a meter thick if liquefied-is driven off: this vapor recondenses in theatmosphere, which \\ .arms more slowly, to form a ground fog of ice particles. [P17487;13 " S, 147" W ]



Early Morning Clouds in Noctis Labyrinthus. Condensate clouds are seen here in early

morning in the canyons of Labyrinthus Noctis, which lies at the western end of theequatorial Valles Marineris system. This picture, which covers about 90 000 km2, wasmade by combining three frames of the same field taken through violet, green, and redfilters. Although these clouds lie mainly down inside the canyons, they evidently extendabove the walls and spill over some of the surrounding plateau. Like most condensateclouds in the Martian troposphere, they are believed to be composed of water-ice crystals.[P18114; 9' S, 95' W]

Dust Storm in Argyre Basin. A local dust storm in the Argyre basin near the end of winterin the southern hemisphere is seen from a relatively high-altitude point in the ellipticalorbit of Viking 2. Winds appear to be coming from the west. The turbulent brown dustcloud near the polar cap boundary is roughly 300 km across. This cloud did not developinto a global dust storm of the type that tends to occur a little later in the Martian year

when Mars is nearer to the Sun. Part of the receding seasonal frost cap covers the lowerhalf of this picture. It appears yellowed by dust in the Argyre basin, but whiter in themountains (at bottom of picture) at the southern rim of the basin. [P18598B; 50' S40" W]



South Pole Dust Storm. This picture ofriphery of the retreating ice cap was taday after perihelion (Mars closest appthe Sun). The cap had shmnk cousisince the time of the Argyre storm obsThe dust storm at the edge of the frostarea, which is just visible in the cornepicture, is ab ou t 20 0 km across. Plumecan be seen outside tlie boundaries of tstorm. This picture shows the first globin its last phase. Su ch sto rms are problated to winds induced by great surfaperature contrasts. [248B57; 70'S, 60'

Local Dust Storms near Noctis Labyrinthus. The region southeast of the NoctisLabyrinthus complex on the slopes of the Tharsis bulge seems to be particularlyconducive to tlle form ation of local dust clouds. These frames were taken in the middleof spring (a) and in late spring (b). Both local dust storms occurred in the period betweenthe trvo global dust storm s. The area in rvhich th e local storms occurred slopes upwardtoward Arsia RlIons. Infrared Thermal Mapper instrument data have shown that becauseof local differences in surface thermal properties, large temperature contrasts occur in.this region. Downslope winds caused by these temperature gradients may be strongenough to create such clouds. [(a) 275B 05-10, (b) 211 B24 ;14' S, 90° W 1



Dust Storm over the ChryseBasin. These two pictures of a dust storm over the Vikinglander site in the Chiyse basin were taken 17 0 seconds apart. Motion of th e clouds can bedetected if the pictures are viewed through a stereo viewer. Analysis of the two picturesindicates tha t portions of the cloud were moving from west to east with speeds rangingfrom 40 to 60 meters per second. This is consistent with westerly winds a t the surfacewith the unusually high speed of22 meters per second as recorded by the lander. The. - .lande r ob servation is, however, possibly in error because its wind sensor was damaged.

[467A69,467A31; 22" N, 48"W ]

DGlobal Dust Storm. The early stages in the development of the first global dust stwere observed by Viking Orbiter2 in the Thau masia region. These images were tak2 days apart. In (a), a single fram e, imaged in red ligh t from a vely high altitud e, incluthe entire weather disturbance; the rest of the southern hemisphere was rather clea

this time. In (b), a mosaic, the frames were also tak en through the red filter. They shan area several thousand kilometers wide seething with turb ulen t clouds of d ust. storm spread rapidly t o higher altitudes, and suspended d ust obscured much of the plfor a period of 5 0 days. Increased so lar heating as Mars nears perihelion is thou ghprovide the energy tha t creates these large-scale disturbances. [(a) 1 76 B0 2; (b) 211 - 542" S, 108" W ]



Low Pressure Cell near Nor th Pole. This R~lartian torm was observed by Viking Orbita t about 65' N latitude. The local season corresponds to late July on Earth. The stis located near Mars' polar front, a strong thermal boundary that separates cold air the pole from the m ore tem perate air to the south. Shadows indicate that the cloudrelatively lo\+ in the atmosp here. Because tempe ratures in this region are well abovcorlderlsatio~l empe rature of ca rbon dioxide, water ice is the probab le constituent oclouds. Water vapor con centrations are high (by 4'Iartian standards) during this seasthe north polar region.

This system strongly resembles satellite pictures of extratropicalcyclo~ies ear tpolar fro nt on Earth. T he counterclockwise circulation is consistent with th e wexpected in a normal low pressure situation.

The frost-filled crater Korolev (approximately92 km in diameter) is located to tnortheast of the storm. The white patches in the top center of the picture are outlierthe north polar remnant cap.[783A42; 70"N , 200' W ]



Cold Fro nt. Viking Orb iter 2 photog raphed this cold fr on t in the Arctic region when theseason on Mars was equivalent to mid-May on Earth. During the2 days between theupper and lower mosaics, the front moved about 9 50 Itm, at an average speed of20 km/hr. The movement may be seen by comparing the two mosaics: lines connectidentical features in the two sets of pictures. Weather systems like this appear to becommon in Mars' northern hemisphere. Viking Lander2 has detected the passage ofsimilar fro nts m any times. Warm, comparatively moist air is lifted over a wedge of colder,denser air as it p ushed sout h. M oisture in th e warm air condenses into ice crystals,forming clouds. Dust, seen frequently in Martian storms, could also be present in theseclouds. Some water must be present, scientists say, because wave clouds seen in bothmosaics result fro m condensation and evapo ratio~l f ice crystals in th e troughs and crestsof pressure waves propagating in the atm osphere. [ 211 -576 4; 65'N , 135' W ]



L ANDING SITES for the Viking landers were chosen before la~unch, utclose inspection from orbit showed tllein to be much too rugged ancl

dangerous to risk landing on.Viking Orbiter 1 searched for almost 3 weeks before finding a new, more

suitable landing site about 800 km west-northwest of the original site, butstill within Chryse Planitia. Because of the unexpected search for a newlanding site, the original goal of landing on the historic date of July 4, 1976was lost; the landing was achieved instead on July 20.

Viking Orbiter 1 took pictures of the proposed landing site for thesecond lander; from these it was determined, even before the secondspacecraft arrived at Mars, that th e site was no t acceptable. Almost half ofthe planet's surface between 40' and 50' N was photographed in the

attempt to find a suitable landing site for the second lander. Finally a sitewas found in Utopia Planitia, and the second successf~ul anding was made onSeptember 3, 1976.



Qlryse Planitia. This map is constructed from a series of images acquired by Vik1 Orbiter L on revolutions 10 , 20, an d 22. C liryse Planitia is an area of plaitis tha t seemN

be volcanic in origin. Th e surface is characterized by mare-like ridges and relatively yoilnpact craters, although subdued or part-filled craters are also present. At the left

1 50 knn ( ltighcr region of cratered terrain in which are several large channels.[USGS map M 1 M

23/5 0 CR~IC, 9 77 ; 1-1068 1

D

High Resolution Mosaic of the Viking Lander 1 Site. Tlie periapsis of Viking Orb iterwas lowered to 300 km to obtain liigli resolutio~images of the su rface. Tlie images hewere taketi by the spacecraft on revolution 452, and craters as small as30 meters acrocan be sees. Craters A , B, C, arid D are th e saliie craters as sllo ~vn n the map of tI'orktown region. K ims of craters C andE can be seen in images taken on the surface Viking Lander 1. [452B09-111



Yorktown Region of Chryse Planitia. This map shows Yorktown Crater-an 8-km-diameter you ng im pact crater-at the top left. Several branchesof the Xanthe Dorsamare-like ridge system are also shown. The major craters on this map have been

tN

tentatively assigned names to commem orate th e 13 American colonies and the ports andcountries with which they traded. Th e craters labeledA, B, C, and D are also shown in 11 0 k m /the m osaic below. [USGS map M 250K 22 /48 CRlIC, 197 7; 1-1059 1



Panorama of Landscape. This view of par t of the horizon a round Viking Land er1 wastaken shortly after landing. At the le ft, about 1 00 meters away, is th e northwest portionof the rim of Crater E in t he high resolution mosaic on th e previous page. At th e right is

the rim of Crater C, about 1.8 km away. The slight depression in the foreground just tothe right of center may be a shallow, 3-meter-diameter secondary impact crater. [VikingLander 1 Camera Event 12A0021

Mosaic of the Utopia Planitia Region. Viking Orbiter 2 acquired these pictures onrevolution 9 during lancling site selection and certification for Viking Lander 2. This part

of U topia Planitia is a polygonally fractur ed surface covered by a thin layer ofpresumably windblo.rvn material that has preferentially acc umu lated in the frac tures. Atthe extrem e u pper right is \!lie, a 100-km-diameter impact crate r, approximately 200km

east-northeast of the lander. VIie Crater is covered by sand dunes ant1 deflation hollo~vs,both of w hich are strong evidence of wind activity. To th e right of bottom center is HradVallis. [USGS map Mlh11461230 Chi, 1977: 1-10611



Mosaic of the C a~ b er ra egion. Tliis mosaic; an rrrlargeme~it f part of the mosaic of theUtopia Plariitia region on the previous page, sllo~v s he d etail aro und tlie sitc of TJikingLander 2. AlIost of the craters in tliis region are the pedestal type, produced when windscoured away th e softer, more erodable ~natc:rialbetwt:ell craters arid left exposedtlteharde r, more resistan t material arou nd tlrc crater rims. The larger craters have beententatively na med for la u~ ic h acilities, tracking stations, and mission control centersused in 1976. [USGSm ap R112501<$81226 CM, 1977; 1-10601a

Panorama of Landscape. This panorama of the terrain surrounding the site of VikingLander 2 wa s talcen by the spacecraft lander sho rtly after touchdo wn. The large numberof rocks and boulders was a surprise because prelanding evidence suggested that the areawould be covered by a thin (tens of meters) sand sheet. FeatureA on the far horizon isbelieved to be ejecta from the rim of Mie Crater. Th e large dip in th e liorizon was causedby the lander being tilted. [Viking Lander2 Camera Event22A0021



GLOBALCOLOR



D URING THE FINAL APPROACH to Rlars, just before going int o orb it,the Viking orbiters took a series of pictures of the planet with different

color filters. As the planet rotated, different parts were photographed untilalmost the entire surface was covered. The pictures taken with the variousfilters were then compared, and the color at each point on the surface wasdetermined by means of a variety of sophisticated computer processingtechniques. The results are displayed here as Mercator maps of the equatorialregions. The approach trajectory prevented good visibility of the polarregions, and they are not shown.

Although Mars has very distinct light and dark areas, differences in colorare quite subtle. The appearance of the planet is dominated by variations inbrightness or albedo that cause the classical markings known from telescopicobservations. The color of the surface is a fairly uniform rusty brown, anddifferences are so small that they can barely be seen. To see the colordifferences, they must be artificially enhanced through computer processing;at the same time, differences in brightness must be suppressed. Two colormaps are shown here, one that approximates the natural color and another inwhich the color differences are artificially emphasized.

Color is of considerable geologic interest because i t allows remotedetection of chemical and mineralogical differences. Only the upper fewmillimeters of the surface contribute to the color, and on Rlars this layermay be mostly wind blown debris. The bright materials that dominate thenorth equatorial zone are apparently aeolian deposits. Two units have been

recognized. The upper unit is discontinuous, very red, and among thebrightest of materials exposed a t the planet's surface. The lower unit isdarker and less red . The boundary between the two is generally serrated andhas no relief. In the southern equatorial belt, the color variations areapparently related to local bedrock and not to randomly dispersed aeoliandebris. he dark highland region (0" o 40" S and 60' W to 30" E ) is dividedinto (a) dark red ancient crater rims, rugged plateaus, mostly riddled withsmall channels, and graben; and (b ) dark "blue" volcanic flows intermediatein age, and show very few channel networks. The large volcanic constructs inthe Tharsis region and volcanic centers in the southern highlands northeastof Hellas are both very dark and very red.

Atmospheric phenomena and surface frost affect the planet's appear-

ance. S ou tl ~ f approximately 40" S, the scene is dominated by t he annualsouth polar carbon dioxide ice cap. Near-surface condensate clouds areabundant in this region, especially in Hellas. Beca~~se onie of the data in thebright areas were saturated, the color balance is distorted; no attempt was



made to correct th is problem. North of abou t20" 1U. condensa te c lo~ td sespecially notice able along the nor the r~l rno st edge w here emission angwe rt c xtrerne. Other clottds are scattcrcd locally th rol ~g ho ut hc equatorregion sout h and s outh we st of V allrs R1larinrris.

Enhanced Color of Mars. In this image, all three color co m po ~ ~ en tsave received tsame contrast enh ancem ent, which approaches saturation in the brightest areas. BecMars is by a factor of two to three more reflective in th e red than in the violet, thecom ponen t is predominant-giving the planet its classic rusty app earance. Some artiof the processing remain in the image, fo r exam ple, diagonal streaks running from ule ft t o lo~v eright.

Color Variations of Mars. Tliis image dramatically enhanc es subtle c olor variations.violet/green ratio is used as the blue comp onent of the final image, the albedo atg reen ~vavcleng th s the g reen componen t, and the r~ d / ~ r e e natio as the red componeHence, the amount of red or blue isconko lled primarily by the slope of the specreflectance curve ; areas with high albedo are also green. Thu s, high albedo blue areasfog, clouds) are blue-green in color, and high albedo red areas are orange and yelbright areas of average color are green. Green is absent in dark areas, so the c

represent the slope from violet t o red ; red areas have a steeper slope, increasing violet to blue; blue areas have a shallower slope.



APPENDIX IGLOSSARY

Aeolian.-A term applied to ~y in d rosion-or deposition of surfac e materials.

Albedo.-The reflectivity of a body com pared with th at of a perfec tly diffusing surface atthe same distance from the Su n, and normal to the incident radiation.

Apoapsis.-That poin t in an orbit farthest from the center of attraction.

Barchan.4 moving, isolated, crescent-shaped dune. The convex surface points towardthe w ind.

Basalt.-A dark, fine-grained volcanic rock. Very com mon.

Bench.-A small terrace o r step-like ledge breaking the contin uity of a slope.

Caldera.-A large volcanic depression conta ining volcanic vents.

Catena-Crater Chain. A chain or line bf craters.

Cavitation.-Plucking of material from th e floor of a channel caused by the sharplyreduced pressures associated with extreme flow velocities.

Chaotic terrain.-A surface consisting of short, jumbled ridges and valleys.

Chasma-Canyon. An elongated, steep-sided depression.

Chondrite.-A ston y mete orite characterized by chondrules embe dded in a finelycrystalline matrix consisting of orthopyroxene, olivine, and nickel-iron, with orwithout glass.

Collapse pit.-A closed, rimless depression caused by subsidence.

Dike.-A near vertical, planar, volcanic intrusion.

Dorsum (Dorsa).-Ridge(s). Irregular, elongate prominence.

Ejecta.-Material thrown ou t of an impact crater during formation. Such material may bedistributed around a crater in distinctive patterns forming "ejecta rays" or "ejectaloops.

"

Escarpment-A long, more o r less continuous cliff or relatively steep slope produced byerosion or faulting. See "scarp."

Esker.-A long, low, narrow , sinuous, steep-sided ridge or mo und com posed of irregularlystratified sand and gravel that was deposited by a subglacial or englacial streamflowing betwee n ice w alls or in an ice tunnel of a continu ously re treating glacier, leftbehind when the ice melted.

Etch pit.-A surface depression caused by the preferential removal of less resistantmaterial.

Fault.-A surface o r zone of rock fracture along which tliere has been displacement, froma few centime ters t o a few kilometers in scale.

Folding.-The curving or bending of a planar structure such as rock stra ta, foliation, orcleavage by deformation.



F os a (Fosae). -Ditches . 1 ,ong, narrow . sliallo~v depression. Tlley occurgroupsa nd are siraigl ~t r culvetl.

Fretted.-Eroded in sucli a manner as to produce t\vo llorizontid planar s~ ~r fa ce separatby near vertical escarpmerils.

Fretted channel.-lo ng, relatively wirle, flat floorcd valley with tributarir:~.klass wastiprobably played a significant role in their formatior].

Gelif1uction.-Creep of froz en mate rial.Glacioaeo1ian.-Material rem oved from a glacier by wind erosio n.

Graben.-An elonga te, relatively depressed crustal uni t or block tha t is bound ed by fon its long sides.

Gradation.-The leveling of the la nd, or tlie bringing of a land surface o r area to a unifor nearly uniform grade or slope through erosion, transpo rtation, and deposition.

Inclination.-The angle betw een th e plane of an orb it and a reference plane. The equator is here used as the reference w hen referring to spacecraft inclination.

Interfluve.-Lying betw een streams.

Labyrinthus.-Valley com plex. Complex, i~ite rsec ting alleys.Laminated terrain.-A surface made of layers of different types of materials: lay

terrain.

Lava.-Rock from a volcano, generally molte n when ejected.

Limb.-The ou ter edge of a planetary disk.

Lithosphere.-The solid ou ter port ion of a planet.Mare.-Low-lying, level, relatively s mo oth, plains-like areas of considerable ext ent.

Mass wasting.-A term th at in clude s all processing by which soil and rock materialsarid are transported downslope pre domin antly en masse by the direct applicatiogravitational body stresses.

Mensa (Mensae).-hiIesas. Fla t topp ed prom inen ce with cliff-like edges.

Mons (Montes).-Mountains. A large topographic prom inence or chain of elevations.

Normal fault.-A break in the surface caused by tensional forces.

0rographic.-Pertaining to moun tains, especially i11 regard to their location adistribution.

Outflolv channel.-A large-scale cllannel that star ts a t full width in chaotic terrain an

few, if any, tributaries.0verthrusting.-A low-angle thrust fa ult of large scale, generally ~n ea su red n kilome

Pahoehoe.-A type of lava having a glassy, sm ooth , and billowy or undulating surfacecharacteristic of I-Iawaiian lava.

Patera.-Irregular crater or a complex one with scalloped edges.

Pedestal crater.-A crater around ~v hic h ess resistan t material has been removed fromejecta leaving an elevated surface of mo re resistan t ejecta material.

Pe1iapsis.-The orbital point nearest the center of attra ctio n.

Penglacial.-Said of the processes, condition s, areas, climates, and topogra phic featurtlie im media te margins of forme r an d existing glaciers and ice sheets, and influeby the cold temperature of the ice.

Perihelion.-That po int in the orbit of a planet w hen it is closest to tlie Sun.

Phase angle.-The angle betwee n a line from the Sun to th e cen ter of a body and afrom the spacecraft to the c enter of the same body.

Planitia-Plain. Sm ooth low area.

Planurn.-Plateau. S m oo tl ~levated area.



Pleistocene.-A reccn t geologic epoch of the Qua ternary period be@runing apprctximatel!one million years ago, the last glacial age.

Polygonal ground.-Patterned ground marked by polygon-like arrang elnents of rock orsoil. Generally producecl on E arthby ice-wedge polygons.

Precessing ellipse.-An ellipse in ~vlvhich he pole is changing dire ctio n.

Rampart.-A narrow , wall-like ridge.

Regolith.-A general term f or loose material overlying bedro ck.Rev erse fault.-S ee tlvhrust fa ult .

Rift.-A narrow cleft, fissure, or othe r opening in rock (as in limestone), made bycracking or splitting.

Rille.-Relatively long, tren ch-like valley; has relatively steep walls and usually flatbot toms.

Runoff channel.-Relatively small channel probably caused by wa ter erosion over a longperiod.

Scabland.-Elevated, essentially flat basalt-covered land with only a thin soil cover.Scarp.-A line of cliffs produced by faulting or by erosion. Th e term is an abbreviated

form of escarpm ent, and the tw o terms commonly have the same meaning, although"scarp" is more often applied to cliffs formed by faulting.

Sediment.-Solid, fragm ental material or mass of such material originating from theweathering of roc ks, e.g., sand, gravel, mu d, alluvinm.

Shield volcano.-A bro ad, gently sloping volcano.

Striae.-Striped ground.Subdu ction zone.-An elongate region in which a crustal mass descends below anothe r

crustal mass.

Subsidence.-A localized gradual clown~vard ettling or sinking of, a surface with little orno horizontal movement.

Tectonic.-A term pertainin g to deforma tion of a planet's c rust , especially the rockstructure and surface forms that result.

Terminator.-An imaginary, diffuse line separating the illuminated and dark porti ons of acelestial bod y. Th ere are tw o terminators: m orning and evening.

Thermokarst.-Rimless depressions caused by the melting of ice an d subse quent collapseof the surface.

Tho1us.-Hill. Isolated domical small mou ntain or hill.

Thru st fault.-A fau lt caused by compressional forces.Trans curren t faulting.-A large-scale strike-slip fault in which th e fau lt surface is steeply

inclined.

Troposphere.-The lowe st layer in an atmo sphere , generally considered to be10-20 kmthick.

Tuff.-Volcanic ash, particle s of 4 mm diameter or smaller.

UTC.-Universal Time Coo rdinated .

Unconformity.-The relationship where the younger upper strata do no t follolv the dipand strike of older underlying strata.

Vallis (Valles).-Valley. A s inuous chann el, many with tributarie s. These are named

"Mars" in m any languages, e.g., A1 Qahira Vallis is derived from th e Arabic word f orMars.

Vastitas.-Extensive plain.

Yardang.-Elongated, sculpte d ridge forme d by wind erosion.



SOURCES

Arnericari Geological In stitute : Glossary of Geology, 1972.'

Robert J . Foster: Physical Geology. Charles E . Merrill Publishing Co., Colurnbus, Ohi1971.

Richard M . Pearl: Geology. Barnes & Noble, Inc., New York, NY, 1969.

G. DeVaucouleurs, et al.: "The New Martian Nomenclature of the InternatioAstronomical Union," Icarus 26,85- 98, 1975.



APPENDIX I1

THE VIKING ORBITERIMAGING SYSTEMS

T HE VIKING ORBITER cameras evolved from t he cameras flown on t heearlier Mariner spacecraf t. Each generation of spacecraft sent to Mars

has featured cameras with vastly improved capabilities, especially higher reso-lution and increased light sensitivity. Basically the cameras are high-perform-ance vidicons, similar to those used in television cameras on Earth, and have a

telephoto lens assembly in fro nt of them. Each orbi ter has two identicalcameras. The cameras, along with instruments to measure the surfacetemperature and amount of water vapor in the atmosphere, are mounted onthe science platform, a device that can be moved about two axes to achieve ascanning motion when viewing the Martian surface from orbit. The drawingshows the arrangement of instruments on the science platform.

Each camera, along with its 475-mm focal length telephoto lens, has afield of view of 1.54" X 1.69'. From an orbital altitude of 1500 km, eachframe covers a minimum area on the surface of 40 X 44 km. Six selectablefilters allow color images to be acquired. Acquisition of a frame andsubsequent readout to the tape recorder requires 8.96 seconds, so a frame istaken on alternate cameras every 4.48 seconds. This alternating pattern,

coupled with motion along the orbit, combines to produce a swath ofpictures, as shown in the illustration of orbiter imagery coverage.

The image on the vidicon is scanned, or read o ut , as 1 056 horizontallines. Each line, in turn, is divided into 1182 pixels (picture elements), andthe brightness of each pixel can range from 0 to 12 7 arbitrary units. Thus, torecord a single frame requires the storage of almost 10 million bits (binarydigits) on the orbiter tape recorder. The pictures are stored on the taperecorder in digital form until there is an opportunity to play back the dataover th e orbiter communications system to a receiving station on Earth.

As the data are received on Earth they are subject to computer-basedimage processing. All images receive first-order processing that consists of thefollowing: noise removal, contrast enhancement, and shading correction.First-order images are the most widely used for scientific analysis and are theversion most commonly seen in this book. Some images also undergo a morecomplex second-order processing tha t includes all of th e first-order pro-



cessing plus sophisticated procedures t o remove geom etric and radiomedistortiolzs and merge black and white images taken throug h diffewavelength filters intoa composite color image. Second-order processing calso include techniques such as generating stereoscopic pairsand pictudifferencing in which an image taken a t one tim e is digitally subtracted fanother image of the same scene at a different t ime t o highlight any chanthat mav have occurred during the interval between th e images.

O R B I T E R S C IE N CE P L A T F O R M



APPENDIX I11

OTHER SOURCESOF VIKING DATA

T HE RESULTS O F PROJECT VIKTNG have appeared in countlesspopular and scientific publications, which it would be futile to list.

However, the reports given below represent the best summary of thescientific findings:

Journal of Geophysical ResearchVolume 8 2, Number 23September 30, 197 7Volume 84, Number B6Jul ie 10, 1979Volume 84, Number B14December 30, 1979

Published by:American Geophysical Union1009 K Stre et, N.W.Wasliington, DC 200 06

A comprehensive bibliography of major Project Viking papers, bothscie~ltifi c nd engineering, can be ordered f rom:

Bibliography of Viking Mars ScienceMail Code 111- 1 0 0Je t Propulsion Laboratory480 0 Oak Grove DrivePasadena, CA 91103

A summary of how the total Viking System works can be found in:

Spitzer, C. R.: "The Vikings Are Coming," IEEE Spectrum, Vol. 6, June 19p. 48-54

Published by:Institu te of Electrical and Electronics Engineers

345 E. 47th StreetNew York, NY 100 17



Qualified scientists and educators may request Project Viking scientificdata from :

National Space Science Data CenterCode 601.4Goddard Space Flight CenterGreenbelt, MD 20771

For the student of Mars interested in examining the companion to thisvolume, which displays selected lander photographs, please see:

The Martian LandscapeNASA SP- 42 5National Aeronautics and Space AdministrationWashington, DC 2054 6

For sale by t he Sup erintendent of DocumentsU.S. Government Printing Office, Washington,D.C. 20402.(Stock number 033-000-00780- 9)



APPENDIX IV

PROJECT VIKINGMANAGEMENT PERSONNEL

Prelaunch, Cruise, and Primary Mission

Walter Jalrobowski, Program ManagerRichard S. Y oung, Program Sc ientistJam es S. NIartin, Jr. , Project ManagerA. Thom as Young, M ission DirectorGerald A. Soffen, Project Scientist

Viking Orbiter Views of Mars

Documents

Transcript of Viking Orbiter Views of Mars