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An Educational Publicationof theNational Aeronautics andSpace AdministrationNF-61/8- 75

condensed from a primordial solar nebula. TheSun also formed from this same material. Afterthe Sun had condensed, planets of different sizesand probably different compositions originatedfrom concentrations of matter present at variousdistances from the Sun. Electric and magneticfields of the original nebula could have forcedthese embryonic planets into orbits around thecentral Sun and spun them on their own axes.

The Sun's Planetary SystemIt is important to be aware that there is no onetheory for the origin and subsequent evolution ofthe Solar System that is generally accepted. All

theories represent models which fit some of thefacts observed today, but not all. Many scientiststoday consider that planets of the Solar Systemprobably formed between four and five billionyears ago; all about the same time, as material

Figure 1. In the latest stages of planet building, some 4 billion years ago, the impact of mountain-sized bodies producedgreat impact basins similar to this on Mercury. Such basins are dust filled on Mars and have been all but obliterated on Earth.

One of a series of NASA Facts about the explora-tion of Mars.

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Matter also condensed within the young SolarSystem in the form of mountain-sized chunks ofrock. Some theories claim that these minor planetswere concentrated in a belt between the large,outer planets and the small, inner planets. Othertheories postulated that the concentration wasbeyond the outer planets. In the former casethe asteroid belt would now represent today'sremnants of this multitude of small bodies. In thelatter case the planet Pluto might be the solevisible remnant of the outer ring of bodies.

formation and bombardment molding, and alsoshowing contraction around a very dense core.Mercury, like the Moon, rotates very slowly on itsaxis; three rotations for every two revolutionsaround the Sun. The Moon rotates once eachrevolution around the Earth. Mars and Earth rotatequickly in close to 24 hours. Venus has a uniquebackward rotation of 243 days.

Such planetesimals would be perturbed by thegrowing planets and sent into elliptical orbits thatwould cross the planetary orbits. They might formthe nuclei of comets, too. It has been calculatedthat within a few million years anyone of thesebodies that crossed the orbits of planets wouldbe either captured by a planet or hurled outof the Solar System. If captured, the planetesimalwould crash into the planet's surface and gougea huge hole, such as a crater or a major impactbasin. (Figure 1).

The very large craters and impact basins onMars, the Moon, Mercury, and Venus are thoughtto be the result of the impact of falling bodiesduring this second period of planetary accretion,as the process of falling together is termed. TheEarth also probably had large craters and impactbasins early in its history.While the inner planets of the Solar System lostlighter elements, such as hydrogen and helium,because they were too hot to hold these gases due

to their proximity to the Sun and their relativelyweak gravities, the outer planets retained theirhydrogen. Thus the planets of the Solar System,separated by the asteroid belt, consist of rockyinner planets, Mercury, Venus, Earth and Moon,and Mars, and fluid outer planets, Jupiter andSaturn, with compositions similar to that of theSun, and Uranus and Neptune, which may beice giants. Pluto is thought by some scientists tobe a large example of an outer belt of asteroidalbodies from which comets may be originatingtoday. In some respects, too, it behaves as avery distant satellite of Neptune.There are striking differences among the fiveinner planets, and particularly between the Earthand the others. These differences are importantto our understanding why the Earth is as it istoday, and why the other planets are differentdespite their formation from common buildingblocks at about the same time.

Planetary AtmospheresAfter the formation of the planetary bodies thereappears to have been a period of planetary heat-ing in which more dense material sank towardthe center of each planet to form a core, whileless dense material rose to form a crust. This istermed differentiation. The Moon and Mercury stinshow much of the ancient cratered surface onwhich there are some lava flows. Mercury exhibitscompressive shrinkage around a cooling, iron-richcore.Volcanic activities on the planetary bodies wouldrelease gases from their interiors; water vapor andcarbon dioxide with traces of other gases. OnMercury and the Moon, these gases escaped intospace; they were lost from Mercury because ofhigh temperatures close to the Sun, from theMoon because of its weak gravity.But on Venus, Earth, and Mars, the gases wereretained, so that these planets still have atmos-

pheres.Today, however, these atmospheres are verydifferent. Venus has an extremely dense atmos-

phere of carbon dioxide at the bottom of whichthe surface of the planet is hot enough for leadto melt (480C~ 900F). On Mars the atmosphereis again predominantly carbon dioxide, but at avery low pressure, and the planet has a coldsurface. On Earth the atmosphere is predominantlynitrogen with some oxygen and traces of carbondioxide. Most of Earth's carbon dioxide has beenbound with the rocks of the Earth's crust ascarbonates because of the presence of muchwater. Both Venus and Mars seem very deficientin water compared with the Earth, which may bewhy their atmospheres are predominantly carbondioxide.

Why are Earth, Mars and Venus so very differ-ent? Their distances from the Sun are not muchdifferent compared with the great distances of theouter planets. Could Earth become like Mars orVenus? How might this happen? These questionsintrigue scientists today and they are importantfor other people too. Perhaps man's industrializa-tion of the Earth, or a series of major volcaniceruptions, could sufficiently change Earth's atmos-phere to push it toward the Mars or Venus state.IVlercury IS Moon-liKe In some respects, a cra-

tered world with virtually no atmosphere, but dis-playing evidence of volcanic activity after its

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away effect to a glaciated planet. The opposite isalso true; a slight rise in Earth's overall tempera-ture could trigger a heating effect that wouldcontinue until the Earth became a hothouse planet.

It is thus important to gather as much informationas possible about the other planets to make surethat man's activities on Earth will not lead to aplanetary catastrophe here.

Planetary ClimatesAt present scientists are concerned that the

climate of the Earth is changing; growing generallycolder. They believe that the Earth is still in an iceage that began about 1 million years ago, thoughnot now at the severest part of the ice age.Although they do not know how quickly climaticchanges occur, they do know from the record offossils that several hundred million years agoEarth experienced a very warm period. Thesechanges to world climate could result from varia-tions in the radiation from the Sun if the Sun is avariable star, as well it might be.The climate of a planet is governed by three

major factors; the amount of radiation it receivesfrom the Sun, the tilt of its axis of rotation, andthe eccentricity of its orbit around the Sun. Whata planet does with the radiation it receives fromthe Sun is also governed by the composition ofits surface and its atmosphere. A reflective planet(high albedo) that sends the Sun's rays back intospace will be cooler than a dark planet (lowalbedo) that absorbs the solar rays. And theatmosphere, too, governs this energy balance be-tween solar radiation received and radiation sentback into space.

Satellites have measured that the Earth shouldreflect about 35 percent of the sunlight fallingupon it. But the balance of the sunlight absorbedby the Earth would only raise its temperature toabout -18C (OF). The oceans would freeze.Why then is the Earth so warm? The incomingsunlight is converted to infrared radiation by theEarth's surface, and because of the presence ofcarbon dioxide and water vapor in the Earth'satmosphere, the infrared radiation is trapped inthe atmosphere. The atmosphere behaves like thewindows of a greenhouse, it allows the solar radia-tion in, but does not let the heat energy out. Theplanet heats up like the inside of an automobile leftin the sunlight on a cold day with all its windowsrolled up. This is termed the "greenhouse effect."

Figure 2. The greenhouse effect is illustrated for the threeplanets, Venus, Earth and Mars, assuming each went througha period when water and carbon dioxide vented from thecrust into a primitive atmosphere. Because Venus was warm,being closer to the Sun, its atmosphere could hold muchwater vapor. The planet heated to the point at which oceanscould not condense. On Earth the temperature was rightfor some water vapor to enter the atmosphere and most tocondense into oceans. On Mars, the temperature was toocold, most water remained frozen and very little was ableto enter the atmosphere.

Such changes might be brought about by achange in the tilt of the Earth's axis which wouldalter the effects of sunlight falling on the poles andthe equator.. They might also be brought about byparticles suspended in the Earth's atmospherewhich would affect the reflecting properties of theplanet. A major volcanic eruption might thuschange the Earth's climate drastically.

Now if the temperature of the Earth shoulddrop a few degrees, the amount of water vaporin the atmosphere would decrease because coldair cannot be as humid at hot air. With less watervapor in the air the greenhouse effect on Earthwould be reduced, and the temperature would fallstill more. In this way a slight change in the overalltemperature of the Earth could precipitate a run-

It is believed that Venus is an example of aplanet on which the greenhouse effect has runaway with itself (Figure 2). When the planetformed, its surface might have been warmer than3

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balance is reached with an atmospheric pressureof less than one hundredth that of the Earth'satmosphere, compared with Venus' surface pres-sure of nearly one hundred times that of Earth.On Mars, slight changes to the overall tempera-ture of the planet because of changing albedo,changing tilt of the planet's axis, or changing ec-centricity of its orbit, could produce vast changes

to its atmosphere. If the temperature on Mars r0sejust slightly, more carbon dioxide would vaporizefrom the polar caps and make the atmospheredenser. Then circulation from equator to the poleswould be able to carry more solar heat to thepoles and vaporize even more carbon dioxide.Thus, the temperature and the atmospheric pres-sure of Mars would continue to rise until all thecarbon dioxide was in the atmosphere. Under suchconditions, the pressure of the Martian atmospheremight be high enough for bodies of liquid waterto be present in all low regions of Mars and forrivers to flow into mini-Martian oceans.

Earth because of Venus' proximity to the Sun.Thus there would be more water vapor in theatmosphere of Venus than in the atmosphere ofEarth. Trapping solar radiation, the atmosphereof Venus would have become hotter and hotteruntil all its primitive oceans boiled off into theatmosphere and the carbon dioxide was not al-lowed to combine into carbonates in Venus' crustas happened on Earth. Over eons of time theoceans in the atmosphere of Venus were brokendown by solar radiation into oxygen and hydrogenwhich escaped into space.

An alternative theory suggests that Venusaccreted from materials of the solar nebula thatdid not contain water and, therefore, the planetdid not possess any water when formed.On Mars, by contrast, the primordial planet wascolder than the Earth and never entered a hotphase. Rather, Mars was forced in the oppositedirection. Water was permanently frozen in the

martian crust, and even carbon dioxide froze intopolar caps until an equilibrium, or balance, wasreached between the amount of frozen carbondioxide and the amount of the gas in the atmos-phere. At the present temperature of Mars this

It is fascinating to see that the sinuous arroyosof Mars (Figure 3), which defy explanation exceptas resulting from running water sometime in thepast, have so few craters on them that they can bedated as extremely young; possibly only 200 to 500million years. And it is known that about that sametime the Earth was suffering a hot spell, perhapsfrom an increase in solar radiation that warmedboth planets at the same time. Now, with the Earthin an ice age, Mars, too, is in an ice age. The bigquestion is; how long and how deep will the iceage become?

There are, however, other ways in which Marscan heat up and have water flow on its surface. Be-cause of the gravitational influence of Jupiter thetilt of the Martian axis can change within a periodof hundreds of thousands of years, and with agreater tilt to this axis the poles would be pre-sented more to the Sun. The consequence wouldbe a runaway heating effect. It has been calculatedthat the polar regions of Mars would need only onecentury of solar heating at 20 percent more thanthey receive today for the runaway heating effectto start. This could result from the Martian axistilting only 5 degrees more than today, which iswell within the calculated amount that could beproduced by Jupiter's gravity.

Today, scientists are becoming concerned thatthe Earth's climate is not as stable as was oncethought. Even very small changes to Earth's climatecan have far-reaching consequences to the peo-ples of Earth since today we are so dependentupon intensive agriculture based on crops that areresistant to disease but have virtually no resistanceto climatic changes. Thus, knowing what is hap-Figure 3. These sinuous river-l ike beds on Mars seem toimply past period of great water flows.

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pening on other worlds assumes increasing im-portance to understanding, controlling, and safe-guarding our own world.And this leads naturally to the major question;Why is there life on Earth? Is there life on otherworlds, particularly on Mars?

Figure 4. This is a simple diagram of the apparatus de-signed by Stanley Miller in 1956 to produce complex mole-cules from simple gases by the energy of an electric spark.This is believed to be one way in which life precursors mayevolve in planetary atmospheres.

Figure 5. The evolution of living systems on Earth. The ques-tion is whether or not a similar evolution has taken placeon Mars, and if so, how far has it progressed?

past. Mercury, too, looks most inhospitable to life,as does Venus. Some of the outer planets arecandidates for life such as Jupiter and largesatellites. But within the inner Solar System onlyMars seems to present a possibility of life besidesthe Earth.

Living Systems-The Searchfor Extraterrestrial LifeLife might be described as an unexplained forcethat somehow organizes inanimate matter into aliving system that perceives, reacts to, and evolvesto cope with changes to the physical environmentthat threaten to destroy its organization.In 1953, a mixture of hydrogen, methane, am-monia, and water vapor-the kind of atmospherethat Earth might have had soon after it was formed-was treated in a laboratory (Figure 4). Scientistspassed electrical discharges through the gas mix-

ture and were surprised to find that the electricalenergy changed some of the simple gases intomore complex compounds of carbon, hydrogen,nitrogen, and oxygen; into molecules known asamino acids which are believed to be the essentialbuilding blocks for all living systems.Today it is generally believed that natural proc-esses such as lightning and radiation from the Suncan produce complex chemicals to form buildingblocks for living things. In fact, some of the com-plex chemicals are found in the space between thestars and on meteorites (the familiar 'falling stars');small rocks that plunge into the Earth's atmospherefrom space, the debris remaining from the forma-tion of the Solar System.At some point in the past, probably about 3V2billion years ago, something organized the com-

plex carbon-based molecules on Earth into livingsystems which were then able to make copies ofthemselves-to reproduce. Life had been created.From then on, according to one theory, that of evo-lution, by slight changes to subsequent copies, bio-logical evolution produced all living things onEarth, all using the same basic 21 amino acids asbuilding blocks. At one stage, the theory continues,a special consciousness emerged that gave rise toMan himself, a living system that can contemplatenot only itself but also the whole of the universe inspace and time (Figure 5). By contrast, most reli-gious theory views Man as the result of a uniquecreative event.

A big question is whether or not life originatedon other planetary bodies as it did on Earth. Weknow from the Apollo program that it did notevolve on the Moon. There is no life there today,nor any trace of life having been there in the

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In earlier centuries there was much speculationabout life on Mars; linear markings, now known tobe mainly an optical effect, were categorized as"canals" (channels) across the surface, and theevidence of an ancient civilization. The close-inphotography by NASA's Mariner 9 shows no evi-

dence of life on Mars; but it is important to realizethat most space views of Earth show no evidenceof life on that planet either!One thing is certain; life as we know it on Earthcan adapt to adverse conditions; it lives in hot

Figure 6. On Earth life adapts to difficult environments. Here a small flower blossoms after spring rain on the cinders of avolcanic crater in Death Valley, California.

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Figure 7. On July 4, 1976, an American spacecraft, Viking,will start to search for life on Mars. This artist's conceptshows the orbiter high above the lander and its jettisonedparachute.

springs and in Arctic ice, it lives in the depths ofthe oceans and the tops of the highest mountains,it lives in sulfuric acid and in the cooling waterof nuclear reactors. Some algae even grow in saltpools and they explode and die if they are placedin fresh water. Flowers will blossom after rainon the cinder cones of desert volcanoes (Figure6). Plants grow where there is never any rain~ theyextract their moisture from dew each night.Kangaroo rats do not drink. They make water bysynthesizing it from sugar in their food.

If Martian life adapted to its environment aslife on Earth did, it must today exist in a vi rtuallywaterless world, drier than the driest deserts ofEarth. It must resist ultraviolet light (the rays thatcause sunburn) stronger than on any mountain-top of Earth. It must resist raging dust storms andtemperatures that dip to hundreds of degreesbelow zero each night.

Experiments in laboratories of Earth have shownthat such adaptation is possible. Earth organismshave thrived under simulated Martian conditions.Only a few inches below the surface of Mars, anorganism might be protected from solar ultraviolet,temperature extremes, dust storms, and be intouch with frozen water that it could tap by biologi-cal heating. It may, indeed, be an ice eater ratherthan a water drinker; a worm-like creature foreverburrowing beneath the frozen soil. There are snowworms on Earth that eat snow algae. There areeven bacteria on Earth that can extract waterdirectly from salt crystals and this needs as muchenergy as extracting water as vapor from theMartian atmosphere.

Mars creatures might also protect their bodiesas many Earth creatures do by growing hard outercases or shells. And just as Earth creatures movedfrom the oceans to escape competition there, soMartian creatures might have evolved to live underthe harsher conditions which prevail today on theplanet's surface.

day and age. If life is discovered on Mars it willbe important to find out if it is based on the sameamino acids as Earth life, or if there was a separateact of creation. It is also of great interest to findout if the life evolved only to the simplest ofbacterial forms, or did it, as on Earth, evolve intomore complex forms to meet competition withother living systems and to survive a changingenvironment.

If there is no evidence of life on Mars, and suchevidence will not be really conclusive until exten-sive searches have been made over large areas ofthe Martian surface, it will be even more importantfor people on Earth to preserve themselves andtheir planet because of their uniqueness. Man-kind may have to be prepared to accept a greaterdestiny than learning to live in harmony on itsplanet of birth. It may have to accept the responsi-bility of spreading living systems throughout theSolar System and into the cosmos.

To search for life on Mars it is necessary toput a lander on the Martian surface (Figure 7).The United States has such a spacecraft-two infact. If all goes according to plan the first will landon Mars on July 4, 1976, the bicentennial day; afitting tribute to the pioneering and forward-lookingspirit of the American people. This NASA projectto land life-seeking spacecraft on Mars is calledViking. It is described in the next booklet of thisseries.

STUDENT INVOLVEMENTProject One

Compare the planets Earth, Mars and Venus interms of size, location in the Solar System, andphysical characteristics. Prepare a table andsketches of these data for reference.

Also, in the past, Mars might have had plentyof water. Life on Mars might have life cyclesgeared to the p1uvial periods of Mars when,thousands of years apart, there are times when theplanet warms up. There are many examples onEarth of this form of life adaptation, though formuch shorter periods. There are Earth creaturesthat hibernate for a single winter and others thatremain dormant for long periods of drought indesert dry lakes. When, scores of years apart,these lakes fill with water for a short season, theyare soon teeming with the previously dormant liferevived by the water.

The search for life on Mars is perhaps one ofthe most fascinating activities of mankind in this7

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a) very little gravityb) crunching gravityc) a blinding sun so you must feel rather thansee.Your planet may be:a) all liquidb) all gaseousc) all solid surfaced) combinations of these

.You may be:a) shaped like a ballb) equipped with many tentaclesc) a single organismd) a group, like a colony of antse) microscopically smallf) gargantuang) a parasiteh) equipped with very different senses using,for example, touch instead of eyes, soundinstead of touch, smell instead of taste,and the like.i) a symbiosis of machine and living system

such as a biological brain in a mechanicalbody, or an extended computerized elec-tronic brain (cerebrum cortex) coupled toa small biological cerebellum in a biologi-cal body.

Project TwoDefine the limits of conditions for life as weknow it on Earth, and compare the extremes fordifferent forms of life on our planet-plants, mam-

mals, man, bacteria, algae, viruses-in terms oftemperature, pressure, water, salinity of water.Are these limits greatest for energy-gathering lifeforms (photosynthesizers) or for eaters?Project ThreeDesign an interplanetary spacecraft either as agroup or individual project.a) Classify the planets in a convenient schemeand identify three major problems in exploringthem-atmosphere, radiation, temperature. Whattechniques can be used to explore the planets?Probes, orbiters, atmospheric probes, landers?b) Define missions for each type of spacecraft inthe exploration of Mars.c) Imagine you are a Martian: define the type ofspacecraft you might use to explore Earth and tryto find out if there is life on Earth.d) For both designs (i.e. to explore Mars and toexplore Earth) give details of the power supply(electrical batteries, solar power, nuclear power),experiments (cameras, life-seeking devices, radia-tion and temperature measurers, meteorologicalinstruments), and the configuration (shape ofspacecraft, size, landing equipment, radio system).e) Prepare a sketch of each spacecraft.Project FourSelect a site for a lander on Earth and on Marsa) Discuss how you would define life.b) What types of instruments would you recom-ment to detect this life:i) In oceansii) In dried-up water channelsiii) In dry ocean basinsiv) In desertsv) On mountain topsvi) On polar capsvii) Elsewhere on the two planets?c) What conclusions could you draw about life onEarth, given that you detected life at any two ofthe chosen sites?d) Where would you select landing sites on Marsas most likely to show evidence of Martian life?Project Five

Space-related drama, dance, or body movementactivity. Imagine that you are a creature of anotherplanet. Show how you move. First describe your-self and your environment, and then portray your-self.Suggestions:.Your planet may have:

-{:( .s. GOVERNMENTRINTI!8 I-or sale by tne superintendent of Documents, U.S. Government Printing OfficeWashington, D.C. 20402 -Price 35 cents

Suggested ReadingMARINER'S EVIDENCE OF A FORMERLY WATERYMARS, E. Driscoll, Science News, v. 103,n. 10, 10 March 1973, pp. 156-158THE CHEMICAL ELEMENTS OF LIFE, E. Frieden,

Scientific American, v. 227, n. 1, July 1972,pp. 52-60THE SURFACE OF MARS, N. E. Howard,Astronomy, v. 1 n. 2, September 1973, pp.4-11MARS STUDIES, various authors, Icarus, v. 18, n. 1,January 1973, enti re issue.EXPLORING THE SOLAR SYSTEM (I) AN EMERG-ING NEW PERSPECTIVE, A. L. Hammond,Science, v. 186, n. 4165, November 1974,pp. 720-724EXPLORING THE SOLAR SYSTEM (II) MODELSOF THE ORIGIN, W. D. Metz, Science, v.186, n. 4166, 29 November 1974, pp. 814-818IS THERE LIFE ON MARS, G. Berry, The Ward

Ritchie Press, L. A., 1973.THE PLANETS TODAY, Various authors, The Royal

Society of London, 1974THE ORIGINS OF LIFE ON EARTH, F. Clark andR. L. M. Syng (eds.) Pergamon Press, N.Y.,1959

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