mars the viking discoveries

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Mars: The Viking Discoveries


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Mars: The Viking Discoveries

Bevan M. French

Chief, Extraterrestrial Materials Research ProgramOffice of Lunar and Planetary ProgramsOffice of Space Sciences, NASA

EP-146October 1977

NASANational Aeronautics andSpace Administration


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Table of Contents

The New Arrival 5

Why Mars? 5

Viking to Seek Answers 6

The Viking Spacecraft 6

A Viking's-Eye View 8

The Winds of Mars 12

The Chemistry of Mars 18

Three Chances for Life 20

From Mars to Einstein 22

What Next? 24

Appendix

Suggestions for Further Reading 32

Experiments and Activities 33

Suggested Viewing 36

Cover photo: Twilight on Mars; from Viking1 Lander, processed by computer.

Inside front cover: Dawn on M ars; Viking1orbiter photo found early-morn ing fog fillingthis network of canyons on a high Martianplateau.

For sale by the S uperintenden t of Documents,U.S. Governmen t Printing OfficeWashington, D.C. 20402

Stock No. 033-000-00703-5


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Preface

Mars: The Viking Discoveries is the17th NASA educational publication to

outline the results of NASA's researchactivities in space sciences.Prepared in a format that will be

useful to the teacher of basic coursesin Earth science, Earth-space science,astronomy, physics, and geology, theyare also written in a style that willappeal to the well-informed,intellectually curious layman.

The author, Dr. Bevan M. French, isa geologist who has studied Moonrocks and ancient terrestrial meteoritecraters for more than 10 years. In 1973he helped discover a Brazilian impactcrater 25 miles in diameter and 150million years old. He now managesNASA's program for scientific researchon meteorites, lunar samples, andother kinds of extraterrestrial materials,as Chief, Extraterrestrial MaterialsResearch Program, Office of SpaceScience. His program is also helpingNASA plan ahead for the return ofrocks from yet another world-Mars-so that scientists can find out directlywhat the Red Planet is really like.

For assisting the author with helpfuladvice, comments, and criticism, we

thank the following: Dr. Richard S.Young, Director, Planetary Biology andQuarantine, Office of Space Science,NASA; Loyal G. Goff, ProgramScientist, and Walter Jakobowski,Manager, Viking Program, Office ofSpace Science, NASA; Dr. HarryHerzer, Senior Specialist, NASAAerospace Education Services Projectand faculty associate, California StateUniversity, Chico; Ms. Carolyn P.Schotter, Teacher of Earth Science,Falls Church High School, FairfaxCounty, Virginia; and Ms. JeanneHewitt, Department of Geology, TheGeorge Washington University. Forextensive editing and revision, weexpress appreciation to Ms. Mary-HillFrench.

October 1977National Aeronautics and SpaceAdministrationWashington, D.C.


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The New Arrival

Exactly 7 years after astronauts firstlanded on the Moon, a new inhabitantarrived on Mars. On July 20, 1976, atop-shaped object dropped from itsorbit hundreds of kilometers aboveMars and streaked. downward into thelate afternoon Martian sky. About 40kilometers above the surface of theplanet, the thin air began to grip andslow the capsule. At an altitude of 6kilometers, a huge parachute unfurled,a protective shell broke away, and themetallic object inside began to drop tothe ground more slowly. At an altitudeof 1.7 kilometers, three rocket enginesfired downward, slowing thespacecraft even more. The parachute

, was cut loose, and the object settledgently to the ground. The rocketengines immediately shut down. TheViking 1 Lander had arrived on Marswith no more shock than a terrestrialskydiver landing in the center of histarget.

Instantly the computer that hadguided the spacecraft on its journeysent a message to Earth that saidessentially: "I am here. I am downsafely. I am beginning my work." Earthwas more than 321 million kilometers(290 million miles) away, and even atthe incredibly fast speed of light(300,000 kilometers per second), itwas almost 20 minutes later, 5: 12A.M., Pacific Daylight Time, before theLander's message reached the Viking

Figure 1. Mars at Close Range. TinyMartian pebbles appear in sharp detail inthis first picture ever taken on the surfaceof Mars . Even after being transmitted for320 million kilometers (200 million miles),the picture is so clear that the observerseems to stand on Mars beside the Viking1 Lander. One of the Lander's footpads(lower right) rests firmly on a surface ma deof fine soil and scattered rocks . Large rock(uppe r center) shows triangular faces thatmay have been cut by wind-driven sand.Another rock is dotted withsmall dark pits that possibly were formedby gas escaping from once-molten lava.The picture was taken with a camera thatscanned the scene vertically, line by line,from left to right, com pleting the picture inabout 5 minutes.

Control Center at the NASA JetPropulsion Laboratory in Pasadena,California. When the applause andcongratulations began on Earth, Viking1 already had been studying Mars for20 minutes. Moments later, Earth's TVscreens began to show the firstplctures of Viking 1's footpad, firmlyplanted on the soil of Mars (Figure 1).A new era in our exploration of theRed Planet had begun.

Why Mars?

Mars has been in humanity'sthoughts since astronomy began. The

Babylonians first began to follow themotions of what to them was awandering red light in the sky, andthey named it Nergal after their god ofwar. Later, the Romans, honoring theirown war-god, gave the planet itspresent name.

A century ago, as the first largetelescopes were trained on Mars,observers saw that the planet had areddish surface, white polar caps, anatmosphere, clouds, and changingpatterns of light and dark that mightbe vegetation on its surface. It seemedto be an Earthlike planet on which lifecould exist, and some astronomersclaimed to see long lines of canalsmade by intelligent beings. Fictionwriters, therefore, needed littleencouragement to populate Mars witha wide variety of creatures:philosophical canal builders, leatherymonsters who invaded Earth, and avariety of humanoids with human traitsof good and evil.

The Space Age methodicallyremoved the basis for much of thiskind of romance about Mars. Nocanals could be seen at close range,and their appearance was explainedas optical illusions that had affected

Earthly astronomers. The Martianatmosphere proved too thin to breathe,and there was very little water. Thesediscoveries only increased ourcuriosity about the real nature of Mars

and its relation to the other worlds weknow. The discoveries did not rule outthe possibility that some form of lifemight exist on the distant planet. Marswas neither entirely dry and airless likethe Moon, nor watery and teeming withlife like the Earth. What was Marsreally like, and what could it tell usabout the Earth and the Moon?

Exploration of the solar system hadearlier been established as a topNASA goal for the period after Apollo.The program was aimed at a betterunderstanding of the solar system'sorigin and evolution, the origin of life,and the planetary processes thataffect life on Earth. Because Mars, of

all the planets, most resembled theEarth and appeared the most likely toharbor some form of life, it was giventop priority for scientific study.

As spacecraft observed the planetin closer and closer detail, wediscovered that Mars is not uniform.The early flybys (Mariner 4 in 1964and Mariners 6 and 7 in 1969) hadproduced photos of a heavily crateredsurface that looked as dead and staticas the surface of the Moon. But thephotographs and maps obtained fromMariner 9 in 1971, as it orbited Marsfor about a year, showed that theplanet is actually a two-part world.

The southern half of Mars, which thefirst Mariner spacecraft had looked at,seems much like the surface of theMoon: ancient, inactive, and stillheavily cratered by an intensemeteorite bombardment that may haveoccurred during the planet's earliestyears.

The northern half of Mars is moreEarthlike: younger appearing,geologically active, and perhaps stillchanging. In this part of Mars theMariner 9 pictures showed hugevolcanoes, great fields of lava, andcracks and fractures in the crust. Most

surprising, and most exciting toscientists, were huge canyons and


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winding, braided channels that

seemed to have been scoured byfloods of running water, although noliquid water can be seen on thesurface of Mars.

By the early 1970s Mars had beenrecognized as an "in-between" world,partly like the Earth, partly like theMoon, yet unique in many ways.

Viking to Seek Answers

The Viking mission would make amore thorough study to answer someof the questions raised by Mariner 9.For example, how old are the hugevolcanoes that Mariner 9 haddiscovered on Mars? It was clear thatMars, like the Earth and Moon,showed evidence of internal heat andvolcanic activity. But the Moon hasbeen dead and quiet for more than 3billion years, since the last floods oflava poured across its surface to formthe lunar "seas." The Earth, on theother hand, has been active for thesame length of time and is still activetoday. If the volcanoes of Mars areold, then Mars may be a dead worldlike the Moon. If the volcanoes areyoung (and to geologists, a fewhundred million years is "young"), thenMars may still be an active planet likethe Earth.

The winding channels carved acrossthe Martian surface are anothermystery revealed by Mariner 9. If thesechannels were cut by flood waters,then where has all the water gone? IsMars now in an ice age like those thatonce chilled the Earth? Was Mars'water now frozen away underground,waiting for a slight warming to bring itrushing forth again? The further studyof Mars might show us how the Earthhad started its long history of volcanic

activity, and we might even learn howclimatic changes and ice ages beginand end.

Moreover, where there are heat andliquid water, there may be life. "Howdid life start?" and "Is there life

elsewhere?" are basic questions that

we continue to ask as we exploreother planets. The scorched andwaterless Moon has yielded no traceof life. On Earth, the records of theorigin of life have been erased by thedevelopment and activities of laterplant and animal life forms. On Mars,where the environment for life isneither as harsh as the Moon's nor asgenerous as the Earth's we might find,still preserved, the answers to how lifecame into being. We might even findlife itself, answering one of our oldestspeculations. Whether Viking detecteda humanoid or invisible microbe, thediscovery of any Martian life would putus forever in a new relationship to theuniverse around us.

The Viking Spacecraft

The Viking Mission to Mars thuscombined two major goals: to studythe atmosphere and geology of theentire planet, and to analyze its soiland search for life in two specificlocations. Each of the two Vikingslaunched toward Mars in 1975 was adouble spaceship. One part, theOrbiter, would circle Marscontinuously, photographing thesurface of Mars and analyzing itsatmosphere from hundreds ofkilometers above the planet. The otherhalf, the Lander, would go down to thesurface of Mars, carrying a battery ofinstruments to probe directly aroundthe landing site. Once down, theLander would never leave. The Vikingswould not return to Earth with a load ofMartian rocks and soil. Instead theywould radio back to Earth theirdiscoveries about the atmosphere,chemistry, quakes, soil, and, perhaps,the life of Mars.

The landing of Viking 1 on thesurface of Mars was a complicatedand ambitious undertaking, moredifficult in some ways than landingastronauts on the Moon. Engineersknew that, at the moment of landing,Mars and Earth would be so far apartthat communications between theViking and its Earth-bound humancontrollers would take about 20minutes for a one-way trip. At such

distances, there is no possibility of

direct intervention if anything goeswrong. Once the computer on theViking spacecraft was given the orderto land, the landing went aheadautomatically, and the people on Earthcould only wait and hope.

Considering these difficulties, it wasnot surprising that Viking 1 wasactually the fourth attempt to land aspacecraft on the surface of Mars.Two Soviet spacecraft, Mars-2 (1 971)and Mars-6 (1973), apparentlycrashed while attempting to land. Athird Soviet spacecraft, Mars-3 (1971)soft-landed safely but stoppedoperating after less than 20 secondson the surface.

The Viking mission had two uniqueand important features designed tomake the landing successful. First, thelanding area was to be carefullyphotographed and inspected while thespacecraft stayed in orbit aroundMars. In addition, the actual landingcould be postponed until anacceptable site was found. Viking 1spent a month circling Mars while thecameras in the Orbiter portionphotographed possible landing sites ingreat detail and transmitted thephotographs back to Earth wherescientists examined them for roughground, boulders, and other possiblehazards.

The Viking photographs weresharper and more detailed thananything obtained during the earlierMariner missions. Landing sites thathad been considered safe becausethey seemed smooth and level in theMariner 9 pictures suddenly displayed

Figure 2. A Viking Robot Ready for Mars.A full-size worklng m odel of the ViklngLander sits on a simulated Martian surfaceat the -NASAJet Propulsion Laboratory.This spacecraft, ab out 1.5 meters (5 feet)across and about 0.5 meters (1.5 feet)high, weighs about 890 kilograms (1 ton).The soil sample collecting arm stretchesabout 3 meters (10 feet) to the lower right.The cameras are the vertical cylinders,each w ith a vertical black slit. The disk,top rear, is the S-b and high-gain radioantenna that transmits to Earth the camerapictures and scientific data. Equipmentand instruments are identified in theaccompanying diagram.


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Radar altimeterelectronics number 1

Radioisotope thermoelectri

Terminal desce

radar (underside ofLander structure) Furlable boom

Collector head

Magnets


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A Viking's-Eye View

deep gullies, scattered craters, andrugged outcrops of rock-no place toland a spacecraft which had only 22centimeters (8 12 inches) of groundclearance. While the photographswere being scanned, the large radiotelescopes at Arecibo, Puerto Rico,and Goldstone, California, bouncedradar waves off Mars, using the radarreflections to measure the roughnessof the planet's surface in differentregions. First one landing site wasrejected, and then another. Finally, anew site was located, photographed,scanned with radar, and foundacceptable, and Viking 1 descendedto a perfect landing in a level rolling

region called the Plains of Chryse. Alittle more than a month later, after asimilar thorough check of a morenorthern region of Mars, the Viking 2Lander made an equally flawlesslanding on the Plains of Utopia. TwoLanders sit on the surface of Mars,while two Orbiters circle overhead,photographing the planet and relayingback to Earth the news of what theLanders find.

The Landers on the surface of Marsare far more complex than anyautomatic spacecraft launched before.Even if the Landers had never leftEarth, their design and constructionwould still be an impressivetechnological achievement. Eachlander looks like a cluttered six-sidedworkbench with three legs (Figure 2)but it contains the equivalent of twopower stations, two computer centers,a TV studio, a weather station, anearthquake detector, two chemicallaboratories (one for organic and onefor inorganic analyses), three separate~ncubators or any Martian life, a scoopand backhoe for digging trenches andcollecting soil samples, and miniaturera~lroad ars for delivering the samplesto the laboratories and incubators.Equipment that would normally fillseveral buildings had been designedin miniature to fit on a spacecraft lessthan 3 meters (10 feet) across.Furthermore, to avoid contaminatingMars with Earthly bacteria, the entire

spacecraft was sterilized by heating itto temperatures above the boilingpoint of water. Each Lander, and all ofits 1 million separate parts, had tosurvive a number of major crises: thesterilization heating, the shock andvibration of launch, a one-year, 400-million mile trip through interplanetaryspace, the passage through Mars'atmosphere, and the landing on itssurface. No wonder there wereheartfelt cheers from the scientists andengineers when Viking 1's first picturesbegan to appear!

The Landers are so well designedthat it is often possible to fix themwhen things go wrong. When the

sampling arm on Viking 1 got stuck, acarefully-planned series of commandsfrom Earth freed it, and the camerasthen showed that a small pin whichhad caused the trouble had fallen freeto the ground. Later, when the armstuck again, this time in an extendedposition, a different series ofcommands brought it safely back intothe spacecraft. Each of these "repairs"was a carefully-planned operation.Each set of commands was first testedon a duplicate Viking Lander sitting ona simulated Martian surface at theNASA Jet Propulsion Laboratory, andthe cameras on the real Viking wereused to check the progress of the"repairs" at every step.

With the minor troubles corrected,the Landers even took on new tasksthat had not been planned before thelanding. After digging up samples ofexposed soil, the Lander's sampl~ngarm was used to push large rocksaside and to collect samples of theprotected soil beneath them (Figure 3).

The safe landing of Viking 1immediately established one basic factabout Mars: the planet's surface isstrong enough to support a heavymachine. The Lander rested firmly ona rolling plain strewn with rocks, andthe cameras on the Lander beganalmost immediately to transmit back toEarth the first views of the Martianlandscape.

Viking's cameras stood about 1.6meters (5 feet) above the ground, andtheir view of Mars was much like whata person standing in the same placewould see. The two cameras could beoperated independently to providepanoramas covering almost a full

circle around the Lander. They couldbe operated together to producestereo pictures from which the shapeof the surrounding surface could beaccurately measured (Figure 4). Mostof the pictures were black-and-white,but different detectors inside thecameras were sometimes used toprovide pictures that reproduced theactual hues of the Martian surface.

The first pictures showed firm soiland scattered rocks immediatelybeneath the Lander. As the cameraslooked out to the horizon, theyphotographed a gently rolling red

landscape that could almost havebeen a desert scene in the AmericanSouthwest. The reddish gray soil wasdotted with rocks of all sizes. Thecolors of the rocks varied from darkgray to light gray to slightly reddish(Figure 5). Some rocks showed up ingreat detail, and many were filled withbubbles. These rocks looked like thelavas produced by erupting gas-richvolcanoes on Earth, and scientiststhink that the bedrock on which bothVikings have landed is made up ofancient Martian lava flows.

The Viking cameras also saw wind-produced features that have familiarcounterparts in Earth's deserts.Although the atmosphere of Mars isth~n, ts winds are still strong enoughto blow dust and fine sand across thesurface. There are dunes of light-colored sand, and detailed pictures ofthe dunes revealed finer ripples withinthem (Figure 6). There are placeswhere the wind apparently scoured outthe fine soil, revealing flat masses of


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Figure 3. "Mr. Badger" Gets a Nudge.Controlled by scientists back on Earth, thesampling arm on the Vrkng 2 Landerreaches out to push aside a large porousrock and to collect a sample of theprotected Martian soil beneath it. Becauseof its shape, the rock was rnformallychristened "Mr. Badger" after a characterin Kenneth Grahame's book, The Wind mthe Wrllows. The Mart~an Mr. Badger" isabout 25 centmeters (10 ~nches) ong andwe~ghs everal pounds

Figure 4. Mars in 3-0 These two imagesof the same scene, one taken by eachcamera on the Vikrng 1 Lander, can becombined, by using a pocket stereoviewer, nto a single 3-drmensional viewthat shows the rolling Martian terrarn andthe shapes of the numerous boulders.(When lookng at the images with a stereovrewer, concentrate on a small object likea rock. Move the viewer around untrl thetwo rmages of the rock come together.Then you should see the landscape in R- d


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The Winds of Mars

grayish bedrock. At other places,streaks of dust have been deposited

over and behind boulders. WhenViking 1 first landed, some of thesepiles of red dust were being erodedand blown away by the summerbreezes th'at swirled around theLander.

Sometimes the Viking camerasturned away from the landscape tostudy in detail the mechanicalproperties of the ground on which theLander rests. The cameras carefullyphotographed the streaks produced inthe soil by the rocket engines whenViking landed. Later, as the soil wastrenched and probed and shaken tocollect samples, the cameras recordedthe appearance of marks, trenches,and clods of soil on the surface(Figure 7). By studying these pictures.scientists back on Earth were able todetermine that the Martian soil is about

Figure 8. A Crowd of Martian Rocks. Likea throng of curious onlookers, thousandsof rocks and boulders surround the Viking2 Lander as it rests on Mars' Plains ofUtopia. The field of rock and soil extendsto the horizon about 3 kilometers (2 miles)away. (The horizon, actually level, appearstilted because the spacec raft is resting o nthe surface at a slight angle to thehorizontal.) Many of the rocks displaysmall pits and holes that may be bub blesformed when the rocks were m olten lava.The rock in the lower right corner is about25 centimeters (10 inches) across, and thelarge rock in the center is about 60centimeters (2 feet) long. The small sandytrough that winds across the picture fromupper left to lower right is part of anunexplained network of such channels ordepress~ ons hat form strange polygonalsurface patterns in the Utopia region.

as firm as good farming soil on Earth.The soil of Mars sticks together in

about the same way, too: smallerparticles clump together into largerclods, and the walls of shallowtrenches remain straight and showlittle tendency to collapse.

Seen from the surface, the twoViking landing sites have theirdifferences. The Plains of Utopia,where Viking 2 landed, are morerolling than the Plains of Chryse whereViking 1 sits. The Viking 1 site (Chryse)apparently has a larger variety of rocktypes, while the rocks at the Viking 2locality (Utopia) are more uniform,generally vesicular (bubble-rich), andmore abundant. There is bedrockexposed at the Chryse site, and nonevisible at Utopia.

There are rippled sand dunes at theChryse location, and none at Utopia.The boulders at the Chryse sitecommonly have flat, polished faces,apparently produced by wind-blownsand.

The Utopia site (Viking 2) shows anunexplained pattern of shallow troughsthat connect to form polygonalpatterns. One of these troughs runsright past the Viking 2 Lander (Figure 8).

For the first time we can nowmeasure and record the weather on

another world. Unlike the airless Moon,Mars has an atmosphere, winds, andweather patterns.

Mars' atmosphere is thinner andcolder than Earth's, and scientistswere eager to study its weatherpatterns in the hope of finding generalprinciples that would help us betterunderstand the weather of our ownplanet. The Viking cameras oftenlooked above the horizon tophotograph the sky, and a battery ofinstruments recorded winds,barometric pressure, and the chemicalcomposition of the atmosphere of Mars(Figure 9).

Viking's first view of the skyproduced a major surprise. Althoughmany scientists had expected that theMartian sky would be blue like that ofEarth, the Viking pictures showedinstead that it has a creamy-pinkishhue (Figure 10). The explanation isthat the Martian atmosphere contains agreat deal of fine suspended red dust.


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Figure 9. " And Now, the Weather forMars." The first interplanetary weatherreports come from a small whiteInstrument box (upper r~g ht jmounted onthe end of a long boom that holds the boxabout 1.3 meters (4 feet) above theMartian surfac e. The boom holds the bo xout of range of most wind disturbancescau sed by the 'body of the V iking Lander.instruments En the box m easure the windvelocity, w ~n d irectton, temperature, andatmospheric pressure. In the backgroun dare sand dunes formed by strong Martianwinds. The parallel bands in the sky arenot real, they were produced by thecomputer processing of the picture.

Figure 10. A Red Sky for a Red Planet.The red surface of Maw lends its color tothe Martlan sky in this view from the Viking1 Lander. Fine red dust from the soil iscarried lnto the atmosphere, giving the skya pnkish hue instead of the blue colorexpected by scientists. Light and darkboulders are strewq on the surface in theforeground,an d light-gray ledges of

1 bedrock appear through She so11 n themiddle distance. The horizon, about 100meters (330 feet) away, may be the nm ofan impact crater. Thls color ptcture wasmade by combining three separateprctures, each taken through a differentcolor filter.The colafs were matched by

comparing similar pictures taken ofcolored objects on the VikingLander itself.


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Although the individual dust particlesare tiny, perhaps only 0.001 millimeter

(1125,000 inch) across, there isapparently enough of this dust in theair to give the whole sky a reddish tint.Earth's sky is generally not so dusty.Only after large volcanic eruptions orsandstorms do we see a reddening ofterrestrial sunsets that producessomething like the color of the Martiansky.

The sky of Mars grew even dustierseveral months after the Vikingslanded, and tlie spacecraft carefullyrecorded the change.

Astronomers have known for yearsthat huge dust storms often comeswirling out of the southern part of'Mars, covering the whole planet andshutting off the surface from the viewof Earth-based telescopes. Such astorm shrouded Mars in 1971 asMariner 9 arrived in orbit, and thespacecraft was able to provide aphotographic record of the storm'ssubsidence and the gradualappearance of the Martian surfacethrough the clouds of dust. These duststorms usually develop as Marsreaches the point in its orbit that isclosest to the Sun, and in the Spring of1977, these clouds arose again andspread over the Martian surface. Highabove the storms, the Viking Orbitercameras photographed the shapes ofthe dust clouds and followed theirprogress. With these data, scientistsare learning more about Martian windsand about the nature of the dust thatthey carry.

After Viking landed, the Martianweather was clear, cold, uniform, andrepetitious. The weather report,recorded by the Viking instruments

and broadcast to Earth on the firstday, remained almost unchanged from

day to day:"Light winds from the East in the late

afternoon, changing to light winds fromthe Southeast after midnight. Maximumwinds were 15 miles per hour.Temperature ranged from minus 122"Fahrenheit just after dawn to minus 22"Fahrenheit in midafternoon.Atmospheric pressure 7.70 millibars."

(On Earth the same day, July 21,1976, the lowest temperature recordedwas minus 100 degrees Fahrenheit atthe Soviet Vostok Research Station inthe Antarctic, and the highesttemperature was plus 1 1 7" F atTimimoun, Algeria. The United Statesrecorded a high of 109" F. at Needles,California and a low of 37" F. at PointBarrow, Alaska.)

Some of the similarities betweenMars' weather and Earth's weresurprising, because the atmos'phereof Mars is less than a hundredth asdense as Earth's. Nevertheless, onboth planets, the atmospherictemperature reached its peak at about3 P.M. local time. The dailytemperature variations recorded byViking showed the same pattern asrecords from a terrestrial desert"control" site at China Lake, California,although the temperatures in the twoplaces differed by more than 83" C.(150" F.). Furthermore, the changingpatterns of wind direction over the flatPlains of Chryse on Mars wereduplicated by the winds blowing overthe equally flat Great Plains of themidwestern United States.

Martian weather includes two otherfeatures familiar to terrestrial weatherwatchers--clouds and fog. The air ofMars contains only about 111 000 asmuch water as Earth's atmosphere,but even this small amount cancondense out, forming clouds that ride

high in the atmosphere or swirl aroundthe high slopes of Martian volcanoes

(Figure 11). In small valleys,atmospheric water freezes out duringthe Martian night and then vaporizesagain when the sun rises, forminglocal patches of white fog that vanishquickly in the relative warmth of theMartian day (Figure 12).

Totally unlike the Earth, however,was the steady decline in atmosphericpressure recorded by the Vikinginstruments. During the Lander's firstmonth on Mars, the atmosphericpressure dropped by about 5 percent.(On Earth, such a large drop inpressure is usually found only in theeye of a major hurricane.) Scientiststhink that the carbon dioxide (CO,)which makes up most of Mars'atmosphere was freezing out as solidCO, (or "dry ice") on the cold southernpolar cap, which was then in themiddle of the Martian winter. TheViking landers thus seem able, fromtwo points on the surface, to detectthe slow growth of an entire polar capthousands of kilometers away, a featthat would be impossible in thecomplex water-rich atmosphere ofEarth.

While one group of scientistsfollowed the changes in Mars' weather,an entirely different group was busyanalyzing the chemical composition ofthe atmosphere itself. The gases in aplanet's atmosphere can come frommany different sources. Some gasesmay have been trapped from theoriginal solar nebula when the planetformed. Others may have beenreleased by heat and chemical


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Figure 11 . Martian Volcano TowersAbove the Clouds. In this Viking 1 Orbiterphotogr aph taken from 8 ,000 kilometers(5,000 miles) away, clouds cover the lowerslopes of Mars' largest volcano, OlympusMons (Mount O lympus), making it look likea satellite picture of a terrestrial hurricane.The huge mass of the volcano is 600kilometers (375 miles) across, and thecliffs that mark its edg e can b e seen in theupper right corner. The summit stands 24kilometers (15 miles) abov e the Martiansurface, and the summit crater, visibleabove the clouds, is 80 kilometers (50miles) across. The clouds in the upper left


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reactions deep inside the planet. Stillothers may be produced by thetransformation (decay) of radioactiveelements in the planet's rocks. Thechemistry of an atmosphere thusprovides unique information about aplanet's origin, its history, and thechemical composition of its rocks.

Earth's atmosphere is composedalmost entirely of two gases: nitrogenand oxygen. Scientists believe that thenitrogen (78 percent of ouratmosphere) came out of the inter ior ofthe Earth billions of years ago, whilethe oxygen (21 percent) has beenproduced gradually by the plant lifethat has existed for billions of years. Asmall amount of the inert gas argon

(0.9 percent) has been formed by thedecay of radioactive potassium atomsin the Earth's crust.

The composition of the Martianatmosphere was measured in twoplaces-at high altitudes as theLander descended, and on thesurface. The composition was thesame in both places, showing that theMartian winds keep the atmosphere aswell-mixed as Earth's.

The pressure of Mars ' atmosphere isonly about 11125 that of Earth's and itschemical composition is totallydifferent. Most of Mars' atmosphere

(95 percent) is carbon dioxide, a gaswhich makes up only 0.03 percent of

Figure 12. Foggy Morning in a MartianValley. White patches of early-morning fogand mist fill a rugged network of Martiancanyons and spill out onto the surroundinghigh, rust-colored plateau. The clouds areprobably formed by water vapor that hasfrozen out of the air during the previousMartian night. In the sunlight, the watervaporizes again, becom ing briefly visibleas mist before being absorbed into the dryatmosphere. This part of M ars, calledLabyrinthus Noctis (The Labyrinth of theNight) was photographed at dawn by theViking I Orbiter; the view covers an areaabout 100 kilometers (62 miles) on a side.The color picture was mad e bysuperimposing three separate black-and-white images taken through color filters.

Earth's atmosphere. The remainder isnitrogen (2-3 percent) oxygen (0.1-0.4percent), and argon (1-2 percent). Thediscovery of nitrogen was excitingbecause this element is an essentialcomponent of the protein moleculeswhich form living things. The smallamount of free oxygen is surprising,but this element can be formed inmany ways, arid its presence does notprove that there is or has been plantlife on Mars.

More precise analyses havedetected traces of the rare inert gaseskrypton and xenon in the Martian air.These two gases make up only about1 part per million of the Earth'satmosphere, and scientists have not

yet been able to measure precisely thetiny amounts present in the air of Mars.

Viking instruments also measuredthe ratios of different isotopes in theMartian atmosphere. (Isotopes are twoatoms of the same chemical elementthat have different atomic weights, forexample uranium-235 and uranium-238.) Isotope ratios of elements in theatmospheres and rocks of otherplanets are important because theyprovide information that cannot beobtained from chemical analysesalone. Isotope measurements canindicate the temperature at which

rocks formed; if two different planetshave similar isotope ratios, then theymay have formed from the same partof the original solar nebula.

The isotope ratios measured by theViking Lander show that theatmosphere of Mars is more Earthlike

than the chemical composition alonewould suggest. The ratio of heavy tolight carbon atoms (carbon-13 tocarbon-12) is 1/89, and the ratio ofheavy to light oxygen atoms (oxygen-18 to oxygen-16) is 11500. Thesevalues are identical to those measuredin our own atmosphere. However, theelement nitrogen is different. The ratioof heavy to light nitrogen (nitrogen-15to nitrogen-14) is 11156 on Mars, whilethe value on Earth is 11271.

The carbon and oxygen ratiosdemonstrate a basic similarity betweenMars and Earth, despite the chemicaldifferences in their atmospheres. Oneexplanation is that both Mars andEarth formed from, similar parts of the

solar nebula which had the sameisotope ratios.

However, the isotope ratios ofnitrogen provide evidence for differenthistories of this element on the twoplanets. If the original nitrogen ratio onMars had been the same as on theEarth, then the light atom (nitrogen-14)must have gradually escaped from theatmosphere of Mars, possibly becauseMars' gravity is not as strong asEarth's.

From these atmospheric data,scientists have calculated that theancient atmosphere of Mars, before


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the nitrogen was lost, might have beenfour or five times as dense as it isnow. This early atmosphere also mighthave contained enough water to forma layer several meters deep over thewhole surface of the planet. Here wasanother indication that the windingchannels on Mars actually had beencarved by water, although theatmospheric analyses could not tell uswhere this water had vanished.

The Chemistry of Mars

While the atmosphere of Mars wasgiving up its secrets, other scientistswith other instruments began to testthe solid matter of the planet, to seewhat could be deciphered from therocks and windblown dust around thespacecraft.

On the eighth day of Viking 1 'sresidence on Mars, a long armreached out from the spacecraft, anda small scoop at the end of the armbegan to dig a small trench in theloose soil about two meters away fromwhere the Lander stood (Figure 7).Continually guided by a computeraboard the Lander, the arm carefullypushed the scoop through the trenchand then retreated slowly back to the

Lander, bringing with it the first sampleof Martian soil ever to be analyzed.Within the Lander, the soil sample wassieved automatically, divided, and senton its way for several diffe rent kinds ofanalysis.

One test of the soil did not require achemical laboratory. Several magnetswere mounted on the scoop, andanother magnet had been p laced onthe outside of the Lander. Thesemagnets trapped and held magneticparticles in the soil and windblowndust. By simply examining thesemagnets with the Viking cameras nowand then, the amount of magneticmaterial in the Martian soil could bemeasured. Early results suggest that

about 5 per cent of the soil ismagnetic material and that it is an iron

oxide like magnetite (Fe,,OJ, themineral that forms terrestriallodestones. This result makes Marsseem rather Earthlike; the lunar soil, bycontrast, has only about 1 per cent ofmagnetic material, and it is all metalliciron.

More precise measurements of thesoil were made with an instrument thatbombarded a soil sample with X-raysand then measured the secondary X-rays given off by the atoms in theMartian soil.

The composition of the Martian soil,as determined by the bombardmentexperiment, is approximately the sameat both landing sites, even though thetwo sites are about 5000 kilometers(31 00 miles) apart.

The chemical elements detected,and their amounts (in weight percent),are: silicon (Si) 21, iron (Fe) 13,aluminum (Al) 3, magnesium (Mg) 5 ,calcium (Ca) 4, sulfur (S) 3, chlorine(CI) 0.7, titanium (Ti) 0.5, andpotassium (K ) less than 0.25.Scientists calculate that, to balancethese elements, oxygen ( 0 ) makes upanother 42 per cent of the soil, leavingabout 8 percent made up of elements(e.g. ; sodium, hydrogen) that cannot

be detected by this method.This composition correspondsapproximately to that of a terrestrial orlunar basalt lava, but there are somestriking differences. The Martian soilcontains less aluminum than aterrestrial basalt and less titanium thana lunar basalt.

The unusually large amount of irondetected confirms the long-held theorythat the red dust of Mars is a red ironoxide similar to terrestrial rust. The redcolor and the small amount (about 5per cent) of magnetic material suggest

Figure 13. The Ancient Crust of Mars.Mars shows a battered and heavily-cratered Moon-like surface in this picturetaken from the Viking 1 Orbiter from18,000 kilometers (1 1,200 miles) away.The flat c~rcular lain at top left is Argyre,a large impact basin about 800 kilometers(500 miles) in diameter and located in thesouthern part of Mars. This basin,surrounded by a rugged range ofmountains, may have been formed by ahuge meteorite impact billions of years

ago. Smaller, younger craters cover theMartian surface outside the basin. The airis clear and cloudless over Argyre, but thebrightness of the distant Martian horizon(top right) suggests that clou8s arepresent there. The parallel white streaksabove the horizon are also clou d layers,perhaps composed of frozen C o p heseclouds are about 25 to 30 kilometers (15to 20 miles) above the surface of Mars.

Figure 14. A View Down a Volcano'sThroat. From 6,000 kilometers (3,700miles) up, the cameras of the Viking 1Orbiter provide a vertical view of ArsiaMons, one of Mars' largest v olcan oes. Thevolcano reaches about 19 kilometers (12miles) above the surrounding Martianterrain, more than twice as high as Earth'sMount Everest. The crcular central area inits summit is about 120 kilometers (75miles) acros s. Around this summit crater,the slopes of the volcano are covered withlava flows that produce distinctive braidedpatterns seen clearly at the bottom of thepicture. At left and right, small craters andcanyons cut into the main cone of thevolcano; these features may be thesources of vast amounts of lava thatspilled out to flood the surrounding plains.


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that the iron must be present in two ormore distinct m~nerals, nly one ofwhlch is magnetic.

The chemical analyses rndicate thatthe Martian soil cannot be made offresh basalt lava alone. The presenceof water (perhaps as much as 1 percent) in the soil, the large amount ofiron, and the unusually large amount ofsulfur, all indicate that the soil is amrxture of original basalt lava withother compounds that have formed asthe rock has been changed or"weathered" by contact with theatmosphere of Mars. The soil could bea mixture of iron-rich clay minerals andother compounds such as Ironhydroxide and magnesium sulfate.

The soil of Mars is much moresimilar to Earth's soil than to the soil ofthe Moon. The lunar soil, as we havelearned from the samples returned bythe Apollo missions, is waterless,unweathered, and formed by thecontrnuous bombardment of large andsmall meteorites. Martian so11 eemsalmost terrestrial, it contains water, itseems to be weathered, and it iscontinually blown about andredistributed by the wind.

Three Chances for Life

A major goal of the Viking missionswas to determine whether the soil ofMars was dead like the soil of theMoon or teeming with mlcroscoplc lifelike the soils of Earth. Soil samplesbrought into the Lander were dividedand sent to three separate biologicallaborator~es o be tested in differentways for the presence of life.

Searching for life on Mars raises abasic problem, best summed up as:"How do you look for life if you don't

know what life looks like?" It was notpossible to build, on one smallspacecraft, enough instruments todetect all the poss~ble orms of life thatsclentrsts could imagine to exist onMars. Before bullding the instruments,the scientists had to make somedecrsrons about what the instrumentsshould look for.

The Viking experiments weredesigned around two assumpt~ons.First, rt was assumed that Martian lifewould be like Earth life, whrch is basedon the element carbon and thr~ves bytransforming carbon compounds.Second, the example of Earth showsthat where there are large life forms(like human beings and elephantsj,

there are also small ones (likebacteria), and that the small ones arefar more abundant, w~th housands orm~llions f them ~n every gram of soil.To have the best possible chance ofdetecting life, an Instrument shouldlook for the most abundant kind of Irfe.If a Martian verslon of Vrking were sentto Earth to look for life, it might easrlyland in a place where there wereneither elephants nor humans, but itwould be very unltkely to land in aplace where there were no bacteria rnthe soil.

The Vik~ng nstruments were

designed, therefore, to detect carbon-based MartIan microbes or s~milarcreatures living ~n he soil. The threelaboratories in each Lander wereessentially incubators, des~gned owarm and nourish any life, living ordormant, in the Martiansoil and todetect with sensitive instruments thechemical products of the organisms'activrty.

One characteristic of terrestrialorganisms such as plants is that theytransform carbon dioxide (CO,) in thesurrounding air into the organlccompounds which make up their roots,branches, and leaves. Accordingly,one Viking biological experiment,designated carbon assimila t~on orpyrolytic release) added radioactive

CO, to the confined atmosphereabove the soil sample. The samplewas then illuminated with simulatedMartian sunlight. If any Martian life-forms converted the CO, into organiccompounds, the compounds could bedetected by their radioactivity.

Lrving terrestrial organisms give offgases. Plants give off oxygen, animalsgive off carbon dioxide, and bothexhale water. A second experiment oneach Lander, the gas exchangeexperiment was designed to detectthis kind of activity. Nutrients andwater were added to the soil, and thechemical composition of the gasabove the soil was continuouslyanalyzed for changes that mightIndicate biological act~vit y.

A third experiment on each Landerwas based on the fact that terrestrialanimals (including humans) consumeorganic compounds and glve offcarbon dioxide. The labeled releaseexperiment added a variety of radio-active nutrients to the soil, then waitedto see if any rad~oactrve CO, (der~vedfrom consumption of this "food") wouldbe given off.

Several soil samples wereprocessed by all three instruments oneach Lander. The results? Puzzling.There is definitely some form of act~vity

In the Martian soil, but it is not yetclear whether this activity 1s caused byMartian life or by some unusualchemical characterist~c f the soilitself.

Viking has glven us some chemicalinformation about the Martian soil, but


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Figure 15 . Landslides Fill Martian"Grand Canyon". This Viking I Orbiterpicture, taken from a range of 2.000

kilometers (1,240 miles) shows a smallsegment of the Valles Marineris ("MarinerValley"), a h uge ga sh that runs east-westfor almost 5,000 kilometers (3,000 miles)along the equatorial region of M ars. Thispart of the canyon is more than 50kilometers (30 miles) across a nd 2kilometers (1 .3 miles) de ep. The aprons ofdebris on the canyon floor show how thecanyon widens as its walls collapse andproduce immense landslides. The largeapron in the center has overridden andpartly covered an older landslide depositto the left. The lines in the depositsindicate the direction in which thematerial flowed after breaking away fromthe canyon walls. Whife streaks in the

middle of the canyon are featuresproduced by winds blowing along thelength of the canyon. Upper walls of thecanyon provide a cross-sectional viewthrough the different rock layers that coverthis part of Mars; hard, resistant rocks(lava flows) at the top overlie less durablerubble (wind-blown dust or volcanic ash)below. Dark circle near center isphotographic flaw.

Figure 16 . The Vanished Rivers of Mars.Cameras in the Viking I Orbiterphotographed this maze of wanderingchannels that cut across the terrain westof the Viking 1 landing site. The surfaceslopes downward, dropping about 3

kilometers (2 miles) in elevation from left toright. Flood w aters once poured acrossthis region from left to right, cuttingthrough a high ndg e (right) to pour out intothe plains to the east. Older craters werecut, filled, and eroded by this flood.Younger craters, formed after the floo d,show sharp outlines. The fate of thesetorrents is unknown; the water may now befrozen as ice in the polar caps or aspermafrost in the Martian soil. Rows ofdark circles are photographic flaws.


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we still do not know enough about itsnature to predict what reactions willoccur when water and nutrients areadded to it in Viking's biologicallaboratories. The Martian soil maycontain many unusual and unexpectedchemicals, possibly formed by therepeated blasts of ultraviolet radiationfrom the Sun that penetrate the t h ~ natmosphere of Mars and blanket thesurface of the planet. Earth's soils arenot affected in this way, because theSun's ultraviolet light is absorbed byour denser atmosphere, and thechemistry of Martian soil could verywell be unpredictably different fromthe soils of our own planet. Even ifMartian soil is completely lifeless, it is

, possible that some reactions with theadded water and nutrients areimitating biological activity.

Because of these uncertainties,scientists are being cautious in theirinterpretations of the biologicalexperiments, even though many of theresults resemble those from testsmade on terrestrial soils rich ~n ivingorganisms.

The carbon assimilation experimentshowed that a small amount of CO,had been converted into carboncompounds, but this conversion couldhave been accomplished by somereducing agent in the soil, such asmetallic iron.

In the gas exchange experiment,both oxygen and CO, were given offwhen water was added to the soil.However, these reactions could havebeen caused by the decomposition of

oxygen- and carbon-rich materials thatoriginally had been produced in thesoil by ultra-violet light.

Finally, the labeled releaseexperiment showed a rapid release ofradioactive CO, that at first seemed tobe caused by biological activity. Butthe release quickly slowed down,suggesting that some chemical in thesoil was being rapidly used up,whereas a biological reaction shouldhave continued as the organisms grewand multiplied.

One problem with a biologicalinterpretation of these reactions is thatanalyses of Martian soil by anotherViking instrument have detected noneof the organic carbon molecules that

make up living things. It is hard tounderstand how these chem~calreactions could be caused by Earthlikemicrobes that leave no other trace,living or dead, in the Martian soil.

At the moment, we know that thereare reactive ingredients in the soil ofMars, but it will take more experimentsand more examination of the Vikingdata before we know just what theyare. As this work goes on, theseparate Viking experiments supportand reinforce each other, each oneproviding data to help interpret theresults of another. The chemicalanalyses of the soil, made by X-raymethods, are used to help interpret thepuzzling results of the biologicalexperiments. The instrument that haslooked in vain for organic carbonmolecules has also measured theamount of such inorganic gases aswater and sulfur dioxide in the soil.When these data are combined andevaluated, we may have some more

definite answers about the chemicals,the minerals, and, perhaps, any lifeforms in the red soil of Mars.

From Mars to Einstein

Late in November, 1976, Marspassed behind the Sun, andcommunications between Earth andViking were cut off until mid-December, when Mars appeared onthe other side of the Sun. As Marspassed behind the Sun, the Vikingspacecraft carried out a majorexperiment to study, not Mars, but thebasic nature of the universe itself.

The spacecraft s'ignals from Marsmade it possible for scientists on Earthto carry out the most accurate testever performed of Einstein's Theory of

Figure 17 . Islands in the Stream Theraised rims of these Martian craters seemto have acted as barriers to floods ofwater that poured across the surface ofMars in the past. The upstream (lower left)sides of all the craters seem erode d, withstreamlined islands left on the downstream(upper right) side. A curiously shapedejecta deposit still preserved around theuppermost crater may have been abovethe level of the floo ds. T h~ s pectacularscenery, photographed by the Viking 1Orbiter from 1600 kilometers (1000 miles)above Mars, is located near the Viking 1landin g site on the Plains of C hryse. (Smalldark rings in the picture were caused by aflaw in the camera.)

Figure 18. The Source of the Flood? Thisstrange Martian valley, more than 50kilometers across, shows a striking changefrom a ch aotic, hilly floor at its hea d (right)to a narrower an d more streamlined shape(left). One explanation is that water, frozenbelow the Martian surface, suddenlymelted and ran out, causing the ground tocollapse and producing a short-livedtorrent that eroded the downstream part ofthe valley. Such "co llapsed terrain" iscomm on in this part of Mars; numerouslarge and small impact craters can also beseen. A smaller valley, possibly produ cedby a smaller flood, is visible near the largeimpact c rater at the top of the picture. Thispicture was taken by the Viking 7 Orbiterfrom a distance of 2300 kilometers (7900miles). (The small dark rings in the pictureare caused by a flaw in the camera.)


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General Relativity. The theory, whichexplains gravitation and the

relationships between space and time,predicts that light waves (or radiowaves) will be slowed down as theypass close to a large and massiveobject like the Sun. Precisemeasurement of the de lay in radiotransmrssion from the Vikingspacecraft as Mars went behind theSun would test whether the GeneralRelativity Theory was cor rect orwhether some competing theory was abetter explanatron of how our universeworks.

On the day of the experiment,November 25, 1976, Mars was about321 million kilometers (200 million

'

miles) from Earth, and the radiosignals took about 42 minutes to makethe round-trip. But the timing andsignalling devices used in Vikingcommun~cations re so accurate thatthe transmission time could bemeasured to one ten-millionth(0.0000001) of a second. With suchaccuracy, rt was not difficu lt todetermine that the radio signals fromMars had been delayed by a full twoten-thousandths (0.0002) of asecond-exactly the delay predictedby the Theory of General Relativity.

This Viking relativity experiment was

also the most accurate measurementof distance ever made; the 321 million-kilometer Earth-Mars distance wasdetermined with an accuracy of about1.5 meters (5 feet)!

What Next?

Although most of the Vikrngexcitement was concentrated in thefirst few months after the landings, it islikely that the Viking Orbiters and

Landers will operate for a year or two,sending back photographs and other

information from Mars. Each Landerhas a long-lived nuclear power source,and each Orbiter gets electricity fromlarge solar cell arrays backed up bytwo nickel-cadmium batteries.

Scientists are eager to use theLander instruments to follow theweather patterns at Chryse and Utopiathrough the Martian fall and winter,and into the spring when the time ofplanet-wide dust storms is thought tobegin. A complete weather record ofthe Martian year, which is two Earth-years long, would be a uniquedocument that could lead to betterunderstanding the weather and climateon Earth and other worlds.

Geologists are also eager for a longperiod of Viking data. The VikingLanders carry seismometers to aetect"Marsquakes" so that scientists candetermine whether Mars is active likethe Earth or dead and quiet like theMoon. Unfortunately, the instrument onViking 1 did not operate.

Since Viking 2 landed in September,1976, ~ ts ensitive seismometer hasbeen steadily recording the tlnyvibrations caused by the wind and themechanical devices on the spacecraft.

A distinctive "event" in early

November, 1976, may have been aquake with a Richter magnitude of 6.4,fully as large as the major SanFernando earthquake that struck theLos Angeles area in 1971.

Overhead, the two Orbiters continueto take pictures of the surface of Marsand to measure the amount of watervapor in the Martian air and thetemperatures of the Martian surface.

Even before the Landers toucheddown, the Orbrters had provided newhigh-resolution photographs of themajor features of Mars: circular basinsand mountains (Figure 13), hugevolcanoes (Figure 14), great canyonsand landslides (Figure 15), mazes of

Figure 19. Splat? The crater Yuty, 18kilometers (1 1 miles) in diamete r, looksentirely different from the numerous cratersthat cover the Moon and Mercury. Thehigh central peak inside this crater, andthe scalloped blanket of ejected materialaround it, make Yuty look like a large-scaleverslon of a crater formed by throwing apebble into thick mu d. One possibleexplanation for this resemblance is thatlarge quantities of frozen water beneaththe Martian surface were instantaneouslymelted by the heat produced by a largemeteorite impact. As a result, huge "m udavalanches," made of water and brokenrock, poured out of the crater to form thecuriously-shaped blanket around it. Thisview was taken by the Viking 1 Orbiterfrom a rang e of 1877 kilometers (1 165miles). (Yuty is named for a village inHonduras.)

Figure 20. Cracks in the Martian Crust?This part of Mars, west of the ArgyreBasin, is cut by numerous parallelfractures (faults) thar run for hundreds ofk~lome ters hrough circular craters and flatplains alike. The dominant set of fractures(top left) runs from lower left to upperright, but other fractures run in otherdirections (see lower right). The fracturesmay be the surface effects of slowmovements in the interior of the planet.The small fan-shaped channels (topcenter) may have been cut by runningwater at some time in the past. The light-colored region (top right) is part of a frostdeposit associated with Mars' south polarcap.


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winding, water-cut channels (Figures16, 17, and 18), and craters with

curiously scalloped deposits aroundthem (Figure 19). Large areas of Marsare covered with strange patterns offractures and joints in the bedrock(Figure 20), and with unexplainedpolygonal mark~ngs hat resemble, ona large scale, the permafrost or"patterned ground" of Earth's Arcticregions.

On September 30, 1976, the orbit ofthe Viking 2 Orbiter was shifted so thatthe spacecraft could swing over thepolar regions of Mars. This maneuvermade it possible to study in detail themysterious white polar cap s that wereone of the first features of Mars to beseen through Earth-based telescopes.

The Orbiter found that the amount ofwater in the atmosphere varies greatly;it is almost zero near the south (winter)polar cap, then increases dramaticallyas one moves northward into thenorthern (summer) hemisphere ofMars. Over the north polar cap itself,the atmospheric water contentdecreases and the instrumentsindicate that the surface temperaturesare about -60" C. (-96" F.).

Cold as these temperatures are,they are above the freezing point ofCO, in the Martian atmosphere, and

scientists are now sure that thepermanent polar caps on Mars aremade of water ice instead of frozenCO, ("dry ice"). The polar caps thuscontain a large reservoir of the waterthat may have cut the channels onMars in an ancient and warmer time.

The cover of ice is not continuous,and the northern polar cap is cut bysteep-sided ice free valleys (Figure21). High-magnification pictures of thevalley walls (Figure 22) revealed tosurprised scientists that the "bedrock"

beneath the polar ice is composed oflayer upon layer of what may be

windblown dust. Here under the polarcap may be preserved the records ofthe changing climates of Mars duringthousands or millions of years in thepast. Some individual layers, as muchas 50 meters thick and coveringhundreds of square kilometers, mayhave been deposited by hugesandstorms far more violent than anyobserved on Mars today.

When the Orbiters finish their task,much of Mars will be photographed,mapped, and studied in great detail,and future missions to Mars will beplanned with better map s than manyterrestrial explorers have had.

The Vikings also explored otherworlds near Mars. In February, 1977,the Viking 1 Orbiter made two closeapproaches to Phobos, one of the twomoons that circle Mars. From asclose as 120 kilometers (75 miles)away, the Viking camerasphotographed the irregular, crateredsurface of Phobos in such detail thattiny craters and mounds a few metersacross can be seen in the pictures(Figures 23 and 24). Many scientiststhink that Phobos, which is only 20kilometers (1 2 miles) in diameter, is anasteroid that was captured by Mars atsome time in the past. If they are right,the Viking cameras have given us ourf~rst lose look at what we will findwhen we venture beyond Mars into themillions of tiny bodies that occupy theAsteroid Belt itself.

Even as the Viking data continue toflood in, there are active discussions

Figure 21. The Land of the (Martian)Midnight Sun. The north polar region ofMars is displayed by the camera of the

Viking 2 O rbiter as the spacecraft passedover the Martian Arctic for the first time inOctober, 1976. Broad regions of white iceare broken by darker slopes and valleyscut into layered rocks that underlie theicecap itself. Individual rock layers appearas curv ed parallel lines that follow thecontours of the slopes and valleys.Measurements of 'the surface temperature,made by instruments carried on theOrbite r, indicate that the ice is frozenwater, not frozen carb on dioxide. Top tobottom of the picture is about 360kilometers (225 miles). The Martian northpole is abo ut 300 kilometers ( I 70 miles)beyond the top of the picture. (Just likethe Earth's Arctic regions in summer, this

part of Mars also has a "midnig ht sunM--ortotal dayligh t--beca use the inclination ofMars' axis and the length of the Martianday are almost identical to Earth's.

Figure 22. A Valley that Cuts into thePast. Walls of a deep valley, eroded in theMartian north polar cap, display thelayered deposits of rock or windblowndust that underlie the polar ice itself.Individual layers as little as 50 meters (165feet) thick can be detecte d, even thoughthe picture was taken from about 2200kilometers (1370 miles) away. The differentlayers may record many past changes inthe Martian climate, by the samemechanisms that produce the changingice ages on the Earth. Dark smu dges onthe ice surface may be recent deposits ofwindblown d ust. This closeup view, takenin Martian mid-summer (Octo ber, 1976) bythe Viking 2 Orb iter, shows an area of thepolar ca p about 60 by 30 kilometers (37by 18 miles). Water ice (white) covers ahigh, level plateau, and the steep wall ofthe valley (top) drops about 500 m eters(1650 feet) from the ice layer to thebottom. This color picture was made bycombining black-and-white pictures takenthrough three different color filters.


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about foilow-up missions to Mars thatcan now be planned on the basis of

what we have already learned. For allthat Viking has done, it is only abeginning; what we have learned fromthe robots on Mars is still not muchmore than we had learned from therobots (Surveyor spacecraft) that wesent to the Moon before the firstastronauts landed there. We know thatthe surface of Mars will support theweight of machines and humans. Wehave the first rough chemical analysesof the soil. We have taken pictures ofthe surface and dug trenches in it.And we can now make excellent mapsof the planet and pick the sites forfuture landings.

To send astronauts to Mars wouldbe a major undertaking. Not onlywould a manned mission requireextensive technological developments,but there are serious medicalproblems involved in keeping the crewphysically fit during a two-year trip inzero gravity.

For the near future at least,machines must do our exploring forus. One possibility would be a robot"rover" that would land on Mars andthen drive across its surface, makingchemical and biological analyses as itwent. Another possible mission would

involve a new kind of Orbiter aroundMars, one that would carry instrumentsto measure the chemical compositionof Mars' surface, just as instrumentscarried on the Apollo spacecraftmapped the chemistry of nearly one-quarter of the Moon. From this Orbiter,probes could be dropped to thesurface, carrying instrumentsespecially designed to survive the

shock of a hard landing . In this way anetwork of instruments could be

place d on Mars to give us globalcoverage of the planet's chemistry,Marsquakes, and weather.

More ambitious, but entirely withinour abilities, is a more complex robotthat would land on Mars, collectsamples of rocks and soil, and returnthem to Earth, where they could bestudied directly with the resources ofall of Earth's laboratories. Only in thisway can we make the thousandnecessary analyses that are toocomplex to be made by machines onthe surface of Mars. Only with suchreturned samples can we determinewith confidence the ages of the rocks,the minerals that compose them, theircomplete chemical composition, andthe weathering they have undergone.With instruments that are nowavailable, we could finally establishbeyond doubt whether such returnedsamples contain any Martian life. Withthe experience of a decade in space,and w ~ t h he knowledge gained fromsampling the Moon, we can collect,preserve, and analyze such samplesfrom the surface of Mars whenever wechoose.

The Vikings have become a bridgeinto the future. When the Landers have

sent their last data back to Earth, theywill remain like monuments on thesurface of Mars, waiting silently untilnew machines, and finally humanbeings, come to stand beside them.(Figure 25.)

Figure 23. A Battered Moon of Mars.Even tiny craters in the surface of Mars'innermost m oon, Phobos, are captured lnthis photograph taken by the Viking 1Orbiter cameras on February 18, 1977. Totake this plcture, the spacecraft came asclose as 480 kilometers (300 miles) to thetiny moon, photographing features assmall as 20 meters (65 feet) across.Phobos 1s elliptical ~nshape; the top-to-bottom diameter is 19 kilometers (12miles), but diameters ~ nother direct~onsare 21 kilometers (13 miles) and 27kilometers (1 7 miles). Be cause of itsirregular shape and an c~e nt, rateredsurface, scientists think that Phobos maybe an asteroid that was captured by M ars,possibly billions of years ago. A largecrater at the lower right is named Hall afterthe American astronomer who discoveredthe two moons of Mars in 1877. Theragged appearan ce at the right slde isproduced by shadows on the unlit parts ofPhobos' ~rregular urface.

Figure 24. A Moon About to Break?Parallel 11nes of fra ctur es an d c rate rsextend across the whole surface of Mars~ n n e rmoon Phobos Some sclentlststhlnk that the whole moon IS graduallybreaklng up from the Impacts of largemeteror~ tes nd from the tldal forcesproduced dur ing ~ t sotations around MarsIn the future, Phobos may dlslntegratecompletely into small fragments, formlng anng arou nd Mars 11ke he fam111ar ~n gs fSaturn or the s~mllar ~ng s ~scov eredrecently around the distant planet UranusThls plcture was taken by the Vlk~ngOrblter as lt passed wlthln 300 kilometers(200 mlles) of Phobos on May 27, 1977The picture on the right 1s the onglnaldata, the left-hand picture w the computer-processed vers~on


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Figure 25. A Viking Sees a Sunset on aNew World. The Sun has set on Mars, buta bngenng twil~ght nngs a reddrsh glowto the Martian surface (left) and to the topof the Vlklng I Lander (lower nght) The11ght of the Sun 1s scattered by red dust ~nthe atmosphere, colonng the surface andproducing a reddlsh color ln the sky wherethe Sun has set Near the Sun, the plcture1s overexposed, and that part of the skyappears whlte (upper nght) The colorednngs around the whlte spot are not real,they are produced dunng the computerprocessing of the camera's pictures Ahuman eye. looklng at the same scene,would see a black nlght sky, grad~ngun~formly nto a reddlsh glow where theSun has set

Figure 26. "Mars, this is Viking .Viking, this is Mars." An apparentwelcoming committee of lava-11ke Martlanrocks 1s framed by the radlo antenna (top)and other ~nstruments n the Vlklng 2Lander The pink color of the sky 1sproduced by hne red dust carried by theMaroan wlnds The Amencan flag (left)and several color cal~brabon harts helpedscient~sts etermine the actual color of theMarhan sky and landscape from theplctures returned by V~kingS ameras


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Appendix

Suggestions for FurtherReading

The Planet Mars

Avener, M.M., and R.D. MacElroy.(1976), On the Habitability of Mars,NASA Special Publication SP-414,U.S. Government Printing Office, 105p., price $5.25. A long-rangeconsideration of whether life cansurvive on Mars and of how we m~ghtbring life . . . Including ourselves . . . tothe planet to change its presentenvironment into something moreEarthlike.

Bradbury, R. , A.C. Clarke, B.Murray, C. Sagan, and W. Sullivari(1973), Mars and the Mind of Man,New York, Harper and Row, 143 p.,price $7.95. A collection of essays

about Mars, in which five scientistsand writers discuss their feelingsabout Mars and what might be foundthere as Mariner 9 went into orbitaround the planet in 1971. The bookalso presents some later reactions ofthe same people to the discoveriesmade by the spacecraft.

Carr, M. (1976), "The Volcanoes ofMars." Scientific American, Vol. 234,No. 1, January, 1976, pp. 3 243 . Adetailed discussion of the hugevolcanoes discovered on Mars in 1971by the Mariner 9 spacecraft: their sizeand appearance, their differences fromterrestrial volcanoes, their ages, andwhat they tell about the history andinternal structure of Mars.

Glasstone, S. (1 968), 7he Book ofMars, NASA Special Publication SP-179. U.S. Government Printing Office.31 5 p., price $5.25. A thoroughcompilation, now somewhat dated, of

our knowledge about Mars in the pre-Mariner and pre-Viking years. Still auseful source of information about thegeneral characteristics of Mars andthe history of study of the planet.

Hartmann, W.H., and 0 . Raper(1974), The New Mars: the Discoveriesof Mariner 9, NASA Special PublicationSP-337, U.S. Government PrintingOffice, 179 p., price $8.75. Abeautifully illustrated textbook thatcombines the early discoveries aboutMars with the findings of Mariner 9'sclose-up pictures. Carefully selectedphotographs highlight separatechapters that describe different

features of Mars. Photographscompare similar views of Mars andEarth.

Mars as Viewed by Mariner 9(1974), NASA Special Publication SP-329, U.S. Government Printing Office,225 p., price $8.15. A detailed "picturebook" of Mars as seen through thecameras of Mariner 9, this documentcontains several hundred captionedillustrations of the craters, volcanoes,canyons, dunes, clouds, and ice capsthat make Mars a complex andfascinating planet, partly like Earth andpartly lhke the Moon.

Hoyt, W.G. (1976), Lowell an d Mars,Tucson, University of Arizona Press,376 p. , price $13.95 hardbound, $8.50paperback. A detailed and scholarlybiography of the astronomer PercivalLowell and his involvement in thecontroversy over the existence ofintell~gent ife on Mars. For peopleinterested in the history of astronomyand the study of Mars in the early 20thCentury.

Mutch, T.A., R.E. Arvidson, J.W.Head Ill, K.L. Jones, and R.S.Saunders (1 976), The Geology ofMars, Princeton, N.J., PrincetonUniversity Press, 400 p., price $35.00.A graduate-level textbook on thesurface features, geologicalprocesses, and rock formations ofMars as determined from spacecraftobservations. (There is a briefappendix containing early Vikingresults.) The book provides a detailedscientific summary of our currentknowledge about Mars. It alsoprovides good comparisons of howthe same geological forces . . .volcanoes, wind, and water . . .

operate in different ways on the Earth,Moon, and Mars.Veverka, J. (1977), "Phobos and

Deimos," Scientific American, Vol. 236,No. 2, February, 1977, pp. 30 37. Adescription of the two tiny moons ofMars, revealed in close-upphotographs taken by the Mariner andViking spacecraft. The moons may becaptured asteroids. They give us anindication of what millions of othersmall bodies in the solar system maybe like.

Viking Results

"Mars: Our First Close Look,"Nat~onalGeographic, Vol. 151, No. 1,January, 1977, pp . 2-31. Handsomelyillustrated presentation of Viking resultsfor the general reader. Scientificresults are combined with beautifulcolor panoramas of the surface ofMars.


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Experiments and Activities

Young, R . S. (1976) "Viking onMars-the First Months," NASA Report

to Educators, Vol. 4, No. 4, December,1976, pp. 1-5. To obtain writeEducational Programs DivisionIFE,National Aeronautics and SpaceAdministration, Washington, D.C.20546.

Two technical summaries of theV~king cientific results are:

Young, R.S. (1976), "Viking on Mars:A Preliminary Survey " AmericanScientist, Vol. 64, No. 6, November-December, 1976, pp . 620-627.

Viking 1: Early Results, (1976),NASA Special Publication SP-408,U.S. Government Printing Office, 67 p. ,orice $2.00.

Technical articles on all aspects ofViking science, written by the scientiststhemselves, have appeared in thefollowing issues of the magazineScience, published by the AmericanAssociation for the Advancement ofScience:

27 August 1976, Vol. 193, No. 4255,pp. 759-815.1 October 1976, Vol. 194, No. 4260,pp. 57-105.17 December 1976, Vol. 194, No.4271, pp. 1274-1 353.

1. Geography and MissionPlanning.

The following are the Martianlatitudes and longitudes of locationsthat were considered as possiblelanding sites for the Viking spacecraft:

Latitude Longitude22" N. 48" W. (Viking 1 landed

near here.)20" N. 108" E.44"N . 10" W.46"N. l lOOW.46" N. 150" E. (Viking 2 landed

near here.)7" S. 43" W.5"s . 5"W.

If MASA (the Martian Aeronauticsand Space Administration) sentspacecraft to land at the samelatitudes and longitudes on Earth,where would each one land? Whathazards would be encountered? Whatwould happen to the spacecraft? Whatwould the spacecraft see? Would itdetect water? life? intelligence?

If you were working for MASA, whatsites would you pick for a landing onEarth? Why? For each site, identify thehazards that your spacecraft landerwould have to survive. What would youexpect to find? Find some pictures ofthe Earth from space to examine for

interesting locations.

2. RetrorocketsDemonstrate the retrorocket

principle by attaching a balloon to awooden block and sliding the blockdown an inclined plane. Determine thevelocity from the length of the planeand the time it takes the block to slidedown it. Repeat the same experiment,

letting the inflated balloon expel air inthe direction that the block is moving(i.e., "downhill"). Show that thisarrangement slows the block down,just as retrorocket motors sloweddown the Viking Landers. Turn theblock around so that the balloonexpels air in the "uphill" direction asthe block slides down the plane.Calculate the amount of velocityadded to (or subtracted from) theblock by the action of the balloon ineach use.

3. Life DetectionCarry out simple versions of the

Viking life detection experiments bymaking chemical tests for thepresence of life in terrestrial soils. Anapparatus to detect carbon dioxide(CO,) or water (H,O) given off byorganisms in the soils can be made byconnecting two bottles with a U-tube.Place a sample of organic-rich soil inone bottle. (Use commercial peat if nosuitable soil is available.)

To detect CO,, place a limewatersolution in the other bottle. The end ofthe U-tube should be placed about 10mm ('12 inch) above the surface of thelimewater. Any CO, given off will reactwith the limewater to produce a cloudyor milky appearance. (Try using a

photographic light-meter to measure


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how rapidly the limewater turnscloudy.)

Although water in the soil may notbe produced by organisms, itspresence indicates the possibility oflife. Water given off by the soil can beobserved by usirig a commercialdrying agent, like "Indicating Drierite"or cobalt chloride paper, instead oflimewater. The drying material can beplaced either in the U-tube or in thesecond bottle. Water can be detectedby weighing the drying material or bynoting the change in color.

In experimenting with heating thesoil, low heat should increase organicactivity and should cause a fasterevolution of CO, or H,O. Too much

heat will kill the organisms and stopgas production. In addition, test a"control sample" that has beensterilized by heating the soil bottle inboiling water, and observe thedifference in behavior.

4. Wind ErosionExperiment with making wind-

produced landforms by blowing anelectric fan (or hair dryer) over a largeshallow box filled with loose sand.Vary the force of the wind, thedistance of the fan from the sand, andthe angle at which the wind strikes the

surface. Try to dupl~cate he sanddunes and ripples seen in Vikingpictures of the Martian surface.

Place rocks on the sand surface,and try to duplicate other featuresseen on Mars: wind-scour under rocks,trails of sand on the downwind side ofrocks, and sand deposits on top ofrocks. How can these features beused to determine the wind direction?Reverse the direction of the wind, andsee how much wind force is needed tochange these wind features so thatthey indicate the new wind direction.Do wind features on Mars necessarilyindicate the present winddirection? Try making and studying3-dimensional stereo photographs ofyour artificial wind features. (SeeExperiment #5 . )

5. Stereo PhotographyDemonstrate how three-dimensional

stereo pictures, like those producedby the Viking cameras, are made andused. Take one picture of a scene(e.g., a classroom), move the camera2-3 feet sideways, and take anotherpicture of the same scene. Examinethe two pictures with a stereoscope,moving them until the picture is seenin three dimensions. Make sketchesand maps of the scene, indicatingobjects that are high and low, nearand far. A print-making color camerais convenient, but any camera can beused.

If you use a color camera, you candemonstrate how the Viking cameras

produce color pictures by combining :photographs taken through differentcolor filters. Experiment by placingcolored filters (of glass or plastic) infront of the camera lens before youtake the picture. Wratten gelatin filtersare best: number 47B (blue), 29 (red),61 (green). Colored acetate can alsobe used; it is cheaper, but it w~ll istortthe image somewhat. Take eachpicture in a stereo-pair throughdifferent colored filters, then "combine"the colors by viewing the stereo-pairwith the stereoscope. Which pair ofcolor filters produces the best match

with the colors in the original scene?Are two colors adequate to make agood match?

6. Magnetic Material in theSoil

Make a synthet ic "Martian soil" bymixing about 5 percent of magneticmaterial (crushed magnetite, FeQ, oriron metal filings) with clean whitesand. Using a large bar or horseshoemagnet, try various methods ofcollecting this magnetic material from

the soil, e.g., scraping the magnetthrough the soil, pouring the soil overthe magnet, or spreading out the soilin a thin layer and passing the magnetover it. First wrap the magnet in paperor plasiir: film so that you can easilyremove the magnetic material thatadheres to it.

Examine the collect ed material witha hand lens or a low-powermicroscope. How much whitenonmagnetic sand was collected withthe dark magnetic material?

Prepare a soil sample that containsa known weight of magnetic material.Try various collection methods andweigh the amount of magnetic materialcollected in each way. Calculate the

efficiency of each method, i.e., theweight collected divided by the weightoriginally present. Discuss why somecollection methods do not approach100 percent efficiency.

Make up several soil samples withvarying amounts of magnetic materia!,e.g., 1, 5, 10, and 25 percent. Processeach sample with the most efficientcollection method, and weigh theamount of magnetic material collected.Calculate the total amount of magneticmaterial present, knowing that:(amount present) = (amount collected)/ (efficiency). Repeat the experiment a

few times. How reproducible are yourresults? How accurate are they?Substitute a different magnetic

material (iron metal filings formagnetite or vice versa) and repeatthe experiments. Does the efficiency ofthe collecting methods change? Why?

7. Mechanical Properties ofthe Soil

Study how the mechanicalproperties of different soils affect theappearance of trenches dug in them.Make a series of soil samples bymixing loose sand with varying

amounts of fine clay or chalk powder.


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Experiment on mixtures that containzero percent clay (pure sand), 25percent, 50 percent, and 100 percent

clay (pure clay).Make trenches by sticking a ruler

into the soil and pulling it through thesoil sample. Note the appearance ofeach trench, and describe whathappens to the walls after the trench isformed. What percentage of clay isneeded to form a steep-walled trenchlike those in the Viking pictures?

Pour a sample of each soil onto a flatsurface from a height of a fewcentimeters. Note how the piles differin smoothness, in slope, and in thenumber and size of clumps formed bythe soil.

Is there any difference in behaviorbetween the soil that has 25 percentclay and the soil that is pure clay?

Examine the Viking pictures of thetrenches dug in Martian soil. Does theMartian soil behave like loose sand?Which soil sample best duplicates thebehavior of the Martian soil?

Study the effect of water bysprinkling a little water on the surfaceof the soil sample before digging thetrench. Through which soil does thewater move fastest? How does thewater affect the shape of the trench ineach soil?

8. The "Canal" IllusionOn a white sheet of paper about 2

feet by 3 feet in size, draw a randomarrangement of dots, circles, ovals,straight lines, wavy lines and irregularsmudges. Make sure that the diagramis a completely random pattern. Hangthe paper at the front of the classroomso that it is well-lit, and have thestudents draw what they see.

Compare the drawings by studentswho are closest to the diagram withthose by students who are furtheraway. Which ones reproduce thepattern best? How many in whichgroup draw straight lines where noneare present in the picture?

(This experiment was first performedmany years ago by the astronomer

Maunder to demonstrate the eye'stendency to produce imaginary lines toconnect objects that are entirelyseparate but poorly seen.)

9. Mars in Fact and Fiction.People have often commented that

science-fiction literature often predictsfuture facts and developments. (JulesVerne's From the Earth to the Moon,and 20,000 Leagues Under the Seaare often cited as examples.)

Science-fiction has been writtenabout Mars for more than three-quarters of a century. A partial list ofbooks, all available in paperback, is:

H.G. Wells, The War of the Worlds

(1 898).Edgar Rice Burroughs. A Princess ofMars (1912).C.S. Lewis, Out of the Silent Planet(1 944).Robert A. Heinlein, Red Planet(1 949).Ray Bradbury, The MartianChronicles (1 950).Arthur C. Clarke, Sands of Mars(1 952).

What conditions of temperature,atmosphere, and climate did thevarious authors att ribute to Mars? Whatkinds of Martians lived in theseconditions? How did Earth peopleadjust to Mars, and Martians to Earth?Did the authors ' view of Martianconditions change as we learned moreabout Mars? How accurately did theauthors predict the real nature of Marsas we have determined it from Vikingand other spacecraft?

How would you write an "accurate"science-fiction novel based on theview of Mars revealed by Viking? Whatkinds of "Martians" could exist? Whatprotection would humans need on thesurface of Mars? What hazards wouldhumans on Mars face from naturalprocesses or from "Martians"?


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Suggested ViewingA series of four films produced by

NASA discuss the planet Mars and theViking life detection experiments. Thefilms, produced before the Vikinglandings on Mars, may be borrowedfrom NASA Regional Film Librarieswithout rental charge.

Life?HQ 261 -COLOR-1 4 % MINS.General characteristics of life are firstdescribed with non-life similaritiesnoted. A number of adaptations areincluded to show how life has adaptedto Earth c~ nd it io n~ , nd how certainindividuals 'ban withstandenvironmental insults. In conclusion,the habitat of Mars is described with

the question raised as to the possibilityof life existing there.

Mars-Is There Life?HQ 263-COLOR-1 4% MINS.Students are introduced to the possiblepast history of Mars, as well as itspresent surface topography-fromvolcanoes, ice caps, stream beds,impact craters, canyons and wind-eroded surfaces. The Viking lander andits biology experiments are discussedin relationship to the search for life onMars. In conclusion, students areasked to consider life forms that mightbe able to survive on Mars, and thepotential significance of theirdiscovery.

Mars and Beyond.HQ 264-COLOR-1 4% MINS.This film traces the Viking mission toMars to specifically explore thebiochemical evidence of life.Elementary chemical components oflife (as we know it) are introduced;these are related to the organicanalysis instrument on board the Vikinglander. The instrument is described bydesign and operations. The programconcludes with the potential

significance of biochemical findings-how they may relate to past, presentand future Martian life.

A Question of LifeHQ 270-COLOR-28% MINS.The film is a composite version of three15-minute films: Life? (HQ 261), Mars:Is There Life? (HQ 263) and Mars an dBeyond(HQ 264). A definition of lifeand general conditions necessary tosustain life are discussed. Viewers areintroduced to the possible past historyof Mars as well as its present surfacetopography and its capacity to supportlife as we know it. Major emphasis isplaced on the Viking life detectionexperiments including the threebiology experiments and the organicanalysis instrument. Consideration isgiven to the po tential significance ofdiscovering life elsewhere in the

universe.

$7 U.S. GOVERNMENT PRINTING OFFIC E: 1977 0-247-91 5


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