sign in sign up
<<
Home , Geology of Venus

CLIMATE HISTORIES OF AND , AND THE HABITABILITY OF

In the temporal sequence that Part III of the book has ¡NTRODUCT¡ON 15.1 been following, we stand near the end of the Archean eon. at the close of the Archean,2.5 billion years ago, By this point in time, the evolution of Venus and its atmo- was a world in which life had arisen and sphere almost certainly had diverged from that of Earth, dominated, the evolution of the crust and the recycling of and Mars was on its way to being a cold, dry world, if volatiles. Yet oxygen (Oz) still was not prevalent in the it had not already become one. This is the appropriate atmosphere, which was richer in COz than at present. In moment in geologic time, then, to consider how Earth's this last respect, Earth's atmosphere was somewhat like neighboring planets diverged so greatly in climate, and to that of its neighbors, Mars and Venus, which today retain ponder the implications for habitable planets throughout this more primitive kind of atmosphere. the cosmos. In the following chapter, we consider why Speculations on the nature of Mars and Venus were, Earth became dominated by plate tectonics, but Venus prior to the space program, heavily influenced by Earth- and Mars did not. Understanding this is part of the key centered biases and the poor quality of telescopic observa- to understanding Earth's clement climate as discussed in tions (figure 15.1). Thirty years of U.S. and Soviet robotic chapter 1.4. missions to these two bodies changed that thinking dras- tically. The overall evolutions of Mars and Venus have 15.2 VENUS been quite different from that of Earth, and very differ- ent from each other. The ability of the environment of a 15.2.1 Onigin of Venus' Thick Atmosphene to veer in a completely different direction from that of its neighbors was not readily appreciated until the eter- The contains somewhat more nitro- nally hot greenhouse of Venus' surface and the cold deso- gen than does that of the Earth: 3 atmospheres of pressure lation of the Martian climate were revealed by spacecraft instead of 0.8 atmosphere. More striking, however, is the instruments. enormous surface pressure of 90 atmospheres of carbon Flowever, robotic missions also revealed evidence that dioxide. The consequence of Venus'massive atmosphere is Mars once had liquid water flowing on its surface. It is an enormous greenhouse effect: Even though the clouds of tempting, then, to assume that the early Martian climate Venus' upper atmosphere, largely sulfur compounds, re- was much warmer than it is at present, warm enough per- flect much more sunlight away than do the clouds of Earth, haps to initiate life on the surface of Mars. However, the Venus has a surface temperature of 730 K. In other words, difficulty of sustaining a warm Martian atmosphere in the even though the surface ofVenus receives less sunlight than face of the faint-early-sun problem of chapter 14 remains a does the Earth's surface, the temperature at Venus' surface daunting puzzle, one that is highly relevant to the broader is above the melting point of lead. Liquid water is not sta- question of habitable planets beyond our . ble on the surface or anywhere in the atmosphere; gaseous !íhat is the range of distances from any given star for water vapor is only 10 parts per million of the atmosphere. which liquid water is stable on a planetary surface and Oxygen is not abundant either, with a pressure of 0.002 Iife can gain a foothold? atmosphere, one-hundredth that in our atmosphere. PLANETS 174 CLIMATE HISTOFìIES OF MAFìS AND VENUS, AND THE HABITABILITY OF

Figune 15.1. Prior to th by hand typically showed telescopic images and ca some as a sign of intellige Lowell intenpneted these illusory features as vast to the parched equatonial deserts, a granden vers the Ari)ona and ialifo¡nia deserts south ând west of his high plateau observatony'

retained' Although alternative models How Venus came to this state is still a subject of heated more likely to be proposed (for example, that the high deuterium debate. Venus is almost the same size as Earth, of similar have been from impacting comets), the density (and hence internal composition), and somewhat abundance is a contaminant at present to be the best expla- nearer to the Sun. One clue is the close correspondence water-loss model appears of the amount of in Venus' atmosphere nation for the deuterium data. have liquid water early in the solar sys- with the amount of carbon dioxide that could be produced If Venus did challenge is to understand how it was from the and other carbon compounds trapped tem's histor¡ the explanation for the loss today in Earth's crust' If Earth's oceans were to boil awa¡ lost and when. The traditional runaluøy greenhouse,featured in many and the hydrological cycle of rainfall end, recycling of cat- lies in the so-called the solar heating at Venus' distance from bonates into the atmosphere might eventually build up a textbooks. Here, a sufficient amount of initial green- massive carbon dioxide atmosphere on our planet as well' the Sun, coupled with carbon dioxide, leads to an The divergent evolutionary paths that Earth and Venus house heating from water and Heating causes more evaporation of have taken apparently have to do with the lack, or early unstable situation: (because the evaporation rate and Ioss, of large quantities of water from Venus. Direct mea- water from the ocean content in the atmosphere are very surement of Venus'atmosphere fromPioneer Venus entry the total water vapor This higher water content' probes in 1,978 revealed a large abundance of deuterium sensitive to the temperature). temperature through (defined in chapter 2) rclative to light in the in turn, increases the atmospheric which in turn causes more water atmosphere of Venus, the ratio of the two being about the greenhouse effect, the atmosphere further' The sys- 150 times that in the oceans of Earth' One interpretation to evaporate, warming leading quickly to the complete of such an overabundance is that large amounts of wa- tem enters a "rrtîaway," ter escaped from Venus early in its history; as the water boiling away oÍ the oceans. of the early history of Venus was lost in gaseous form from the atmosphere' the heav- Very careful modeling a runaway greenhouse was marginal ier deuterium atoms in HDO and D2O (versus H2O) were shows that at the time, 15,2 VENUS 179 for that planet. The reason lies again in the faint-early- transparent to infrared radiation, the amount of water va- sun problem. Although toda¡ Venus receives 1.9 times por drops very steeply. At about 10 km above the surface the amount of sunlight that Earth does at the top of the lies a boundary between the lower atmosphere, the tropo- atmosphere (remember much of this is reflected by Venus' sphere, and the stratosphere above it. This boundary, the clouds), in the earliest period of solar system history the tropopause, is defined by the altitude at which the tem- sunlight that Venus received was only 1,.4 times that re- perature stops falling and begins rising at higher altitude ceived by Earth at present. Below a certain threshold sur- as the air becomes transparent to most infrared radiation, face temperature, the greenhouse effect does not evaporate and some molecules selectively absorb sunlight in the ul- enough water to initiate a runaway. traviolet wavelengths. Above the tropopause, water vapor So how did Venus arrive at its present state? The solu- no longer decreases with increasing altitude; its minimum tion to this puzzle lies in considering the effect of water value is determined by the temperature at the tropopause. vapor on the entire atmosphere, as shown in figure 15.2. In Earth's atmosphere toda¡ the dropoff in temperature On Earth toda¡ because the temperature drops rapidly with height leads to a very sharp decline in water vapor with altituCe as the atmosphere thins and becomes more with altitude. The water vapor condenses as clouds and these eventually are lost as rain. The Earth's stratosphere is extremely dry today, about as dry as the present bulk atmosphere of Venus. What water vapor does exist in the stratosphere is subject to being broken apart by ultraviolet photons from the Sun to form oxygen (Oz) and hydrogen;

É because hydrogen is a light molecule, it moves upward in -: 100 the atmosphere and eventually is lost to space. The ul- õ traviolet radiation is restricted to high altitudes precisely because it is absorbed there by molecules such as water 50 and ozone; the vast majority of Earth's water is protected from such destruction by being resident in the oceans and lower atmosphere. Consider now what would happen if Earth's surface temperature were increased, simulating what might have happened on Venus if it once had had liquid water oceans. More water vapor is put into the troposphere, allowing formation of more massive cloud decks. Clouds can warm or cool the climate, depending on their altitude, but their

v formation by condensation always releases heat, which o¡: 100 E causes the temperature profile to fall more gently with altitude. Because of this effect, the temperature profile for higher surface temperatures declines more gradually 50 than for lower surface temperatures, and the tropopause boundary between the troposphere and the stratosphere 0L moves upward as the surface warms (figure15.2). More 10-6 10-s l0-4 10-3 l0-2 10-r 1 eventually HzO Volume Mixing Ratio water is admitted into the stratosphere, and large amounts of water are present at altitudes accessible Figure 5.2. A moist greenhouse atmosphere in action. The 'l to solar ultraviolet photons. For a surface temperature just tempenatune [top] and amount of water vapor [bottomJ ar^e plot- ted versus altitude for diffenent values of the sunface temPera- 80 K above Earth's current global mean value, the water ture. Each pr"ofile is marked with its particulan surface tempen- vapor at high altitudes increases by a factor of 10,000. atune, Is. The water-volume mixing natio is simply the number In effect, then, a global surface temperature above 340 K of water molecules divided by the numben of all molecules (of all budget the atmosphere, chemical speciesJ in the atmosphere at a given altitude. Hence "pops the cork" on the water of a waten mixing ratio of 1O-3 means that one out of every thou- allowing large amounts of water vapor to flow to altitudes sand molecules is waten The stnatosphere is simplified in the where solar ultraviolet radiation breaks it apart, and the calculation by assuming that ib has a constant tempenatune of hydrogen escapes. This moist greenhouse crisis operates at 2OO K; in neality, its temperature is not constant. See text fon a descniption of the moist gneenhouse loss of waten Bepnoduced lower solar fluxes than is required for the runaway green- from Kastlng (19881 by permission of Academic Press, house; for an Earth-like atmosphere with and a c¡ (-, --,t -sl-- -'_ : -. a ¿ = , 'a' : i -- -) )- l/ /> = ,t \ i. \ ll I I -a= ì Ir .,4, "l I = .. |..:- t\ 2 ' 'tt' 2.. ' "r¡"lc: I : zi t-- .; Z.tl\ t rl9o el ,!_:, - / ;, '- i, -r I --s 2 :.9 aç- /." iF =er cð| -. : jtj.i- ^&- I i = / , I /. I I

I I ; I {¿- 3tt= l/ / t; | 'l Ita- It \ a,

lr I : I = I l. , .1 I I lr

I | 7, s9 l\ 't 1l: \ ;l ¡g I 2=Í -l I J /7 ¿al 15,2 VENUS 181

of COz, the threshold for the moist green- small amount 15.2.2 Overview of the Surface of Venus house is just 1.1 times the present solar flux received by Earth. This flux is well below that which was received by Although early Soviet and U.S. probes measured the at- Venus during the faint-early-sun epoch, but above that for mospheric composition and temperature of Venus, map- Earth throughout its history. ping the of the surface was hindered by a global 'We can imagine what happened to Venus early in its mass of sulfurous clouds at high altitude. First from history. Possessed of an atmosphere with at least as much Earth, then radar from two Soviet orbiters Veneras 15 COz in gaseous form as Earth possesses toda¡ Venus' sur- and 16 and the IJ.S. spaceuaft have enabled face was above the temperature threshold for the moist mapping of the surface. A radar mapper functions like a greenhouse even when the solar flux was only 70"/o of camera that provides its own flash or source of illumina- its present value. If liquid water did exist on the surface, tion. Photons at radio wavelengths (chapter 3) can pene- the atmospheric temperatures were high enough to allow trate the clouds, and the radar transmits such photons to evaporated water to flow freely to the tenuous upper atmo- the ground surface. These are reflected and scattered, and sphere. Ultraviolet photons broke up the water molecules, some are received back at the radar antenna. By coding causing most of the hydrogen to be lost and eventually or shaping the transmitted pulse of photons, and taking depleting the planet of water. The signature of this lost advanfage of the orbital motion of the spacecraft, the re- ocean is with us today in the form of a high Venusian ratio ceived photons can be arranged or mapped by computer of deuterium-to-hydrogen, because the heavier deuterium into an image of the surface at radio wavelengths. For de- tended to be left behind in the atmosphere as hydrogen tailed geologic work, the very-high-resolution Magellan escaped. images, collected at Venus ftom 1990 through 1993, arc Once bereft of surface water, the die was cast for Venus. of greatest use. Carbon dioxide in the atmosphere had no means of being The geology of Venus, on a broad scale, looks at first locked up in surface rocks because liquid water was not glance like the Earth's with highlands rising out of a low- available to make bicarbonates. The carbon dioxide that land plain, akin to rising above the ocean floor. we see today in Venus' atmosphere cannot escape from However, the proportion of land on Venus that rises above the top of the atmosphere, cannot be trapped in rocks at the mean surface elevation is far smaller than on Earth; the surface, and remains as a massive gaseous memento of likewise, there are few long, deep cuts in the crust like the early loss of water. Earth's submarine trenches (figure 15.3). Thus the signa- How quickly could the water have been lost? Obser- tures of mature plate tectonics - massive continents and vations of young stars suggest that the early Sun put out subduction zone trenches - are largely missing. It is as if more ultraviolet radiation than it does today though, as we were to look at Earth in the Archean eon of time, discussed in chapter 1.4, its overall energy output was when plate tectonics was just getting going and - lower. Based on the amount of ultraviolet radiation avail- al masses were small. Soviet probes have sampled several able at Venus from the early Sun, less than 100 million regions on the surface; all of the analyses are consistent years were needed to remove the equivalent of an Earth's with basaltic compositions (close to that of Earth-ocean ocean-worth of water. Flence there was little or no time crust). No strong evidence for granites has been found to lock up carbon dioxide as carbonates before the wa- anywhere on Venus. ter was lost, and because accretional heat likely was still The surface of Venus contains impact craters. Although contributing to a very hot early crust for Venus' most or the number of these is far larger than on Earth, it is less all of Venus' carbon dioxide complement washkely neuer than that of the and Mars. The number of craters is locked up in the crust. The 90 atmospheres of COz present consistent with a surface that has renewed itself through today probably is close to the original atmospheric abun- volcanic flows over geologic time, with the last overall re- dance, although some of the carbon dioxide could have newal of the surface being perhaps 300 million to 600 been added later from the Venusian by volcanoes' million years ago. (rühether the surface is continually or The moist greenhouse model has important consequences episodically active geologically is addressed in chapter 16.) for the habitability of planets in general, a point we return This is long after the loss of any putative Venusian water to in section 15.6. ocean, which occurred in the first few hundred million years of Venus' history. Flence, we expect to see no evi- dence of ancient oceans, and indeed there is no evidence Figure 15.3. Global topognaphy of Venus fnom the Magel- lan radan pnobe, with vanious features labeled. Countesy of for the past existence of liquid water anywhere on Venus' NASA,/JeI Propulsion Labonatory. surface. Figure 15.4. tal Sapas Mons, a 6OGkm-diameten, t .-S-t

high. Close examination of the caldera and flanks re- u"ã1" n"ttvo.ks of water-canved channels. lNote that this has a new, young cone perched in its caldera.l CourtesY of NASA.

E -l

15.3 MAFS 183

The thick atmosphere prevents small bodies from reach- bilizing effects of our large Moon. There is some weak ing the surface as well; however, the largest impactors go evidence in geological features across the Martian surface unimpeded. Because there is no surface water on Venus, that past tilt may have exceeded 50 degrees (the current craters and other landforms that are not buried in value is 24 degrees). erode very slowly. The mean slope of features is therefore About one year out of two, heating during the south- larger on Venus than on Earth, and the images of mountain ern hemisphere spring drives large quantities of dust into ranges are eerie in their evident absence of water the atmosphere, allowing more sunlight to be absorbed in (figure 15.4). the atmosphere and moving dust across the planet' These The lack of plate tectonics and its accompanying ge- global dust storms may last for weeks or months. 'Water ologic signatures on Venus is perhaps the most profound is present today on Mars as ice trapped at one or difference betweenVenus and Earth. Remarkabl¡ the pres- both polar caps, but probably is more abundant as ground ence of water is apparently an important condition for ice trapped in a zone of permanent freezing (permafrostl sustaining plate motions, and certainly for the formation throughout high- and mid-latitude regions of Mars' crust' 'SØater of continental masses that are present on Earth in abun- ice also condenses out in the thin atmosphere; storm dance (and may only exist on Venus in one location, if systems occasionally have been seen in orbiting spacecraft 'We at all). defer a more detailed development of this idea images. The search for life on Mars culminated in the land- to chapter L6, where the origin of Earth's plate tectonic ing of two sophisticated robot laboratories, Vikings 1 and geology is explored. Significant and striking geologic dif- 2, ín L976. These laboratories sampled Martian and ferences are appareît on these two planets that should be tested for chemical reactions that might indicate living pro- ridding themselves of the same amount of internal heat; cesses. No evidence of life was found in the dry regions understanding the origin of these differences is perhaps to which the landers had been targeted, sites that were the most important question in Venusian geology' chosen to maximize the chances of safe landings. Further- more, the abundance of organic molecules on the surface was so low as to be undetectable. The thin atmosphere of MARS I5.3 Mars, with no ozone shield, allows solar ultraviolet radi- ation to penetrate to the surface and break apart chemical Mans Today 15.3.1 bonds; organic molecules arc readlly destroyed in such an The Martian atmosphere is, in composition, very simi- environment, and much of the hydrogen is lost to space. lar to that of Venus, with carbon dioxide most abundant, Additionally, the in the soil is combined with oxy- nitrogen the secondary constituent, and water and oxy- gen in such a way as to make an extremely reactive mix- gen in minor abundance. Mars' atmosphere is diminutive ture that would quickly oxidize organic molecules. The compared to those of Venus and Earth, however. The sur- present surface of Mars, at least in the high plains, is an face pressure is only 0.006 of an atmosphere. The thin inhospitable location for life. atmosphere means that Mars has hardly any greenhouse warming. This, combined with its greater distance from 15.3.2 Geological Hints of a Wanmer Early Mars the Sun, results in a temperature range from as much as is at all times except 270 K at the equator to only 150 K at the polar caps. Unlike Venus, the Mars surface visible Cameras sent to Mars on robotic mis- Mars is a true opposite of Venus: a cold dry planet, with during dust storms. have mapped the surface in great detail from orbit air so thin that ultraviolet rays from the Sun penetrate to sions sites. The has been the surface, effectively sterilizing its uppermost soil' and at three landing Survey scientist Mars is so cold that the carbon dioxide atmosphere eloquently summarized by U.S. Geological Michael Carc (L984): freezes out seasonally at the poles. The Pressure in the at- mosphere therefore varies significantly over the Martian Mars, like the Earth, has had a long history of volcanic and year, is about twice an Earth year. The tih (oblï which tectonic activity, and its surface has been modified by wind, as Earth's; the sum- quity) of Mars currently is the same wâter, and ice. However, the fwo planets differ greatly in ge- mer sun shines on one pole, evaporating carbon dioxide ologic style. Materials that form at the surface of Earth are and driving it to the winter pole. Mars' axis, however, continually being cycled deep within the planet as a result of may undergo large shifts in its obliquity caused by grav- plate tectonics, and are continuâlly being redistributed across itational tugging of the other planets, principally ; the surface as a result of the vigorous action of liquid water' On Earth would suffer the same fate were it not for the sta- Mars, no deep recycling occurs; volcanic material brought to 18'4 CLIMATEHISTORIESoFMABSANDVENUS'ANDTHEHABITABILITYoFPLANETS

Valles in 1997, identifred one with the surface simply remains in huge volcanic piles' Moreover, al- ing in Ares July consistent with andesite, which though some water erosion has occurred, the cumulative effect an elemental composition because over the life of the planet has been trivial' Once relief is cre- would be suggestive of plate tectonics' Flowever, on ated, it largely remains. The result is a planet on which a record only the elemental abundances could be determined of sustained geologic activity is beautifully preserved in huge this mission, and not how the atoms are structured in a and volcanoes, enormous canyons' extensive fracture systems' mineral, the finding was ambiguous: The rock could be an giant flood channels. amalgam of basaltic material and more silica-rich debris from an impact. On the large scale, Mars shows no evidence for con- The lack of plate tectonics likely is caused by the small tinents and lowland (ocean-type) basins' The southern size of Mars, and hence less vigorous than on stands several kilometers above the northern hemisphere Earth. The lower amount of heat coming out of Mars hemisphere; the south is very heavily cratered in contrast results in a thick crust, which resists breaking into plates' to the north. This asymmetry may be the result of a gi- The style of geology is more like that of Venus than Earth, ant impact early in Mars' history; it is not at all what one but far less active than either' get with Earth-style plate tectonics' Two extensive would Evidence for water on Mars abounds' Impact craters uplands on Mars are sites of past volcanism' The largest appear to have melted ground ice; their peripheries show Tharsis, contains huge shield volcanoes, giant ver- one, ,þ, of extensive mudflow. Volcanoes heat the ground sions of the Hawaiian volcanoes. Again, they are clues to and release water; a number of runoff channels reveal that the static nature of the crust: llith no lateral movement' water was melted by the eruptive heat' Most intriguing is magma welling up from the interior keeps spewing out ma- evidence for a sustained warmer period on Mars contained terial on the same part of the nonmoving crust' building up in channels and canyons' The evidence is threefold: huge volcanoes in isolated locations. The Viking robotic landers sampled the at two widely separated landing (i) Networks of dry channels and valleys are present on sites in the northern hemisphere and found the rocks to Mars. Three basic forms can be identified (figure 15'5): be basaltic in composition. The Pathfinder lander, arriv-

images: outflow chan- Figure 15.5. Three types of dnied-up channels on Mars, in viking onbiten [a] nels. t 15.3 MARS 185

Figure 15.5. [Continued] Three types of dried-up channels on Mars, in Viking Orbiten images: [bJ Valley netwo¡ks in the ancient southenn highlands. [c] Runoff channels on the volcano Alba Patena. Countesy of NASA,/JeI Propulsion Laboratory. 186 CLIMATE HISTORIES OF MARS AND VENUS, AND THE HABITABILITY OF PLANETS

outflow channels, uølley netuorks, and runoff chan' An early period of warm conditions on Mars, with nels.The outflow channels appear to have been formed liquid water, requires a thicker atmosphere of carbon by the very rapid release of large quantities of wateS dioxide, perhaps several atmospheres or more of pres- or might have been carved by flows of debris (rocks' sure. Because it formed farther from the Sun than did mud) mobilizedby water. The flows in such channels Earth, in a cooler part of the solar nebula, Mars prob- were sufficiently energetic that they could have been ably started out with at least as much water and car- sustained under virtually any atmospheric conditions, bon dioxide as did Earth. An early thick atmosphere is including the cold, dry climate existing now on Mars therefore possible. During such a period, life could (under which slowly flowing water would quickly freeze have developed. Unlike on Earth, the climate apparently and then sublime to water vapor). The wide variation changed because carbon dioxide disappeared and tem- in abundance of craters on surfaces in and around the peratures fell below the freezing point of water' perhaps channels suggests that the channels formed episodically terminating Martian life. Vhether warm conditions oc- over the history of Mars. The valley networks, on the curred in multiple episodes, and how recent the last such other hand, have a form that indicates they were carved episode was, remain controversial. The interpretation of by slowly flowing liquid water or, alternativel¡ by col- some Martian features as glacial in nature is an impor- lapse of the surface (sappingl caused by groundwater tant part of the debate, because such features appear flow. The possible sources of the water include melting to be much younger than the bulk of the valley of buried ice and expulsion to the surface, melting of netvvorks. surface ices, or even precipitation of sno\M or rain, The The cause of the climate cool-downs might be tied valley networks occur primaril¡ but not entirel¡ on sur- to Mars' small size. On early Mars, carbon dioxide faces that are yety heavily cratered, and some of the im- could have been progressively locked up as carbonates pacts clearly occurred after the networks were formed. in much the same way as on Earth (probably without Most are therefore very ancient - dating to the end of the mediating step involving life). Mars, however, is much the heavy bombardment some 3.8 billion years ago. smaller than Earth and therefore has cooled more rapidly Because their formation requires conditions very dif- than our planet. The result seems likely to be a very thick ferent from those present today (much more restrictive crust that cannot slide horizontally in the form of re- than required for the outflow channels), they could be a cycling plates. The presence of a few massive volcanoes record of a time when the atmosphere was thicker and where heat is removed and the lack of typical plate tec- the climate warmer. A few younger valley networks, as tonic features attest to this interpretation. Thus, on well as runoff channels seen on the slopes of some volca- Mars, there was no plate tectonic activity and hence no noes, suggest that warm conditions (possibly localized) significant recycling of the crust: Carbon dioxide locked may have occurred multiple times in Martian history. up as carbonates would have remained that way' Loss (ii) Massive canyon systems formed by geologic processes of atmosphere by impacts was also important, since the show evidence of modification by liquid water' The small size of Mars and hence weak gravity (one-third the canyons merge into numerous channels that show fea- Earth's) encouraged escape of gases heated by impactors. 'lVhether tures caused by the flow of liquid water, Sedimentary formation or impact escape was the deposits within the canyons have been seen on orbiting more important loss process is a matter of current spacecraft images which suggest the former presence of debate. standing lakes. Even as surface temperatures fell below the freezing (iii) Some geologic features in various areas of Mars appear point of water, further (slower) formation of carbonates to have been carved by gløcial action, that is, the move- was possible. The result was the nearly complete loss of ment of massive amounts of surface ices under their atmosphere via escape to space and permanent storage own weight. The features include certain kinds of ridges as carbonate sediments. Perhaps changes in Mars' axial and troughs that resemble terrestrial landforms carved tilt or episodes of volcanism temporarily released carbon

by glaciers and called mor aine s and e s k er s (fi gure 1 5. 6). dioxide from the crust in subsequent warming episodes. Such interpretation is difficult to make without ambigu- Such a model for the evolution of the Martian atmosphere ity; other nonglacial causes for the features might have can be tested by searching for carbonates exposed on the been at work. If the glacial interpretation is correct, surface in selected locations on Mars. This is best done however, it implies surface conditions in which water by a combination of orbital surveys, using infrared spec- ice was stable against rapid sublimation, and hence re- trometers, and robotic visits to promising locations on the quires thicker atmospheric conditions. surface, Figure '1 5.6, Examples of unusual Mantian featunes captuned in Viking Orbiter images and interpreted to be glacial in origin by Kargel et al. [1995], tal Genenal negion in Arcadia Planitia. [b] Detail of the centen-left pontion of panel [a] showing troughs with central nidges that might have been fonmed by the action of glaciers on the underlying and surrounding rock and dirt. [c) Further enlargement of the uppen-centen portion of panel [bJ showing, below the ridge and trough, a feature that is similar to mounds formed where the glacial ice stops moving. 188 CLIMATE HISTORIES OF MARS AND VENUS, AND THE HABITABILITY OF PLANETS

Tunnel channels

d Figure 15.6. [Continued] Examples of unusual Martian featunes captuned in Viking Orbiten images and intenpreted to be glacial in origin by Kangel et al. [1995]. [d] Map of a region north of Lake Ontanio on North America, showing glacial tnough and ridge systems on a scale similan to the image in panel [cJ. Figure pnovided by J. Kangel, U.S. , Flagstaff.

pressure to sustain a given atmospheric temperature. Car- 15.4 WAS MARS REALLY WARM bon dioxide, like water, can form clouds, though much IN THE PAST? lower temperatures are required for a given amount of carbon dioxide to condense than for the same amount 15.4.1 Limits to a Carbon Dioxide Greenhouse of water. It is possible to plot the carbon dioxide pres- The picture of a warm early Mars is drawn by analogy sure for various temperatures at which carbon dioxide with Earth - a thick carbon dioxide atmosphere sustain- cloud formation will occur - the lower the temperature, ing a greenhouse effect in the face of a faint early Sun. the lower the pressure at which such clouds will form. Because of Mars' greater distance from the Sun compared An equivalent curve for water determines the altitude at to the Earth's - yielding only half the sunshine that Earth which water clouds form in Earth's atmosphere for par- receives - a higher carbon dioxide pressure is required to ticular conditions on any given day. Figure 15.7 shows sustain a certain temperature at any given epoch in the that CO2 cloud formation occurs on Mars, for present Sun's history. At least several atmospheres worth of car- solar luminosities, when the surface pÍessure of carbon bon dioxide, or more, were required for Martian surface dioxide exceeds 0.35 bar. This is many times less than temperatures to be above the freezing point of water early the pressure of carbon dioxide needed to warm the sur- in its history. face to the water melting point, and is a direct conse- As shown in figure 1.5 .7 , a potentially serious flaw arises quence of Mars' greatef distance from the Sun compared for a Martian greenhouse. For progressively smaller with Earth's. The carbon dioxide pressure needed to amounts of sunlight, one requires a higher carbon dioxide keep liquid water stable on early Mars is higher still 15.4 WAS MABS REALLY WABM IN THE PAST? 189

(CH+), ammonia (NH:), and various compounds of sul- fur have been proposed. The first two could plausibly be present only in the very earliest history of Mars, perhaps the first few million years, because they would quickly be ^40 broken apart by sunlight or surface reactions to form other species. The sulfur compounds might be more stable, but o ! their effectiveness as greenhouse gases is limited. An al- the of 4zo ternative way out of the problem comes from work Francois Forget of Univ. Curie of Paris and Ray Pierrehum- bert of the University of Chicago. They find that clouds made of large particles of COz ice can actually warm the Martian surface. Predicting particle size in clouds is dif- 0.1 1 10 r00 1000 ficult, but it is at least possible that a greenhouse atmo- P"ut / P sphere on early Mars was supported by CO2 clouds with property. Figure 15.7. Gneenhouse pnoblem for Ma¡s. The amount of can- this bon dioxide in the Mantian atmosphere is shown for various val- The existence of an early Martian greenhouse to sus- ues of the canbon dioxide sunface pnessure. Each pnofile is the tain warmer temperatures is a problem that has yet to be natio of the saturation pnessure of canbon dioxide to the actual satisfactorily resolved, in contrast to the equivalent issue pnessune as a functton of altitude; whene this ratio equals one [vertical dashed line], cloud formation occurs. The present-day for Earth. Our planet's closer proximity to the Sun ensured carbon dioxide pnessure nean the sunface [0.006 banJ leads to that only modest amounts of carbon dioxide were required pnoduce carbon dioxide clouds an atmosphene that does not [ex- to sustain equable temperatures, not enough to condense cept near the polesl. To get a Martian sunface wanm enough fon a po- liquid water, over 2 bars of pnessure is needed at the sunface out as carbon dioxide clouds and hence precipitate [5 bars during the faint-sun epoch]. Howeven, for any surface tential crisis. The search continues for plausible additional forma- pressure above O.35 ba¡ acconding to the figure, cloud infrared-absorbing gases and better understanding of CO2 tion will occun Repnoduced fnom Kasting t19S1lby permission properties, the skeptics warn that early Mars of Academic Press. cloud while may not have been warm for extended periods of time, or at all. (since the Sun was fainter), ensuring that CO2 cloud formation must be considered in models of a warm early 15.4.2 Abodes fon Life on Early Mans atmosphere. 'llhere The effect of carbon dioxide clouds on the early Mar- might life have begun and been sustained on early tian greenhouse is not fully understood, because both scat- Mars? The answer to this question depends on whether tering of sunlight and absorption of infrared energy (heat) conditions were very different from those at present. If may occur within such clouds. Most simple models Mars was much warmer, with a thick atmosphere, run- suggest that the net effect of the clouds is to cool the atmo- ning or standing water at the surface would have provided sphere. This, in turn, requires higher carbon dioxide pres- similar environments to those on the early Earth for the sures to achive a given surface temperature, which cannot formation of life. Flowever, if standing or running water be obtained because the gas simply condenses into thicker was transient or episodic, conditions may have been too and thicker clouds, and eventually to carbon dioxide snow unstable for prebiological chemical reactions to develop or rain. toward sustained, biochemical systems. Carbon dioxide cloud formation represents a poten- If life did form in lakes at the Martian surface, the ad- tially serious problem for the concept of an early Martian vent of freezingconditions would not have meant the im- greenhouse. One plausible way to circumvent the problem mediate end to life. NASA Ames scientist Chris McKay is to posit other greenhouse gases that enhance the effect and others have studied lakes in Earth's Antarctic conti- of the carbon dioxide. 'Water vapor is not a candidate' nent that are covered by a Iayer of ice year-round. They because the low temperatures at which the carbon diox- sustain photosynthesizing algae. The liquid region below ide cloud formation occurs are such as to keep the atmo- the ice is maintained in large measure by the warming ef- sphere extremely dry (water condenses out at much lower fects of sunlight, transmitted through the clear ice into the pressure, for a given temperature, than does carbon diox- liquid water below. In this respect, the ice-covered lake is ide). Vhat is required is a gas that condenses out at much analogous to a greenhouse atmosphere. However' the lake higher pressure, and is a good infrared absorber. Methane liquid also is stabilized by the warming effect of freezing 190 CLIMATEHISToRIESoFMABSANDVENUS'ANDTHEHABITABILITYoFPLANETS

for evidence of past Martian life would involve itself, which releases heat' (This is the converse process search for physical evidence (for example' stromatolites to the cooling of a drink by the melting of ice cubes') looking sediments)' chemical evidence (ratios of car- and field observations in the Antarctic sug- i.r Calculations ".t.i.nt unusual except in as bon or other isotopes in rocks that are gest that such lakes are stable for temperatures as low biological processes), and mineralogical evidence (types 240 K, fully 33 K below the freezing point of water' that are not normally found together' except Another possible birthplace and abode of life is hy- of minerals formed through the mediation of biological activ- drothermal systems in the early Martian crust' regrons when for extant life requires techniques to stim- liquid water circulates in the rock and is warmed ity). The search where or (more diffrcult)' to the interior and by the in- ol"t. urrd detect metabolic activity both by heat flowing from of isolate, reproduce, and then study the genetic materials sulating effects of being underground' At Earth's mid- The more exotic a Martian organism is o..".r ridg.r, hydrothermal systems are rich in the chemi- living orgãnisms' to Earth life, the more challenging is its detection' Such cal and thermal energy needed to support an array of living must be conducted in regions of Mars where life complete absence of sunlight' Such could searches organisms in the and was most likely to have formed and been sustained' have been the case on earlY Mars' are invariably the most difficult and dangerous sites It is difficult to estimate how long either of these two these reach. For cost reasons' the search will be conducted fypes of ecosystems might have lasted on Mars' The to vehicles. hydrothermal systems are particularly prob- using robotic gÅrrnd*"a.. for past life on Mars do not understand the Although the discovery of evidence i.rrr"ti. in this regard; we simply histor¡ pre- would be a profoundly moving moment in human details of Martian geologic history sufficiently well to the odds are against such an event in the near future' How- dict where such systems could have been most long-lived' equally important (but less dramatic), is the explo- 'llith regard to the ice-covered lakes, McKay and col- ever, for radon ;f Mars to understand the history of its climate and leagues arju. that they could have been maintained campargn to do so will pay off rcgard- years aftet mean annual surface tem- geology. A vigorous soÃe 700 million the 1".r, oi*h.ther evidence for life is ever found, because peratures fell below the fteezing point on Mars' If' for ^".grr.rr.rrt', and demise of putative warm conditions on early sake, Mars had a warm climate up through nature Mars remains one of the great mysteries in solar system 3.5 billion years ago, then ice-covered lakes might have exploration. To understand whether and why it happened contained liquid water as late as 2'8 billion years ago' to comprehend the remark- Earth, life was still solely in the form is L gain the insight necessary At this time on billion in able stability of our own planet's climate over four of single-celled prokaryotes, and the dramatic changes years. our atmospheric composition wrought by such organisms (chapter 1'7) had yet to take place' 'We re- thus expect that any life on Mars would have 15.5 PUTTING A MARTIAN HISTORY of its extinc- mained at the single-celled stage at the time TOGETHEH tion. Life might have been sustained to somewhat more Although speculative, here is a possible interpretation of recent times ifthe ice-covered lakes provided a bridge bet- the spacecraft data from Mars: After formation of Mars ween episodes of warmer conditions' triggered by release ã.t early epoch of heavy cratering' a period began of groundwater, but such conditions were likely short- utd was characterízedby high carbon dioxide abundance' lived and geographically isolated' The recent discoveries that the possible presence of other greenhouse gases' warm of bacteria living several kilometers beneath the surface of temieratu.es, and liquid water' This may have lasted for the Earth, in rocks that allow access to water and nutri- half a billion years, and primitive life might have formed' ents, hint at possible environments for life to exist on Mars Over time, the COz in the atmosphere was progressively today. Perhaps beneath the surface of Mars, warmed by carbonates or lost by impacts' and to recent volcanism, simple Martian locked up in surface heat sources related tem- atmospheric pressure and temperature decreased' As biota carry on. peratules dropped below the fteezing point, glacial ero- sio.t b..a-. more importânt than erosion by liquid wa- Life, and the as 15.4.g Searching fon Evidence of ter, Perhaps several subsequent warm epochs occurred Eanly Climate chance large impacts or volcanic activity broke the crust' carbon dioxide back into the atmo- to explore other worlds, and even releasing water and It is a daunting task Even- sphere and allowing liquid water on the surface' more so to search for evidence of microscopic life-forms however, these reprieves ended' The atmospheric that might be rare or hidden in inaccessible places' The tuall¡ 15.6 IMPLICATIONS OF VENUSIAN AND MAFìTIAN HISTOBY FOFI LIFE ELSEWHEFIE 1sJ1 carbon dioxide continued to be progressively locked up in Because evidence exists for liquid water on Mars in the the sediments, and water ice became trapped in polar caps distant past and isotopic data suggest ân ocean on pri- and as permafrost, in a process that continues up to the mordial Venus, one might argue that the range of habit- present cold, dry state. Life, initially retaining a toehold ability around stars like the Sun is from 0.7 to 1.5 AU. in lakes capped by ìÃ/ater ice, or in subsurface hydrother- However, life must have time to develop and evolve; if mal vents, eventually ran out of sustainable climates and we are interested in advanced life-forms we must look for became extinct or relegated to a few sites deep below the planets with stable climates for liquid water over billions surface, of years. Because the Sun is thought to be typical, most One possible geologic and climatologic histor¡ com- stars should slowly increase their luminosity over time, pared to that of Earth, is shown in figure 15.8. The speci- and hence the zone of "continuous" habitability around ficity of the figure, however, should not be construed to each star must be much narrower than the current Venus- mean that we understand Martian history in detail. Infact, Mars range. 'We there remains an important controversy about whether consider for the moment a star like the Sun, that is, early Martian climate was warm and equable over long of similar mass, composition, and luminosity history. The periods of time, or just in brief episodes. Furthermore, the requirernent of abundant liquid water early in a planet's questions of how late in geologic time the episodes of wa- history dictates that the outer edge of the habitable zone ter or debris flow in the outflow channels extended and be inward of the current Martian orbit because, during the origin of such outbursts are unresolved; such events the faint youth of the star, any planet at Mars' distance may be less relevant to the issue of life because they do may have the same difficulty in sustaining a greenhouse as not imply a global change of Martian climate. In one ex- Mars had. Computations by Pennsylvania State scientist treme view, Martian climate was neuer clement, and only Kasting suggest that an early carbon dioxide greenhouse impact- or volcanically induced outbursts of water broke is readily sustainable within 1.15 AU of a Sun-like star; the monotony of a dr¡ cold geologic history. The val- this could be extended outward for planets with other ley networks seemingly argue against such a vieq but greenhouse gases or "warming" CO2 clouds. some geologists have argued that processes associated with The inner edge of the habitable zone must be much groundwater release and flow could carve such valleys, closer to Earth than it is to Venus, because Venus suffered -We even under conditions not too different from today's. a moist greenhouse loss of water early in its histor¡ and will not know for sure until Mars is thoroughly surveyed as a solar-type star heats up, planets progressively more from orbit and on the surface over the coming decades distant than 0.7 AU will suffer the same fate. Computa- (figure 1.5.9 and color plate M). tions by Kasting and others suggest that planets inward of 0.95 AU will suffer a moist greenhouse crisis for a lu- minosity of the central star equal to that of the Sun to- IMPLICATIONS OF VENUSIAN AND I5.6 day. Hence, for a planet to sustain a liquid water surface MARTIAN HISTORY FOR LIFE ELSEWHERE over 4.5 billion years - the current age of the solar sys- The search for evidence of life beyond our solar system tem and the length of time for sentient life to develop on is among the most daunting technological challenges ima- Earth - it must be beyond about 0.95 AU from its Sunlike ginable. Direct evidence of life could come in the form of star. a radio beacon or even a visitation, but only from living The resulting zoîe of continuous habitability extends forms advanced enough to do so and motivated enough from 0.95 AU to 1.15 AU, that is, a width of 0.2 AU.'llhat to make contact. Absence of evidence of such contact is is the likelihood of finding a planet in another solar system not evidence of absence of life-forms, by any means' An orbiting at that distance from its parent star? In our own alternative approach, to examine neighboring systems for solar system, four rocky planets orbit the Sun between planets of the right size and in the right location for har- 0.4 and 1.5 AU. The mean spacing of the planets - four boring life, will yield interesting results even in the case in planets over 1.5 AU-is about 0.4 AU. If our system is which few or no such planets are found (the conclusion typical, we can say that the likelihood of other planetary then being that Earth is a rare pearl). systems having a continuously habitable planet is just the The exploration of Mars and Venus, and their scientific width of the required zone divided by the typical (defined results, provide a framework within which to estimate the by our system) mean spacing, or 0.2/0.4: 0.5. This is a regions around neighboring stars wherein habitable plan- probability of 50% - quite high indeed! ets might be found. Habitable is defined by most planet- Of course, other factors must come into play. Other ary scientists as being capable of harboring liquid water. systems will have planets whose sizes differ from those in I ,/ Formation of Polar ActivitY Tharsis Diminishing Volcanic Layered Terrain Valley Networks - -

Outflow Channels

\ P".io¿ of Warm Moist Conditions

<-- Higher Life Forms MicrobialLife +

BuilduP of 02 CO2Æ',i2 AtmosPhere Possible Life

Definite Evidence Ø for Life 9 C,) (Stromatolites, Fossil Algae) x Vascular tEl'e

2.0 1.5 1.0 15.7 THE FINITE LIFE OF OUR BIOSPHERE 193 I our solar system, Several stars are known to have planets ethical dilemmas associated with transforming a vast nat- comparable to the mass of Jupiter within the terrestrial ural environment. planet zone (as defined by our solar system). A system in which a Mars-sized body occupied Earth's orbital posi- tion might not have hosted life because such a small planet 15.7 THE FINITE LIFE OF OUR BIOSPHERE would not possess the plate tectonic recycling of carbon The evolution of our Sun has one more consequence for dioxide needed to sustain an environment stable for liquid life on Earth. From now to the end of its stable hydrogen- water. Conversel¡ a more massive planet possessing plate fusing stage, the Sun will continue to increase in lumi- tectonics but occupying the orbit of Mars might develop nosity. As it does so, the climate of the Earth will edge a sustainable greenhouse atmosphere, possibly a bit later closer to the point at which a moist greenhouse is initi- than did our Earth as its parent sun increased in luminos- ated and rapid loss of the Earth's water ensues, as ap- it¡ and then could sustain a clement climate over billions parently occurred early in Venus' history. A natural de- of years. laying tactic is the feedback process described The mass of the central star itself also determines hab- in chapter 14 wherein, as the brighter Sun warms Earth, itability. More massive stars are more luminous and hence more rainfall and more erosion will occur, and hence the habitable planets must be in proportionately larger orbits carbon dioxide budget of our atmosphere will decrease. than Earth's orbit around the Sun. F{owever, more impor- Flowever, a point will come when rising temperâtures can- tant, the higher luminosity comes from a more rapid rate not be buffered by the decreasing amounts of atmospheric of hydrogen fusion, so that massive stars are shorter-lived carbon dioxide, and rapid loss of Earth's oceans to space (chapter 4) and hence provide their planets a much smaller will begin. time for biological evolution. Happil¡ the most abundant Models by Caltech and Pennsylvania State scientists of stars in the galaxy are the least luminous and longestJived: the Sun's luminosity history and the response of Earth's the M-dwarfs. The¡ however, harbor another difficulty: atmosphere suggest that this crisis will be reached in For a planet to gâin sufficient wârmth from an M-dwarf 1 billion to 2 billion years from now At that point, if to be habitable requires that it be as close to its star as the biosphere has not collapsed aheady from decreasing is from the Sun. In that tight orbit, gravitational amounts of atmospheric carbon dioxide, the lack of liq- tugging will force the planet toward a state of synchronous uid water will finally kill off all living organisms. On the rotdtion where one side faces toward the star - iust as our other hand Mars will enjoy more clement conditions, if Moon keeps one face toward the Earth. The resulting cli- enough water and carbon dioxide are stored in the crust mate will be vastly different from Earth's, and perhaps not to be partially liberated into a thicker atmosphere by the conducive to stable liquid water. brighter sun. Returning to our own solar system, human technology Life began on Earth some 3.8 billion to 4 billion years might expand the habitable zone in the not-too-distant ago, and complex eukaryotic cells appear in the fossil future. A number of futurists have proposed seeding the record from 2 billion years ago. Therefore, we are more Martian atmosphere with efficient greenhouse gases such than halfway through the time period during which life, âs methane and chlorofluorocarbons, in an effort to warm even complex life, can flourish on Earth. Our time here the surface, release water, and generâte conditions that is not forever. After the brightening Sun drives water, and are more Earth-like. Although beyond the reach of cur- hence life, from Earth, it will continue to shine by hydro- rent space transportation capabilit¡ such a terraforming gen fusion for another 2 billion to 4 billion years. For of Mars might be possible for a future generation, which those last several billion years of the Sun's histor¡ Earth's will have to weigh the advantages against the potential surface might hold a fossil record of its long springtime of clement conditions, during which it teemed with living

Figure '1 5,8. Comparative geologic and climabologic histories of organisms that eventually looked upward to contemplate Mars and Earth, in a view taken by NASA scientists Chris McKay the stars. and Carol Stoken "Formation of Tharsis" is the uplift of a broad Mantian highland containing several giant volcanoes. The "Polar Layered Ternain" is a set of altennating layens of dust and ice seen in the polar regions, thought to have formed as the tilt of Mars' 15.8 OUEST¡ONS pole changed on a frequency of millions of yeans and altered you search for evidence of past, the pattern of dust and ice [waten or carbon dioxide] deposition. a. How and where would From McKay and Stoken (1 98S1. and present, life on Mars? \. I 15.9 BEADINGS 195 b. Can you think of any refugia for carbon-based life on Search for Extra-solar Terrestrial Planets. Techniques and present-day Venus? Technology (M. Shull, H. Thronsoan and A. Stern, eds.). Kluwer, Dordrecht, pp. 3-24. Kasting, J.F., !Øhitmire, D.P., and Reynolds, R.T. 1993. Habit- able zones around main sequence stars. Icarus 101,708- 15.9 READINGS 128. Am¡ P.S., and'Waldeman, D.L. (eds.l. 1997 The Microbiol- McKa¡ C.P., and Stoker, C.P. 1989. The early environment ogy of the Terrestrial Deep Subsurfac¿. CRC Press, Boca and its evolution on Mars. Reuietus of Geophysics 27, Raton, FL. 1.89-21.4. Caldeira, K., and Kasting, J.F. 1,992. The life span of the bio- McKaS C.P., and David, !í.L. 1991. Duration of liquid water sphere revisite d. Nature 360, 721-723. habitats on early Mars. Icarus 90,2'1,4--221. Carr, M.H. 1984. Mars. In The Geology of the Terrestrial Moroz, V.I., and Mukhin, L .M.1,977 . Early evolutionâry stâges Planets,M.H. Carr (ed.). NASA SP-469, lflashington, DC, in the atmosphere and climate of the terrestrial planets. pp.207-263. Kosmich esþie Issledouaniya 75, 901,-922, Forget, F., and Pierre Humbert, R.T. 1997,\larming earlyMars Phillips, R.J., and Hansen, V.L. 1994. Tectonic and magmatic with carbon dioxide clouds that scatter infrared radiation. evolution of Ve nus. Annual Reuiew of Earth and Planetary

S ci ence 27 8, 1.27 3-1.27 6. Sciences 22, 597-654. Kargel, J.S., Baker, V.R., Begé, J.E., Lockwood, J.F., Péwé, T.L., Rampino, M.R., and Caldeira, K. 1,994. The Goldilocks prob- Shaw, J., and Strom, R.G. 1995. Evidence of ancient con- lem: Climatic evolution and long-term habitability of ter- tinental glaciation in the Martian northern plains. Journal restrial planets. Annual Reuiews of Eartb and Planetary of Geopbysical Research 100, 5351-5368. Sciences 32,83-1.1.4. Kasting, J.F. 198 8. Runaway and moist greenhouse atmospheres Sackmann, I-J., Boothroyd, 4.I., and Kramer, K.E. 1993. Our and the evolution of Earth and Venus. Icarus 74, 472494. Sun. III Present and future. Astropbys. J. 418 457- Kasting, J.F, 1991. CO2 condensation and the climate of early 468. '1,972. Mars. Icarus 94, 1,-1,3, Sagan, C., and Mullen, G. Earth and Mars: Evolution Kasting, J.F. 1,997. Planetary atmosphere evolution: Do other of atmospheres and surface temperatures. Science 177 52- habitable planets exist and can we detect them? ln The 56.

Figure 15.9. The Mans of today, captured by the Mans Pathfinder landen after its successful landing on July 4, 1997, shows hints of eanlien times when waten nushed acnoss parts of the sunface. lt also shows the tracks lefb by the Sojournen nover in its tnavels across the landing site, the finst mobile exploration of Mars. fhe Pathfinder imager was developed fon NASA by Pe- ter Smith [Univensity of Anizona] and colleagues. lmage countesy NASA,zJet Propulsion Laboratony and Mark Lemmon [University of Arizona).