SPAT IUMPublished by the Association Pro ISSI

No 5, June 2000

INTERNATIONAL SPACESCIENCEINSTITUTE

Earth,Moon and MarsEarth,Moon and Mars

Mankind finds itself thrown into a hostile universe. While our bod-ies are subject to the soulless lawsof chemistry and physics govern-ing physical processes on even themost distant star, our minds, ourhopes and fears find no spiritualequivalent. The intriguing sketch-es on the walls of the Altamiracave, for example, may have beenthe attempt of our early ancestorsto create humanlike beings simplyin order to alleviate their spiritualloneliness. The Greeks later filledtheir heavens with a great varietyof deities who, by virtue of theirhuman features, clearly betray theintent of their creators.

Little has changed since. The pasttwo thousand years have seen man-kind struggling to find a rationalefor its existence, hopefully givenby an entity beyond its transitoryexistence. These days, evidenceseems to take precedence over be-lief. Hence, modern science seeksto provide answers to these age-old questions. It is only logical tobegin with the nearby celestialbodies, i. e. with the Earth, Moonand Mars, to which the presentissue of Spatium is devoted, and tosearch there for traces of life. But,while this endeavour has providedus with deep insight into the his-tory of our own planet, the feasi-bility of life on the Moon hasmeanwhile been excluded, andcorroboratory proof of life onMars has not yet been found. We are still alone.

Prof. Johannes Geiss is the spiritusrector of the International SpaceScience Institute in Bern and itsExecutive Director. We are very

grateful for his kind permission to publish herewith an adapted ver-sion of his address at the FourthAnniversary of the I S S I founda-tion.

Bern, June 2000Hansjörg Schlaepfer

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FrontcoverMars: South of Candor Chasma(NASA / J P L)

INTERNATIONAL SPACESCIENCEINSTITUTE

Editorial

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Introduction

The ultimate goal of planetary re-search is to find answers to the bigquestions: What happened when4’600 million years ago the Sunand the planets formed out of aninterstellar cloud? What were theexternal circumstances and specif-ic processes that governed the ori-gin of the Earth and determinedits evolution to the present state?Is there – or was there – life in thepast on another planet or on oneof the moons in the solar system?From the presence or absence ofsigns of life outside the Earth, canwe draw conclusions about theorigin of life on our own planet?

To an interplanetary traveller,Earth, Moon and Mars wouldpresent themselves as entirely dif-ferent worlds (Figure 1). For earthand planetary scientists, however,these three members of the solarsystem have important similari-ties. Their sizes, their chemicalcompositions and their distancesto the Sun are relatively similar, sothat comparative studies yieldresults on their origin and theirevolution. Moreover, Earth, Moonand Mars are the only objects inthe universe where we havedetailed geological observationsand at the same time possess rocksamples for laboratory analysis.

Nineteenth century scientists real-ised that evolution is central to bi-ology and geology. On the basis ofcharacteristic fossils, the sequence

of geologic epochs from theCambrian to the Quarternary wasidentified. Then, at the turn to the20th century, radioactivity was dis-covered, opening up a new era formany branches of science, includ-ing the earth sciences. The decayof uranium, thorium and potassi-um isotopes was found to be themain source of energy in the inte-rior of the Earth, maintaining therea high temperature and drivinggeological processes. The applica-tion of radioactive decay for meas-uring ages of rocks was an equallyimportant breakthrough. By usingvarious “radioactive clocks”, theage of the Earth was derived andan absolute time scale was estab-lished for the larger part of its history (Figure 2). Thus, it becamepossible to gain a quantitativeunderstanding of the causes ofgeological evolution and identifythe underlying physical processes.

In the second half of the 20th cen-tury, Apollo and other pro-grammes enabled scientists to usethe experience gained from theearth sciences and apply their con-cepts and methods to the investiga-tion of the Moon. By combiningobservations at the lunar surfaceand from orbit with the chemicaldata obtained from the analyses oflunar rocks brought back to Earth,scientists reconstructed a largepart of lunar history.

Thirty years after astronauts setfoot on the Moon and explored itsgeology, scientists and space agen-cies are turning their attention in-creasingly to Mars, the Red Planet(Figure 1). Let me emphasise thatthe exploration of Mars for quite awhile will be by robots. Humanexploration will be a great chal-lenge, because Mars is at least 200times more distant than the Moon.

Earth,Moon and Mars *)

Johannes Geiss, International Space Science Institute,Bern

Figure 1Earth, Moon and Mars. The “Blue Planet" Earth, the “Red Planet” Mars and thegrey Moon. The relative sizes are to scale. We have evidence that on Mars there werelarge quantities of water at some time in the past. Today we see only small ice caps ofCO2 and H2O, which are seasonally distilled from pole to pole, giving rise to strongwinds.

*) Keynote address at the Fourth Anniversary of the International Space Science Institute, 21th October 1999

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We have on Earth a dozen Mar-tian meteorites. There is littledoubt that their origin is the RedPlanet, but we do not know theexact location they came from.The Mars meteorites were ex-pelled from the planet by a largeimpact, as a result of which theywere badly shocked. This makesradioactive dating difficult. Still,ages of Mars meteorites havegiven us a general idea of thetimes of formation of geologicalfeatures on Mars. However, sam-ples returned by spacecraft from afew well-defined geological unitswill probably be needed to obtaina definite time scale for Martiangeology.

The Early Daysof the Solar System

Sun and planets

Stars are created when an inter-stellar molecular cloud collapses.We can observe this process in theOrion Nebula, where star forma-tion is presently going on. TheSun was born 4’600 million yearsago in a collapsing interstellarcloud that was dispersed long ago.Once initiated, it took only a fewmillion years to form the Sun andto allow the nuclear fusion pro-cess in its interior to start workingand hence for the Sun to shine.

As predicted by Pierre Laplaceearly in the 19th century, a portionof the gas and dust of the protoso-lar cloud remained in a rotatingdisc from which the planets wereformed by accretion (Figure 3).Thisprocess followed the principle of“big fish eats small fish", wellknown in the behavioural andsocial sciences. It began with dustparticles attaching to each other.After they reached a certain size,gravitation took over, and largerbodies attracted and swallowedsmaller ones. In the end, a numberof planets were left, most of themwell separated from each other, sothat they could safely orbit theSun for more than four billionyears.

The Earth

In the case of the Earth, the accre-tion process did not go so smooth-ly. A second object, about the sizeof Mars, formed on a nearby orbit,so that the two were bound to col-lide.All the evidence indicates thatthis collision did occur, and it cre-ated the pair of Earth and Moon aswe know it, circling around thecommon center of mass, thusavoiding any further collision.

Earth 12’756 km

Moon 3’476 km

Mars 6’860 km

0Million Years

1’000 2’000 3’000 4’000 5’000

Ages of Rocks

4’600

Figure 2The ages of Earth, Moon and Mars.This diagram shows the distribution ofages of rocks from Earth, Moon andMars. Earth has been geologically veryactive throughout history. As a result,the geological record of the early timeshas largely been overwritten. The Moonis much smaller than the Earth, and con-sequently, the geological activity of theMoon ended about 3’000 million yearsago. We have, however, a good geologi-cal record with absolute times by radio-active dating for the epoch 4’100 – 3’200million years ago for our companion.For 3’000 million years, the Moon hasbeen geologically dead. For Mars, wehave only a rudimentary time scale ofgeological events, since we only have 13Mars meteorites for radioactive dating.

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The Origin of the Earth–MoonSystem

For centuries, scientists have won-dered about the large size of theMoon. Many planets in the solarsystem are circled by moons, but,with the exception of the satellitesof Earth and Pluto, all thesemoons are tiny in comparison tothe planet they accompany (Figure4). The giant planets, Jupiter andSaturn, have satellite systems con-sisting of more than ten moonseach. They were probably formedfrom protoplanetary discs thatsurrounded Jupiter and Saturn at

the time of their formation. Theexceptional size of our Moon,however, calls for an entirely dif-ferent origin.

The Giant Collision

Among the various hypothesesabout the origin of our Moon,there is only one, the “Giant Col-lision Theory" that explains allrelevant observations. This theorywas originally proposed by Wil-liam K. Hartmann and colleaguesin the United States. The sequence(Figures 5 to 7) depicting three stagesof the creation of the Earth–Moonsystem were painted by him. Theyare based on quantitative model-ling by A.G.W. Cameron,W. Benzand others.

Figure 4Jupiter with its moons Io and Europa. The giant planet Jupiter with two of itsmoons, Io (reddish) and Europa (white). While Io and Europa are nearly the samesize as our Moon, they are tiny in comparison to the planet they orbit.

Figure 3Our Sun is born. The Sun was born bya collapse of an interstellar cloud 4’600million years ago. Some of the materialcould not be collected into the centralstar, the Sun, due to the law of physics,which requires conservation of angularmomentum. This material eventuallyaccreted into planets (painting by Wil-liam K. Hartmann).

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The Giant Collision Theory is con-sistent with all the relevant piecesof evidence we have. In particular,it explains in a natural way someunexpected results of the analysesof lunar rocks and some obser-vations made by the Apollo astro-nauts and by photogeologists.These are:

— The lunar rocks are completelyfree of crystalline water andhydrogen-containing minerals,such as mica or clay minerals(Figure 8). Not only hydrogen,but also the other volatile ele-ments such as carbon and nitro-gen are virtually absent. Astro-naut observations and all the

pictures taken from orbit andon the lunar surface did notreveal any traces of past waterflows or products of sedimenta-tion. The complete lack ofvolatile material and the rela-tively low abundances of sodi-um, potassium and other alkalisare readily explained by theextremely high temperatures ofthe material ejected by the giantcollision.

– Relative to the Earth and mete-orites, the Moon has much lessiron. This would be expected, ifthe giant impact occurred afterthe Earth had formed its ironcore and if the impact ejectedonly material from the Earth’scrust and mantle.

– The relative abundances of thethree isotopes of oxygen showsmall characteristic differencesin the material of differentobjects in the solar system. Thusmeteorites coming from differ-ent asteroids, meteorites com-ing from Mars and rocks onEarth all have their characteris-tic, but different isotopic signa-ture. The exception is that oxy-gen in terrestrial and in lunarrocks has exactly the same iso-topic composition. The GiantCollision Theory explains thisfinding, because most of thelunar material was originallypart of the Earth. Moreover, thetwo collision partners accretedfrom the same region in theprotosolar disc.

Figure 5A Giant Collision created the Moon:half an hour after impact. All evi-dence indicates that our Moon was cre-ated by a collision between Earth andanother large object, ten percent of theEarth’s mass, formed in a nearby orbit(painting by William K. Hartmann).

Figure 6Five hours after the Giant Colli-sion: Because of the enormous heatcreated by the impact, all volatilesescaped, so that only non-volatile mate-rial remained in Earth orbit (painting byWilliam K. Hartmann).

Figure 71’000 Years after the Collision: Theformation of the Moon is essentiallycomplete. In its early history, the Moonwas molten (“Magma Ocean”). The“tidal forces” at that time were morethan a thousand times stronger thanthey are today and contributed to heat-ing the Moon. Data from the Apollomissions showed us that the Moon iscovered by a crust of light material.Thus, the Moon is chemically highlydifferentiated, and this could only havehappened if the Moon was molten earlyon (painting by William K. Hartmann).

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Figure 8Landscape on the Moon: This beautiful landscape in Mare Imbrium was visited by the Apollo 15 crew. Like everywhere elseon the Moon, the rocks in this mare plain and in the distant highlands are extremely dry. There is no trace of past water flows norhave we found crystalline water in rocks. The rocks and the soil are also devoid of carbon and nitrogen. Thus, there was never anychance for life on the Moon. Small amounts of water ice or other condensates exist in polar areas. They are probably mostlysecondary, stemming perhaps from impacting comets (NASA Photo AS-15 9212430).

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The accretion process likely leftbehind some sizeable objects thatwere bound to collide at the endof the epoch of planet formation.There are indeed indications thatlarge collisions happened else-where in the solar system, thoughthe collision producing our Moonmay have been the most violent. Ithas been postulated that the highdensity of Mercury and the highinclination of the rotation axis ofUranus were caused by large colli-sions at the end of the accretionprocess.

According to the Austrian-Britishphilosopher Karl Popper scienceprogresses by falsification of theo-ries. The story of the origin of theMoon is a good example of thispostulate.As data,observations andtheory increased in quality andquantity, one hypothesis after theother about the Moon’s originencountered severe difficulties,the giant collision theory beingthe only one that remained con-sistent with observation.Of course,we do not know what kind of newevidence may turn up, but most ofus think that with the giant colli-sion theory we have the right sce-nario for the origin of the Moon.

Evolution of the Moon

The time scale of evolution of theMoon was reconstructed from theradioactive ages of lunar rocksdetermined by several laborato-ries in the United States and inEurope.

Accretion of the Moon from thematerial circling the Earth proba-bly took only a few years. Theoriginal heat content, the gravita-tional energy liberated in theaccretion process and heatingfrom the extremely strong tidalforces caused by the proximity ofthe Earth combined to melt theMoon in its early history. Chem-ical differentiation set in and pro-duced a crust of light material thathas been preserved up to today,especially in the oldest region onthe Moon, the highlands on itsbackside.

The early Moon

Initially, the tidal forces betweenEarth and Moon were more thana thousand times stronger thanthey are today, causing significantheating of the Moon and enor-mous tides on Earth. These tidalforces led to a transfer of angularmomentum from the rotation ofEarth and Moon to the orbitalmotion of the Moon. WhenMoon and Earth were close, thetransfer was fast; the Moon reced-ed quickly from Earth, the dayslengthened, and soon the Moon’srotation slowed down, so that

we always see the same side of it.With the receding of the Moon to its present distance, the tidalforces lost most of their strength.Dangerous tides occur only in afew coastal regions, but there maybe another important effect: it hasbeen calculated that the orbitingMoon has stabilized the rotationaxis of the Earth, and this in turnhas a stabilising effect on theEarth’s climate.

Using various ‘radioactive clocks’,laboratories in the United Statesand Europe measured the abso-lute ages of numerous lunar rocksbrought back by the Apollo astro-nauts and reconstructed the timescale of lunar evolution. The fig-ures 9, 10 and 11 show the Moon atthe end of important epochs in itsevolution.

The accretion process in the solarsystem did not end abruptly: largechunks of material were stillroaming around in the spacesbetween the planets, even severalhundred million years after theSun was born. On the Moon, the last major impacts occurred inthe time-span 4’050–3’850 mil-lion years ago (Figure 9). These lateimpacts excavated the large marebasins that were later filled withbasaltic lava rising up from below.More than ten impacts, formingcraters with diameters of morethan 500 km, occurred during thistime-span. On Earth, we have norecord of this catastrophic epoch.However, taking into account thelarger size and gravity field of theEarth, we estimate that more thana hundred craters of these dimen-sions were excavated on our plan-

9SPAT IUM 5

et during that period. For theemerging atmosphere and oceans,this must have been a catastrophicepoch.

Even before the last mare basinswere excavated on the Moon, lavahad begun to rise and fill thesebasins. These were basaltic lavas,richer than the crust in iron, mag-nesium and titanium. The darkcolour of the Maria on the Moon

is due to minerals containing theseelements, in particular the titani-um-iron oxide called ilmenite.

The ages of the mare basaltsbrought back by the Apollo astro-nauts and Russian Lunar missionswere found to range from 3’800to 3’200 million years (Figure 10).A few small rock pieces in theOceanus Procellarum were asyoung as 3’000 million years. Fur-

ther east there may be some areascovered with even younger basalts,but there are no samples and noabsolute ages from this region. Inany case, the formation of marebasalts that marks the last majorgeologic epoch on the Moonended not much later than 3’000million years ago. Since then, theMoon has been geologically in-active, its energy reserves exhaust-ed.

Figure 9The Moon 3’800 million years ago.500 to 700 million years after its forma-tion, the Moon was still bombarded bylarge chunks of material, forming thehuge basins that are still distinguishable.Within about 200 million years, morethan ten craters with diameters of morethan 500 km were formed. On Earththe record of this bombardment hasbeen lost, but we can estimate that morethan 100 such craters were formed onEarth during the same epoch. This musthave been a fantastically catastrophicperiod in the Earth’s history, at a timewhen oceans and atmosphere were inan early stage. We would not knowabout this catastrophe if we had notinvestigated lunar rocks in Earth-boundlaboratories.

Figure 10The Moon 3’000 million years ago.At about 3’800 million years ago, thebasins began to fill with lava. Shown arethe ages of the local mare basalts in mil-lion years, determined by radioactiveclocks. From West to East: OceanusProcellarum (3'200 million years), MareImbrium (3'300), Mare Serenitatis(3'800), Mare Tranquillitatis (3'600–3'800), Mare Crisium (3'600). Theseages show that this process was largelycompleted 3’000 million years ago. Thelarge plains, which Galileo called Maria,have existed on the Moon since thattime. We know now that their darkcolour is due to a titanium-iron mineralthat is relatively abundant in the marebasalts.

Figure 11The Moon today. About 3’000 millionyears ago, the Moon cooled down suffi-ciently, so that geological activity after-wards did not change the face of theMoon very much. The main changesare from occasional impacts. On thefront side of the Moon, the largest‘fresh’ crater is Copernicus, which wascreated by an impact about 900 millionyears ago.

The Moon 3’800 million years ago

Time of Impact (million years)(Formation of Basin)

Mare BasaltsAges in million years

Age of Copernicus (million years)(Note: Age of Cambrium is 500 million years)

The Moon 3’000 million years ago The Moon today

3’850

3’890

3’890

3’910

3’300

3’200

3’800

3’600

3’600–3’800900

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The serene Moon

In the absence of geological pro-cesses and without water orwinds, the face of the Moon hasnot changed very much duringthe last 3’000 million years, as acomparison of figure 11 with fig-ure 10 shows. Erosion at the lunarsurface is now mainly caused bysmall impacts (Figure 12). This pro-

cess is extremely slow, so thateven small surface features last fora long time (Figure 13). Major im-pacts were very rare during thelast billion years. On the frontsideof the Moon, the impact creatingthe Copernicus crater is the mostprominent among them. It oc-curred about 900 million yearsago (Figure 11). Sometimes Coper-nicus is called a young crater. Thisexpression illustrates the funda-

mental difference in the evolutionof Earth and Moon. Copernicuswas formed before the Cambrian,the earliest of the classical geolog-ic epochs. Since on Earth plate tec-tonics and volcanism continue tothe present time, the effect of im-pacts on the geologic evolution isminor. However, the catastrophescaused by large impacts have had amajor influence on biological evo-lution (Figure 14).

Figure 13The footprint of Neil Armstrong.There are neither winds nor rain on theMoon and erosion is very slow. Thus,the chances are good that this footprintof Neil Armstrong, the first man on theMoon, will still be visible a millionyears from now. There is, of course, aminute statistical chance that this foot-print will be impacted by a largerobject, but then other footprints of the Apollo crews would survive (NASAphoto).

Figure 12A microcrater in a piece of lunar glass. In the present epoch, small impacts are themain cause of erosion, “gardening” and lateral transport (photo by Norbert Grögler,University of Bern).

10 µm

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Mars

Mars is intermediate in size be-tween Earth and Moon, and itseems to be intermediate as wellin the degree of geological evolu-tion (Figure 2). This is to be expect-ed: Small bodies cool faster thanlarge bodies, since energy produc-tion and energy content are basi-cally proportional to the volume,whereas energy losses are propor-tional to the surface. It is for thesame reason that warm-bloodedanimals need a minimum size tobe able to maintain a body tem-perature that is above the ambienttemperature.

Asteroids have diameters below1’000 km. Thus, they cooleddown and became geologicallyinactive only one or two hundredmillion years after they wereformed. This has been confirmedby radioactive dating of mete-orites that are rock pieces fromasteroids. The Moon, with a diam-eter of 3’476 km, became geologi-cally inactive about 3’000 millionyears ago. Because of its size, theEarth has remained geologicallyactive throughout its history. As aconsequence, the geological rec-ord of the first 1’000 million yearsof its lifetime is essentially lost.

The ages of the few Mars mete-orites range from 4’500 to 150million years. Apparently, somerock-forming processes have beengoing on quite recently. On theother hand, the old ages indicatethat the record of the earlyMartian history was not overwrit-

ten as much as on Earth. This is inline with the size of Mars, whichis in between the sizes of Earthand Moon (Figure 1).

The Exploration of Mars

The radioactive dating of Martianmeteorites gives a general idea ofthe absolute ages of geologic fea-tures on the Red Planet. A relativetime scale of the sequence of geo-logic epochs identifiable on Marsis being set up by scientific groupsat the Deutsches Zentrum fürLuft- und Raumfahrt, Berlin, andthe Planetary Science Institute inTucson, Arizona. The method isby counting craters, which hasbeen successfully applied on theMoon.

Presently, earth and planetary sci-entists are engaged in reconstruct-ing the geological evolution ofMars and in estimating absolutetime scales for the most promi-nent events in the Martian history.These issues were discussed in arecent workshop on “the Evol-ution of Mars” at the InternationalSpace Science Institute. Theresults from Martian meteoriteswere compared with photogeo-logical evidence obtained fromorbit and with the data obtainedby the Sojourner, the vehicle land-ed by NASA on the Martian sur-face on July 4, 1997. The wholeworld watched as the Jet Pro-pulsion Laboratory in Pasadenasteered the Sojourner to somenearby rocks, so that the chemicalsensor of the Max-Planck-Institutfür Chemie, Mainz could analyse

Figure 14The Cretaceous/Tertiary boundary.On Earth, impacts did not have a largeeffect on the geological or geographicalevolution of the surface, because vol-canism and plate tectonics were domi-nant. Impacts of meteorites, however,had a major effect on the biological evo-lution. The best-documented case is theevent that occurred 65 million yearsago, marking the boundary between theCretaceous and the Tertiary age. At thattime, a large impact – probably locatedat the edge of the Gulf of Mexico –caused the extinction of many species,giving way to the evolution of newspecies. Thus, in sedimentary deposits,the CT boundary is marked not only byimpact-specific materials (iridium,soot), but also by an abruptly changingmicro-fauna over a very short interval(Figure by O. Eugster).

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them. At first, the outcomeseemed surprising: the chemicalabundances were completely dif-ferent from the abundances in the Martian meteorites. However,the Sojourner investigated rocksin one of the dry riverbeds, the Ares Valley, whereas most of the Martian meteorites probablycome from volcanic highlands.Thanks to the Sojourner we nowknow of the very large variety inthe composition of Martian rocks.This indicates that chemical dif-ferentiation was much moreintense on Mars than on theMoon.

The evolution of Mars

The two most outstanding geo-logical features on the Martiansurface are the huge volcanoesand the evidence of the existenceof liquid water in a past epoch.The largest of the Martian volca-noes, Olympus Mons, is shown inFigure 15. It came as a great surprisethat the volcanoes on Mars arehigher and more massive than thehighest mountains on Earth (Fig-ure 16). The principal reason is de-picted in Figure 17. On Earth, platetectonics causes continental driftsas predicted by Alfred Wegenerearly in the 20th century. Thus,plate tectonics determines theglobal geography on Earth, and itproduces the high continentalmountain ranges. On Mars, theinternal energy is not sufficient todrive plate tectonics. Thus, volca-noes are the dominant mountains,and they can grow without beingcut off from their hot spots below.

Figure 15The Olympus Mons. The most outstanding geographical feature on Mars is thelarge volcanoes (photo by Viking Orbiter, NASA) .

Figure 16Olympus Mons versus Mauna Kea: Olympus Mons and other volcanoes on Marsare much higher than the largest volcanoes on Earth, even if we measure these fromthe bottom of the ocean. This may seem surprising, since one might expect less in-tense volcanic activity on Mars than on Earth, because of the difference in size.

Olympus Mons 15 miles high

45 miles

Mount Everest 5 1/2 miles high

Mauna Kea 61/3 miles high

Sea Level

OceanFloor Vertical Scale exaggerated 2 times

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Contrary to the Moon, wherethere are no traces of past waterflows, we see large dry riverbedson Mars. The difference betweenthe lunar rills and the Martianriverbeds is demonstrated in pic-tures taken from orbit: Martianriverbeds have extensive tributarysystems (Figure 18). The long lunarrills on the Moon have no tribu-taries, the rills are thought to stemfrom lava flows (Figure 19).

The larger riverbeds on Mars com-pare with the Nile valley. At some– as yet – unknown time in the past,large amounts of water were trans-ported, and they must have filledsignificant portions of the lowlands. Figure 20 is a topographicmap, with the area that mighthave been temporarily filled by

the rivers shown in blue. The exactextension of this temporary oceanis still debated, but all indicationsare that it covered a significantportion of the Martian surface.Jim Head of Brown University,Phillip Masson of the Universitéde Paris and other photogeol-ogists are working to identify theancient shorelines. If successful,we can quantitatively determinethe amount of water that waspresent.

Where did these water masses go?A considerable fraction has proba-bly been lost, but some must haveleaked through the ocean bottomand is preserved as a permafrostlayer.

Life on Mars?

Liquid water is a prerequisite forthe emergence of life, at least ofthe form of life we know. Sincewater is liquid only in the tem-perature range of about 0 °C to100 °C, we could expect life on a planet only in a narrow range ofdistances from the central star.Thus in the solar system, we havepresently only one body, theEarth, with liquid water on its sur-face. Because of radioactive heat-ing, planetary temperatures in-crease with depth, and therefore itis believed that water could be liq-uid at certain layers below the sur-faces of Mars and some moons ofdistant planets with icy surfaces,such as Europa.

Figure 17Origin of the Hawaiian volcanic islands: Plate tectonics (“continental drift”) onEarth is the answer to the apparent contradiction shown in Figure 16. The oceanicplate moves with a speed of a few centimetres per year over a hot spot in the mantle,creating a chain of volcanic islands. On Mars such plate motions are absent and,therefore, the lava coming from a hot region in the mantle is piled up in one placeover a long period of time.

Figure 18River system on Mars: Sometime inthe history of Mars there was a fluvialperiod. The extensive river systems areevidence of this.

Figure 19The Hadley Rille: On the Moon thereis no indication of liquid water, presentor past. But how do we know this?Hadley Rille and the other lunar rillshave no tributaries: they were probablymade by lava flows. On Mars, the tribu-tary systems of valleys are the most ob-vious proof that they are dry riverbeds.

plate moving over the hot spot in the mantle

relative position of hot spot (million years)

53

1.5today

hot spot

Motion of the Plate

P a c i f i cO c e a n Ki laueaHawai i

Maui

Kahoolawe

Molokai

Lanai

Oahu

Ni ihau Kauai

N

We have strong evidence that dur-ing an earlier epoch there werelarge volumes of liquid water onthe surface of Mars. It is not yetknown during which epoch inMartian history the rivers and theocean existed (Figures 18 and 20),nor do we know whether therewere several fluvial periods or onlyone. In any case, since Mars is rel-atively close, the Red Planet offersthe best chance for discoveringextraterrestrial life or for identify-ing relics of extinct life.

Traces of life in Martianmeteorites?

Reports on the observation of longstraight “channels" on Mars werenot confirmed by orbital photo-graphy. The Viking spacecraft,landed by NASA in the 1970s, didnot turn up any evidence of life,although they were equipped withsome suitable instruments. Then,a few years ago, a group of scien-tists in the United States foundspherules in the oldest Martian

meteorite, and after elaborate mi-croscopic investigations and chem-ical analyses, they concluded thatthese spherules could very well berelics of life. The evidence is in-triguing, but the discussion of itssignificance is continuing. Indeed,we cannot expect to have a con-sensus concerning the biologicalnature of these spherules unlesscorroborative evidence is foundon Mars.

If life on a planet were confined toa limited area, a puddle of wateror a cave, it would be extremelydifficult to discover. After all, thesurface of Mars is nearly as large asall continents on Earth taken to-gether. Fortunately, however, oncecreated or imported, life seems tobe extremely adaptable and pro-lific. This opens up another meth-od of searching for life, which we may call the geochemical ap-proach. On Earth, this approachhas proven to be valuable forsearching for early traces of life.Large ferric iron deposits that

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Figure 21Buhwa iron ore deposit.This sedimentary iron ore deposit inZimbabwe is about three billion yearsold. The iron in this ore is highly oxi-dised. It is thought, that these ores arethe oldest indication for the presence ofoxygen produced by photosynthesis ofalgae (photo by Jan D. Kramers, Uni-versity of Bern).

Figure 20Ancient ocean on Mars. The dry riverbeds on Mars with their tributaries gotowards the large low-altitude plains in the north. Considering the amounts of waterthat have cut the large valleys, the water masses must have fed large oceans. Theestimated extent of the ocean is shown here in blue. Elevation is given in kilometers(from J.W. Head, Science, 286, pp. 2134 – 2137, 1999).

–8 –4 0 4 8

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240°

60°

formed only in the early Pre-cambrium (Figure 21) are thoughtto be the oldest evidence of life.Most limestone deposits, such asthose lifted to the top of MountEverest or the Bernese Alps, areproduced by life. If life existed onMars during a fluvial epoch, itwould have spread over a largearea. Thus, searching for geo-chemical traces of life on Marsshould be considered. G. Turnerand J.D. Gilmour of the Uni-versity of Manchester proposed tolook for a life-specific iodine dis-tribution on Mars.

Life on Earth!

Let me finish this evening’s lecturewith a tale that may illuminate thegeochemical search for life. Thesociety on a planet orbiting a near-by star decided to explore theirstellar neighbourhood. In a plan-et-wide effort, they built a fleet of extraordinary spaceships thatwould enable them to reach adozen or so stars in their neigh-bourhood during a crew’s life-time. However, by giving so muchpriority to rocketry, they neglect-

ed to develop other equipment,such as high-performance tele-scopes and sensitive detectors.Ulysses, when cruising in theMediterranean, found himself in acomparable situation. He had amarvellous ship, but no telescope,poor maps, and no idea of the dan-gers awaiting him. The spaceshipwith the extraterrestrials on boardgoing towards the solar systemwas successful. After short, unre-warding visits to the Jovian moonsand to Mars, they turned towardsthe Earth. In a direct approach andwithout much “reconnaissance”,they chose to land on the westernflank of the Jungfrau, because theBernese Alps were among the fewplaces that protruded through thecloud layer covering all Europe atthe time of their landing. All theycould see was snow and rocks(Figure 22). One of them tested therock with a knife: “It’s limestone,there is life on this planet or atleast there was in the past!” Cau-tiously, he opened his suit andfound he could breathe. So therewas oxygen. On their home-planet the oxygen was produced and continuously replenished byplants. This convinced them thatthere was a rich flora below theclouds. Slowly, they climbed down.Suddenly, the cloud cover brokeup, and there it was: grass, cows

and even some people at a dis-tance. When the visitors cautious-ly approached, one of the earth-lings began to talk, and it soundedstrange, but friendly. “So that isthe language of the people on thisplanet, for us impossible to under-stand”, they concluded. It was onlymuch later that they found outthat almost nobody else on Earthunderstands these nice people liv-ing in the foothills of the BerneseAlps either.

Acknowledgements

I am indebted to HansjorgSchlaepfer, the editor of Spatium,for his help with the manuscriptand his perseverance in gettingthis issue published. I thank SilviaWenger, Ursula Pfander andGabriela Indermuhle for theirhelp with the manuscript and thefigures, and Diane Taylor for criti-cally reading the manuscript. Withthis issue of Spatium I would liketo remember two distinguishedcolleagues and friends, Paul Gastof Columbia University andNASA Johnson Space Center andNorbert Grogler of the Universityof Bern, with whom I had closecooperation during the Apolloperiod.

Figure 22The Bernese Alps shown above theclouds From right to left: Jungfrau,Mönch and Eiger.

SPAT IUM

The author

Prof. Johannes Geiss received hisdoctorate in 1953 from the Uni-versity of Göttingen and, afterseveral research positions in theUSA, became a lecturer (“Hab-ilitation") at the University ofBern in 1957. After a professor-ship in oceanic science at theUniversity of Miami, he returnedto the University of Bern as a pro-fessor in 1960, where he wasnamed director of the Physi-kalisches Institut in 1966. In1982 / 83 he was rector of theUniversity of Bern, as well.

During his tenure at the Uni-versity of Bern, he had extendedstays as a visiting scientist inFrance and the USA. The publicconnects the name of ProfessorGeiss with the solar wind experi-ment, the first experiment theAmerican astronauts set up on theMoon in 1969.

Since then, Professor Geiss hasbeen heavily involved in scientificprojects of NASA and the Euro-pean Space Agency, E SA.

Johannes Geiss is co-founder ofthe Association Pro I S S I and pres-ently the Executive Director ofthe International Space ScienceInstitute, I S S I , in Bern. I S S I issupported by a foundation of thesame name, endowed initially byContraves Space in Zurich. TheInstitute is primarily funded byE SA, the Swiss Confederation,the Canton of Bern and the SwissNational Science Foundation.ISSI is dedicated to interdiscipli-nary research of the solar systemand the universe, inviting scien-tists from around the world toworking group meetings andworkshops and offering them theopportunity to exchange ideas andevaluate their newest researchresults. The basis for I S S I 's re-search activity is the data providedby the scientific missions of thespace agencies of Europe, the US,Russia and Japan as well as labora-tory experiments and computersimulations.