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3 Early Planetary Environments: Implications for Chemical Evolution and the Origin of Life INTRODUCTION Life originates and evolves on planets. Depending on the location with respect to its central star, the endowment of elements and energy sources, and the evolutionary path taken by a particular planet, the surface environ- ments may become either hospitable or inimical to the origin of life. The comparative study of planets then is essential to understanding the relation- ship between planetary development and the origin and evolution of living systems. As a minimum for life to arise and evolve on a planet, the persistence of liquid water and a hydrologic cycle operating in concert with geochemical cycles of the biogenic elements would appear to be required. Physical processes occurring in surface environments also had to sustain the chemi- cal synthesis of structures that would become capable of metabolism and self-replication. The central role of organic chemistry in life on Earth underscores the importance of understanding how processes operating ini- tially in an essentially inorganic realm could have led eventually to the organic structures that are now recognized as life. In a more general context, the organic chemistry of planetary environ- ments is an extension of the cosmic evolution of the biogenic elements (see Chapter 2) into the planetary epoch. Knowledge of the processes that pro- duce organic matter, wherever it occurs in the solar system, is central to our understanding of chemical evolution. The planetary bodies and satellites in the outer solar system are of pri- mary interest in this context because they are natural laboratories in which the chemical evolution of organic matter can be studied directly. Investiga- tion of their present state can yield insight into the complexity of organic 56
EARLY PLANETARY ENVIRONMENTS 57 chemistry that can be attained in the outer reaches of the solar system. A variety of environments exist within which these experiments are taking place today. The occurrence of selective pathways for the synthesis of organic compounds related to abiotic processes on the primitive Earth can also be investigated by studying the processes and products found in these environments. Furthermore, it may be possible to determine the relation- ships among the materials in asteroids, the satellites and planetary rings of the outer solar system, and the components of primitive meteorites and comets. There has been speculation about the possible existence of living organisms in Jupiter's atmosphere and also about a hypothetical ocean that could provide an environment for life beneath the ice cover of Europa. Both notions suffer from serious theoretical objections. The life histories of the terrestrial planets Earth, Mars, Venus, and Mer- cury and of the solid satellites, Moon and lo, underscore the relationship between planetary processes and the origin of biological processes. These histories may be compared in terms of hypothetical stages in the life of such planets, as indicated in Figure 3.1. For example, Earth passed through to the stage of plate tectonism; core formation and accretion played major roles in determining its early thermal history and constraining the time of origin and the conditions of the early milieux of atmosphere, ocean, and crust in which living systems arose; and plate tectonism maintains the bio- 1. CONDENSATION AND AGGREGATION OF DUST IN SOLAR NEBULA 2. PLANETESIMAL INTERACTIONS 3. PLANET FORMATION a. DISTRIBUTION OF BIOGENIC ELEMENTS CORE FORMATION OUTGASSING ,:7 4. VIGOROUS MANTLE ~ ,' CONVECTION = ,' CRUSTAL As_ ~ DIFFERENTIATION PLATE TECTONICS `~ GEOCHEMICAL CYCLES 6. TERMINALVOLCANISM ~ THICKENING OF LITHOSPHERE 7. QUIESCENCE COSMIC EVOLUTION OF THE BIOGENIC COMPOUNDS 1 ? FIGURE 3.1 Stages of planetary evolution (after Kaula, 1975). PREBIOTIC EVOLUTION ? BIOLOGICAL EVOLUTION
58 THE SEARCH FOR LIFE'S ORIGINS geochemical cycles essential for its global ecosystem. A planet may not progress through all stages in Figure 3.1, depending on these same factors. For instance, the Moon and Mercury appear not to have gone beyond core formation. Mars does not show convincing evidence of plate tectonics at any time in its history. Venus may have slow plate tectonics. lo has intense volcanism. The causes of divergences in development among the terrestrial planets have to be understood because they provide bounds on any general theory relating the origin of life to the origin and evolution of planets. Two major goals can thus be formulated for studies on early plane- tary environments that are crucial to our understanding of organic chemical evolution and the origin of life. GOAL 1: To understand the processes responsible for the chemical evolution of organic matter in the outer solar system. GOAL 2: To understand how the conditions for chemical evolution and the origin of life were influenced by the physical and chemical development of the terrestrial planets. THE OUTER PLANETS OBJECTIVE 1: To determine the origin and distribution of organic matter and disequilibrium products containing the biogenic elements in the hydrogen-rich atmospheres of the outer planets. The giant planets Jupiter, Saturn, Uranus, and Neptune are composed of large amounts of gas. The icy bodies of the solar system, the outer satellites of Jupiter, the satellites of the other giant planets, and Pluto all have substantial amounts of ice and variable amounts of silicate and iron in their interiors. According to one current theory, formation of planets in the outer solar system began with the aggregation of ice-rich grains from the solar nebula into cometlike planetesimals (see Chapter 2) followed by ac- cretion of the small bodies into Earth-sized planetary cores. The accretion process generated secondary atmospheres around the cores as these grew in size and mass. These cores then acquired thick gaseous envelopes by gravi- tational attraction of surrounding nebular gas. The moons of the giant planets presumably accreted in similar fashion but with differing propor- tions of ice and silicate and without accumulation of nebular gas. Astronomical observations of Jupiter, Saturn, and their moons have been made from Earth and by the two Voyager spacecraft. Both Jupiter and Saturn have atmospheres that are enriched in carbon and nitrogen with respect to the solar values, by factors of about two and three, respectively, whereas oxygen appears to be deficient in Jupiter's atmosphere for reasons
EARLY PLANETARY ENVIRONMENTS 59 that are not yet understood. Hydrogen is, of course, the dominant constitu- ent of both atmospheres and the major constituent of Uranus and Neptune as well. The atmospheres of these smaller giant planets are less well under- stood, but it is apparent that carbon, at least, is even more enriched on Uranus and Neptune than on Jupiter. This enrichment is most likely a result of degassing of the cores during accretion. Chemistry in these atmospheres is limited by the extreme overabundance of hydrogen; by the absence of a long-lived solid surface on which material could collect, become concentrated, and undergo further chemical reactions; and by the increase in temperature with depth that, along with atmospheric convection, will lead to the breakdown of compounds produced photochemi- cally at higher altitudes either in the gas phase or on grain surfaces. Never- theless, some nonequilibrium chemistry is certainly expected in these at- mospheres as a result of energy supplied externally from above (solar ultra- violet light; bombardment by electrons, protons, cosmic rays), reactions driven by lightning discharges, and the effects of convection in the atmo- spheres themselves, which can bring species formed in one thermal regime into another where they will be out of equilibrium. Indeed evidence of these processes is available in the form of ethane and acetylene, formed from methane in the upper atmospheres of these planets; as yet unidentified col- ored material in atmospheric clouds and hazes; and constituents such as phosphine, germane, and possibly CO that have been formed at low eleva- tions and brought up to visible levels by vertical currents. All of these products and processes are worthy of additional study for what they can tell us about the origin of organic material in hydrogen-rich environments. Some regions in these planetary atmospheres may resemble conditions in the primitive solar nebula, which in turn were probably repre- sentative of a large class of cosmically abundant environments. Hence understanding these chemistries what drives them, what they produceis of considerable interest for attempts to constrain possible starting condi- tions in the early solar system. OBJECTIVE 2: To elucidate the organic chemistry and the origin of carbon oxides on Titan. The complexity that can be achieved in the organic chemistry of a small planet with a strongly reducing atmosphere holds perhaps even greater inter- est than the giant planets. In this context, "small" means a body whose mass is meager enough to allow hydrogen to escape easily from its atmo- sphere. This is one of the distinguishing characteristics of the Earth and the other inner planets. It also applies to the satellite of Saturn, Titan, whose present atmospheric composition has been thought to resemble the highly reducing end member in the spectrum of possible models of the primitive Earth atmosphere. This satellite has a solid surface that is at a low tempera-
60 THE SEAR ClI FOR LIFE'S ORIGINS sure, thereby avoiding the other two factors limiting organic chemical evo- lution on the giant planets. The problem is that the temperature of Titan's surface is so low 94 Kthat liquid water is out of the question. Liquid ethane, however, may exist dimly illuminated by light from the distant Sun that filters through dense, ubiquitous layers of smog. Much interesting chemistry is indeed taking place on Titan. The atmo- sphere exerts a surface pressure of 1.5 bars, consisting primarily of nitrogen with 1 to 6 percent methane. Ten to 15 percent argon may also be present: the current uncertainty in the mean molecular weight would petit it (direct detection is very difficult), and arguments based on cosmic abundances and the trapping of gases by water ice (clathrates) support this possibility. Of greatest interest are the trace gases shown in Table 3.1. Here are some of the results of spontaneous chemical reactions in a reducing atmo- sphere. Not only are hydrocarbons and nitrites present, both CO' and CO are also present. - The first two classes of compounds are expected in such an atmosphere as a result of reactions between fragments of nitrogen and methane. This chemistry is driven primarily by precipitating electrons from Saturn's mag- netosphere, but there are also contributions from solar ultraviolet photons TABLE 3.1 Composition of Titan's Atmosphere Species Name Abundance Major Components N2 Nitrogen 73-99% Ar Argon 10-15% CH4 Methane 1-6% H2 Hydrogen 0.1-0.4% Hydrocarbons C2H6 Ethane 20 ppm C3H~ Propane 20-50 ppm C2H2 Acetylene 2 ppm C2H4 Ethylene 400 ppb C4H2 Diacetylene 30 ppb CH3C2H Methylacetylene 30 ppb Nitriles HCN Hydrogen cyanide 20 ppb HC2CN Cyanoacetylene 10-1000 ppb C2N2 Cyanogen 10-100 ppb Oxygen Compounds CO2 Carbon dioxide 10 ppb CO Carbon monoxide 60 ppm
EARLY PLANETARY ENVIRONMENTS 61 and cosmic rays. Presumably these atmospheric reactions also give rise to the ubiquitous haze layer of organic matter, which almost certainly consists of condensed simple organic compounds and photopolymers. If over geo- logic time ethane were produced as a by-product of methane decomposition in the upper atmosphere, it could accumulate into liquid bodies on Titan's surface. What is the source of oxygen for the two oxides? Given the low surface temperature, water vapor can be excluded from the ices on Titan's surface. Water from the outside, in the form of infalling meteorites or icy debris from the Saturn system, is a good possibility. Another source would be primordial CO along with CH4, trapped in the ices that formed Titan. Titan offers us a very interesting natural laboratory for testing ideas about chemical evolution in reducing atmospheres. The gases listed in Table 3.1 can only be a subset of the total constituents to be found. The smog itself is likely to contain more complex substances than those listed in the table, which formed as a result of polymerization in the atmosphere. This material, as well as the condensable forms of the species given in Table 3.1, will settle on Titan's surface, sinking or dissolving in the seas of ethane or accumulating in drifts on icy outcrops. It would be of great interest to determine the level of chemical complex- ity achieved in this natural environment. Is there evidence of any preferred pathway? Are unlikely reactions catalyzed in some unforeseen way? Are compounds of biological interest, such as amino acids or adenine, pro- duced? Are any of these results relevant to the events that preceded the origin of life on Earth? A beginning for answering these and other related questions could be provided by the NASA-ESA (European Space Agency) Titan Cassini mission, currently under study, because extensive chemical analysis of the atmosphere and some surface science are being considered for inclusion in its instrument package. OBJECTIVE 3: To characterize the organic matter on the dark sur- faces of the asteroids, satellites, and planetary rings of the outer solar system. Atmospheres are not the only locales for organic chemistry in the outer solar system. The dark surfaces of Phoebe, the leading hemisphere of Iapetus, the satellites and rings of Uranus, and comet nuclei all seem to require the presence of carbon compounds, but not necessarily the same compounds. The dark coatings found in the Uranus system seem distinctly more neutral in tint than those found on Iapetus and Phoebe, which absorb more in the blue than the red. Phoebe and the dark side of Iapetus do not match either. Were these coatings formed by physical processes acting on the surfaces, or were they formed elsewhere and accreted along with other components at the time these various objects formed? What is their rela-
62 THE SEARCH FOR LIFE'S ORIGINS tionship to the coatings on comet nuclei, which in the case of Halley has a reflectivity of about 3 percent? What is this dark material? The organic residue from the Murchison meteorite provides a reasonably good spectral match to Iapetus (but not Phoebe). In addition to the dark dust revealed by the Halley encounter, radicals such as C3, CH2, C2, and CN, seen in comet spectra for years, indicate the presence of organic "parent molecules" trapped in or on the ices of the nucleus. Surprisingly, CO2 was identified as a parent molecule in the Halley nucleus. Its source remains controversial, but attempts to find evidence of solid CO2 in the outer solar system seem worthwhile. Can cometary ices accumulated in the solar nebula have supplied the building blocks for the asteroids, satellites, and planetary rings of the outer solar system? We have come full circle and, once again, are considering conditions that existed in the primitive solar nebula and may have afforded the components accreted into bodies of the outer solar system, some of which could even have originated in the interstellar medium before the nebula formed. These conditions should be understood not only because of their intrinsic interest; incorporated in comets and meteorites, the components must have been brought to the primitive Earth. Hence, the proportions of the biogenic elements and the nature of the compounds they formed, wherever they may be found in the current epoch, are of considerable importance to under- standing the chemical evolution of organic matter that led to the origin of life. THE TERRESTRIAL PLANETS Like all the terrestrial planets, the two discussed here Earth and Mars- are composed mainly of silicates and iron and contain only trace to minor amounts of the volatiles and the biogenic elements that are necessary for life. Historically, two scenarios have been proposed to explain the extreme depletion of the terrestrial planets in gases and volatile compounds. (1) The planets originally condensed as giant gaseous, protoplanets, and the volatile material was subsequently lost, possibly by action of the solar wind. (2) The solid material that formed the planets condensed from or partially equili- brated with the solar nebula at high temperatures and then accreted into rocky planets. The first scenario involves difficulties in getting rid of gas against the gravity of the planet and has not been widely accepted. The mechanics and chemistry of the accretion of planets from planetesimals have been studied extensively and are discussed below. Condensation or equilibration at a single temperature cannot explain the composition of Earth, however. The Earth is strongly depleted in rock- forming elements as volatile as potassium and may be depleted somewhat in silicon relative to magnesium. This would imply quantitative depletion of
EARLY PLANETARY ENVIRONMENTS 63 the biogenic elements hydrogen, carbon, and nitrogen, which are obviously present as H2O, CO2, and N2 and are bound in minerals at the surface and which may be present in the core. Analogous situations hold true for Mars and Venus. The conclusion that condensation is not the whole story is supported by the fact that even individual primitive meteorites (see Chapter 2) consist of mixtures of minerals and inclusions that require a wide range of condensation temperatures. Mixing of volatile-rich and nonvolatile ma- terials on the respective bodies must have occurred to account for the com- positions of Earth, Mars, and Venus, as well as for these meteorites. The endowments of biogenic elements in volatile-rich bodies (e.g., comets and asteroids) supplied to the terrestrial planets were prerequisites for the origin of life, yet their origin and delivery were the results of astrophysical pro- cesses operating elsewhere in the solar system. Earth Accretion by Impacts According to the scenario described earlier, the terrestrial planets grew as a result of larger planetesimals capturing smaller ones by gravitational attraction and collision. During this process, near misses among the bodies tended to perturb their orbits so that the approach velocities were a fraction of the escape velocity of the largest planetesimal in an orbital zone of accretion. Once the Earth and Venus had grown to a significant fraction of their final size, orbits were strongly perturbed and the planetesimals re- maining in the inner solar system were mixed. Accretion models typically yield a virtually fully formed Earth in 107 to 108 years. Such accretion, however, has important implications for the timing of the development of environmental conditions suitable for the origin of life, which is discussed in more detail below. Model calculations indicate that several lunar- (0.01 Earth mass) to Mars-sized (0.1 Earth mass) objects would have impacted the Earth. The number, size, and position of the terrestrial planets are partly a consequence of the randomness associated with the small number of these bodies. Indeed, the Moon is believed by some to have been formed by a very large impact with the Earth. The conditions associated with a very large impactor of lunar or Martian size are extreme and would have wiped out much of the earlier structure of the planet to a depth of many kilometers, with concomitant loading of the atmosphere with hot rock vapor. Any life forms that may have existed at the time would not have survived. The rate of impact of numerous smaller objects would have affected the amount of volatiles retained in the Earth's atmosphere. This process can be visualized bv comparing the energy fluxes per unit area with the present
64 THE SEAR ClI FOR LIFE'S ORIGINS heat flux supplied by the Sun, 1340 W/m2, and the current heat flux from the interior of the Earth, 0.08 W/m2. Sustained heat fluxes much larger than the solar flux would have heated the atmosphere, so that liquid water could not have existed. Fluxes larger than the interior heat flux would have af- forded significant geological effects. For example, accretion of the Earth in 10 million years would correspond to a heat flux of about 4000 W/m2, which may have been enough to form a massive water greenhouse with molten rock at the surface. Late in the accretion, especially after the last major impact, the energy fluxes were likely to have been much lower, and water greenhouse atmospheres could exist only after fairly large impacts. The size distribution and the flux of the objects that formed the Earth in the first 108 years are thus important. With a continual flux of smaller objects, water would have been kept in the atmosphere and hydrogen would have been lost to space at a significant rate. The water that dissolved in molten rock would have been transported to the interior by convection. Below about 100 km depth, the water would have been lost by reaction with metallic iron to form iron hydride in solution in the iron phase and ferrous oxide in the silicates. Carbon as CO, CO2, or CH4 is not readily lost to space, but carbon is soluble in iron and some should have entered the core. In these regards the case of very large impacts has not been modeled. Would a very large impact into an existing atmosphere and hydrosphere have ejected a portion into space and injected another into the upper mantle? How much rock vapor would have been produced and what influence should it have exerted on the resulting composition of the gases in the atmosphere? How long would it take to cool the surface sufficiently to allow liquid water to form again? More studies of the size distribution of impacting bodies and the effects of very large impacts are needed to address these questions. OBJECTIVE 1: To determine when and how the volatile elements neces- sary for life were added to the surface regions of the Earth. In the solar nebular region where the silicate- and metal-containing build- ing block for Earth condensed, the temperatures were unfavorable for re- taining water and other volatiles, as noted above. Thus, it has been pro- posed that volatile elements were supplied by impacts of icy or volatile-rich rocky objects that accreted in the outer solar system. These objects were scattered into the inner solar system, as well as farther from the Sun, by gravitational perturbations resulting from formation of the giant planets. The remnants of these icy bodies exist today as comets, but they visit the inner solar system infrequently. Some theoretical modeling of the process of comet accretion in the solar nebula has been done, and these models suggest that a maximum size between 10 and 100 km could exist under some boundary conditions. In this case, comets are not likely to have been the projectiles for major impacts. However, Pluto's icy satellite, Charon, is
EARLY PLANETARY ENVIRONMENTS 65 0.06 times the diameter of Earth, has about the mass of the Earth's oceans, and may represent the largest end member of the comet size distribution. The expected total supply of material by comets to the Earth during and after accretion is poorly constrained. Direct observation of the maximum size of comets, although very difficult, is highly desirable. Even if comets were the major source of volatiles, it is not clear whether they would have supplied much complex organic matter to the Earth. Large objects impacting Earth at cometary velocities are expected to vaporize completely, and the survival of organic compounds under such conditions would appear problematic. Small meteorites derived from asteroidal parent bodies can pass through the atmosphere without significant heating of their interiors. The bulk of the incoming mass, however, is in much larger but infrequent projectiles, whose high-velocity impacts with the Earth should also lead to vaporization. For large cometary and asteroidal objects, it would be of considerable interest to know the composition of the resulting vapor clouds. Studies should be carried out to determine the bounds on the input of intact cometary and asteroidal organic compounds to chemical evolution. Tectonics and the Development of Earth's Early Atmosphere and Hydrosphere The timing of the last lunar- or Martian-sized impact is unknown; it could have been as early as 4.5 billion years ago, the age of the oldest Moon rocks. The origin of an atmosphere and an ocean following the last major impact was probably rapid, but quantitative observations exist only for radiogenic rare gases. The massive water atmosphere would have con- densed into oceans more or less the present size, leaving behind a thick atmosphere containing the noble gases as well as hydrogen-, carbon-, nitro- gen-, and sulfur-bearing species. What was the composition of that atmo- sphere? Accretion and the formation of the Earth's core are now thought to have occurred simultaneously. Giant impacts supplied their already differenti- ated cores to the Earth. Enormous amounts of heat were added to the Earth by these processes. Most of this heat had to escape to space for the Earth to cool below the melting point. That is, the heat flux from the Earth's inte- rior during the first 100 years was probably greater than all the heat flux since. Plate tectonics (or any similar broad convective overturn) is much too slow to release this heat. For example, a heat flow 100 times the present one would require 10,000 times the current rate of plate motions or subduction of the 10,000-year-old lithosphere. The time is much too short for the more dense oceanic mantle to begin cooling. A more likely form of convection is by total melting of material at depth and eruption of the
66 THE SEARCH FOR LIFE'S ORIGINS material to the surface. Such silicate melts have extremely low viscosities and spread out in thin flows that cool rapidly. For example, the eruption rate of 20 mm per year necessary to supply 6 W/m2 is obtained by noting that the heat content of such flows is about 3 MJ/kg. This rate would recycle the mantle in 150 million years, a duration compatible with the probable time interval. Implicit in the idea of early core formation and mantle melting is that preexisting metallic iron carried in by accreting bodies would have been removed from near-surface regions. If segregation of iron into the core was inefficient and metal remained in the upper mantle to buffer the redox state of magmas, or if the metallic core, mantle, and outgassed volatiles were all in thermodynamic equilibrium during the period of rapid heat loss, CO and CH4 would have been the predominant thermodynamically stable forms of carbon injected into the atmosphere from the interior. The primitive atmo- sphere (highly reduced) would have been rich in H2, CO, C~4, NH3, Ad H2S (hydrogen sulfide). Furthermore, the impacts of large iron-beanng as- teroids comparable in mass to that of the oceans may have yielded pulses of highly reduced gases as a result of equilibration between vaporized iron and elemental hydrogen, carbon, nitrogen, and sulfur. In the absence of metal, the composition of the atmosphere would have been determined by the redox state of the near-surface metal-free silicate melts. Under this circumstance (neutral redox composition), CO2 would have been the predominant carbon-bearing gas; other major atmospheric constituents would have been N2 and H2O; sulfur would have emerged from hydrothermal vents as H2S; and CH4, CO, and H2 would have occulted in trace amounts at best. Synthesis of organic compounds for chemical evolution would have oc- curred more readily under highly reduced conditions (see Chapter 41; how- ever, there is no direct evidence bearing on the composition of Earth's earliest atmosphere. A better understanding of the geophysical and geo- chemical history of metallic iron in the upper mantle during tectonic evolu- tion is required. Such understanding may be gained by further study of Archean mantle-derived rocks, upper mantle xenoliths, lunar basalts, and igneous rocks from Mars, including the so-called SNC (Shergotite, Nakhlite, and Chassignite) meteorites of putative Martian origin. Although no direct evidence exists concerning the relative abundances of CO2, CO, and CH4 in Earth's earliest atmosphere, modeling of atmospheric photochemical processes indicates lifetimes of the order of tens of years for a primordial endowment of CH4, and steady-state sources for this gas are problematic in the absence of metallic iron (projectiles may have supplied some metallic iron). Until further insight into the primitive atmospheric composition emerges, the assumption that it was highly reducing cannot be
EARLY PLANETARY ENVIRONMENTS 67 taken for granted, and pathways for organic synthesis starting from CO2 should be explored (see Chapter 4~. Also, a sedimentary rock column, brought back from Mars as a documented core, could provide direct infor- mation about the early evolution of that planet's atmosphere, which must have paralleled our own. As at present, competing processes would have tended to both degas volatiles and return them to the Earth's interior. Lavas from total melting at depth were probably hot enough and had a low enough viscosity to lose most of their volatiles even beneath the ocean. Once cooled at the surface, olivine-rich glass would have tended to react with H2 and CO2 to form hydrous minerals and carbonates. Stirring of the surface by impacts would have aided penetration of water into warm rock, as well as hydration and carbonation reactions. The latter reactions would have begun to remove CO2 from the early massive atmosphere. What was the size of the atmo- sphere during this early stage of tectonism? If a substantial fraction of the carbon presently tied up in carbonate minerals was originally in the atmosphere as CO2 after accretion, a massive CO2 atmosphere of the order of several tens of bars could have existed. The greenhouse effect associated with this atmosphere would have countered the glacial temperatures inferred to have been the consequence of the lower luminosity of the early Sun; CO2 dissolved in the water would have precipi- tated CaCO3 at the moderate temperatures of the marine (shallow) hydro- thermal circulation system. If this mechanism for removal of CO2 from the atmosphere was not supplemented by its weathering counterpart on land (as happens today) because land surface was lacking, a thick CO2 atmosphere and warm surface temperatures (<100°C) could have persisted into the Arch- ean. The implications of such an atmosphere for the geochemistry of sea- water and prebiotic chemical evolution should be investigated. Once the last pockets of very hot material in the mantle erupted and cooled, some form of plate tectonics became the dominant mode of heat transfer from the Earth's interior. With the onset of plate movements resembling today's, volcanic activity and metamorphism released gases, and lower-temperature alteration returned volatiles into the interiorparticu- larly at midoceanic ridges. Although the rates of the processes were proba- bly enhanced by the higher temperature of the Earth's interior, and perhaps by greater global rates of plate tectonics, it is unclear whether there was a net gain or loss of near surface volatiles during this epoch. If the redox state of the magmas during this stage of tectonic development was similar to that recorded in the geological record from the present back to 3.8 billion years, N2, CO2 and H2O would have been the dominant gases in the atmo- sphere. It is unknown at what time such a tectonic regime took hold, but the evidence suggests possibly earlier than 3.8 billion years ago (see below).
68 THE SEARCH FOR LIFE'S ORIGINS OBJECTIVE 2: To constrain the conditions on the early Earth for de- termining the timing and probable environments for the origin and mainte- nance of the first organism. Because a fundamental characteristic of all life is to be far from equilib- rium with its surroundings, geophysically active boundary regions that are also far from equilibrium would appear to be particularly interesting envi- ronments for the origin of life. These regions would have included fuma- roles and volcanic vents on continents and continental shelves, deep-sea plate spreading centers, island-arc volcanic vents, the land-air interface, the sea-air interface, and especially the sea-land-air interface. The geophysics and geochemistry of these environments warrant careful study to determine their potentiality as sites not only for producing organic molecules, but also for producing self-organizing structures as precursors of self-replicating systems (see Chapter 4~. The aspects of plate tectonics that do not involve continents, including midoceanic ridges with hydrothermal vents, oceanic islands, and island arcs, probably existed as early as did liquid water and tectonism, but there is little hard evidence for this. The origin of continents is more poorly under- stood, and the best estimates of continental growth and recycling come from geological data. The earliest (3.8 billion years old) preserved rocks from Greenland resemble those from active continental and island-arc re- gions. Detrital zircons in ancient sediments indicate continental or island- arc environments as early as 4.1 billion years ago. It is especially noteworthy that the Earth's microbial ecosystems revealed in 3.5-billion-year-old sediments from Western Australia appear to have occupied shallow marine hydrothermal environments dominated by island volcanism. Moreover, the contemporary microorganisms with the most an- cient lineages based on molecular phylogenies are anaerobic, thermophilic, sulfur-metabolizing archaebacteria. These organisms were isolated from hot springs and hydrothermal vents where they thrive up to 105°C. Further inferences about the environments and environmental conditions that may have spawned Earth's earliest organisms must be gained by more micropale- ontological and phylogenetic studies of past and present life, respectively (see Chapter 5~. Dry land probably existed on oceanic islands, island arcs, small conti- nental masses, and the rims of impact basins. As noted below, in addition to the likely intense ultraviolet flux, rock vapor, ejecta, and tsunamis from large impacts would have made dry land an unfavorable place for the main- tenance of life. Chemical weathering from dry land contributed chemical fluxes to the ocean as at present, but the size of these fluxes was relatively small. In contrast, hydrothermal vents in the ocean probably contributed
EARLY PLANETARY ENVIRONMENTS 69 more of such fluxes than at present because plate tectonic rates were proba- bly at least somewhat higher than now. The precise differences in ocean chemistry are not known, but they may not have been extreme because the ocean even at present is dominated or strongly influenced by hydrothermal vents for many elements. A reducing ocean depleted in sulfate and en- riched in calcium, manganese, and iron seems likely. The presence of ultramafic rock, either from ejecta or lava flows on land, also seems likely. At present, groundwater in such environments is basic and strongly enriched in calcium hydroxide. If calcium hydroxide is a necessary catalyst for organic synthesis (e.g., the formose synthesis of sug- ars from formaldehyde; see Chapter 4), it was probably more abundant in the Archean than now. Streams and freshwater springs draining ultramafic rock probably flowed both into the ocean and into more normal bodies of fresh water. Impacts and Their Influence on Environmental Conditions for the Origin and Maintenance of Life In theory, life could have arisen at any time after Earth was fully ac- creted and liquid water appeared on its surface (i.e., by about 4.5 billion years ago). At the same time, an early appearance of life would also have been subject to numerous environmental perturbations resulting from im- pacts, many of them potentiallyif not outright lethal. Small bodies, up to tens of kilometers in radius, produced craters and may have perturbed the atmosphere in ways similar to those postulated for the impact at the end of the Cretaceous period. Among the global killing mechanisms for such impacts is believed to be the modification of the climate by the ejection of dust into the atmosphere, whereas direct impact, rock vapor, and ejects would have resulted in only local effects. The tidal wave generated by an impact in the ocean was at least hemispheric. As proposed for the end of the Cretaceous period, small impacts may have killed many organisms but do not appear to have been a danger to the existence of life in general. The case with larger bodies is less clear. Earth-like conditions would have returned after a brief interval after impact because the collisional en- ergy was relatively small. For example, the energy of a collision with a 140-km-diameter object is equivalent to that of 100 years of sunlight on Earth. The conditions over the entire Earth's surface during the interval after such impacts may have been lethal because no place on Earth is far from the effects of a major impact. The most lethal aspect of a large impact is the production of rock vapor. For modest impacts at asteroidal velocity, about 10 percent of the energy of
70 THE SEARCH FOR LIFE'S ORIGINS the impact is consumed in vaporizing a mass equivalent to that of the pro- jectile. In a small impact, the vapor spreads out and cools by radiation. The rock vapor from a sufficiently large projectile overwhelms the Earth's at- mosphere. Because the mass of the atmosphere is 5 x 10~8 kg, the poten- tially lethal size of a projectile is somewhere between 30-km diameter, the rock vapor from which would heat the atmosphere 100 K, and 140-km diameter, the rock vapor from which would equal the mass of the atmo- sphere. The effect of rock vapor on life in the deep ocean is less clear because the mass (1.4 x 102 kg) and heat capacity of the present ocean are much larger than those of the atmosphere. For large impacts a transient water greenhouse traps much of the energy. A 400-km-diameter object would evaporate the ocean and evaporate life. A second lethal aspect of large impacts is the generation of worldwide tsunami and the fouling of oceans with ejected material. These processes have not been modeled for large impacts, but it is clear that the tsunami heights are comparable to the ocean depth and that the ejecta mass is sev- eral times the mass of the projectile. The effects of rapid pressure change during the passage of tsunamis (many modern microorganisms can survive such pressure variations); chemical changes from catastrophic mixing of the ocean with condensed rock vapor, stirred up sediments, material eroded from the land, and ejecta; and longer-term heating of the ocean by hydro- thermal circulation in the ejecta pile would almost certainly have been le- thal even to some deep-sea life forms. Too little is known about the physics or the biology in such conditions to conclude whether all life was elimi- nated by large impacts early in the Earth's history. It is evident, however, that deep-sea organisms would have been much less vulnerable to the ef- fects of such impacts than either life on land or life dependent on sunlight in shallow water. The size and frequency of large impacts with the Earth can be con- strained by studying the Moon because the record there has not been re- moved by later events. The youngest basin larger than 300 km in diameter, Orientate, is about 3.8 billion years old. There are 13 basins between it and Nectaris, which is between 3.9 and 4.1 billion years old. The surface is saturated with basins older than Nectaris, of which 30 have been identified. The largest confirmed (3.85 billion years) basin, Imbrium, is 1200 km in diameter. The largest postulated basins, South Pole (2500 km) and Procel- larum (3200 km), formed very early and have been obscured by late events. Better dating of the lunar surface is needed to refine impact rates. Obtaining an impact frequency for the Earth from the lunar cratering record requires adjustment for the different surface area and gravitational attraction of the planets. The lunar cratering rate at the time of Imbrium corresponds to about one large impact on the Earth every million years. (This corresponds to a rate of resurfacing less than that of present-day plate
EARLY PLANETARY ENVIRONMENTS 71 tectonics.) It is much more difficult to compute the size of the projectiles that hit the Moon and, hence, the energy of similar objects hitting the Earth. Estimates of the energy involved in the formation of the larger lunar basins differ by two orders of magnitude, and no serious mathematical models have been made for large lunar impacts. Without such studies, one can only speculate whether large impacts precluded life or merely decimated it on Earth. Prebiological chemical evolution and the origins of life could have oc- curred at any time after Earth accreted. If the gestation period for the planet to spawn life in near-surface environments was shorter than the inter- val between large-scale impacts, life may have arisen many times and been obliterated, or it may have arisen in surface environments and migrated to deep-sea niches following the development of a network of hydrothermal environments that bridged shallow and deep marine locations. Once pro- tected in deep ocean niches, it might have survived numerous impacts and possibly served as dispersal centers to repopulate shallow marine environ- ments. These speculations on chemical evolution, multiple origins of life, and models of early environmental conditions in the atmosphere and oceans can only be substantiated by the geological record. Ancient rocks of all types- especially samples of sedimentary environments are critical, and efforts to find them are of high priority. The very processes of impacts and tectonism appear to have obliterated all but traces of the early record of Earth, how- ever. For this reason it is essential to extend the search to Mars, where an early planetary history parallel to but less violent than Earth's may have prevailed and where the record must be better preserved. Mars Mars continues to be the extraterrestrial body that holds greatest promise of scientific return on fundamental questions about the origin of life. A1- though the results from the Viking Biology and Molecular Analysis experi- ments were not necessarily representative of the planet as a whole, the likelihood of extant life on Mars appears low. On the other hand, there are reasonable prospects that evidence of chemical evolution and fossil life might be found. As is true for Earth, the key to understanding the occur- rence or absence of chemical evolution, the origin of life, and extant life on Mars lies in deciphering the planet's history of water, its geochemical cycles, and its atmosphere. OBJECTIVE 3: To assess the isotopic, molecular, morphological, and environmental evidence for chemical evolution and the origin of life on Mars.
72 Aqueous Environments TlIE SEARCH FOR LIFE'S ORIGINS The conditions over most of the surface of Mars today are extremely hostile to life. Of particular importance are the low temperatures and the apparent lack of liquid water. Several lines suggest, however, that local and seasonal aqueous environments may be possible at the present time. The partial pressure of CO2 (and, therefore, the total atmospheric pres- sure) is probably dynamically maintained near the triple point of water by the weathering of silicates and the release of CO2 from the interior by volcanism and metamorphism. At equilibrium, weathering reactions, such as CaMgSi2O6 + CO2 = CaCO3 + MgSiO3 + SiO2, would reduce the partial pressure of CO2 by orders of magnitude. These reactions, however, proceed very slowly in the absence of liquid brine (or water). An increase in atmospheric pressure leads to greater stability of brines, more weathering, and then a decrease in atmospheric pressure. Con- versely, a decrease in atmospheric pressure retards weathering and allows metamorphism and volcanism to recharge the atmosphere. A similar situation exists on the Earth. The bulk of the CO2 is in sedi- mentary rocks. A decrease, for example, in metamorphism would lead to less gas in the atmosphere. The lack of a greenhouse would then cause lower temperatures. This would retard the loss from weathering and stabi- lize the cycle. The channel systems on Mars strongly suggest that an aqueous surface environment existed in the past in many areas. The poorly developed na- ture of the drainage networks indicates that springs, rather than rain, fed these channels. These fluvial features appear to have formed early on, probably in the first billion years of Martian history during the period when life arose on Earth, and may represent sites occupied by the first organisms to have arisen on Mars. In addition, photogeologic evidence suggests the presence of stratified sedimentary deposits on the floors of canyons in the Valles Marinaris system. These deposits imply sedimentation in standing bodies of water, in which case lake environments may also have served as habitats for Martian life. The low rate of crustal recycling on Mars might have affected the main- tenance of life. Surface layers might have become leached of nutrients. However, if life exists only in limited areas, wind-blown dust might repre- sent a reservoir for nutrients, provided that the trace amounts of oxidants found in the soil by the Viking experiments did not exert a sterilizing effect at the surface. The rugged topography, numerous faults, and frequent sedimentary or volcanic stratification imply that aquifers may channel brines into springs
EARLY PLANETARY ENVIRONMENTS 73 on Mars. Warm (~273 K) springs are possible where high topographic relief causes deep circulation of the brines. Wet muds can exist seasonally in such environments, but more rapid recharge is needed to maintain pools. Radar reflectivity appears to be the best method of searching for the distribution of brines. Seasonal reflectivity anomalies have been interpreted to indicate widespread brines at shallow depths. Very high resolution meth- ods may be needed to find springs and brine pools. Differences Between Mars and Earth The Earth has large oceans, whereas no large bodies of surface water exist on Mars. A drastic difference in the amount of degassing is not required to explain the lack of water on Mars because of the different style of tectonics on the two planets. Consider the difference between the rock cycles on the Earth and on Mars. To start with mass balances, the oceans on Earth are equivalent to a uniform layer 2500 m thick and the water in sediments is equivalent to an additional 200 m. The total water on Mars at the time the channels formed has been estimated to be approximately 500 m or one-third of the Earth's per mass. On the Earth, sediments are deposited preferentially along continental margins. The pore space in these sediments is destroyed by low-grade metamorphism, and the water in hydrated miner- als is released by higher-grade metamorphism. Metamorphism is particu- larly frequent for sedimentary rocks near continental margins. The sedi- ments themselves are uplifted and eroded so often that most sediments are recycled sedimentary rocks. There are no deep depressions in which sedi- ments can accumulate and remain undisturbed. The ocean basins are con- tinually swept clean and renewed by plate movements. Old unmetamor- phosed sediments are thus rare on Earth. Dry sediments are similarly rare. In contrast, there are no plate tectonics on Mars. Other tectonic and igneous processes are also much less active on Mars than on Earth. Thus, sediment can accumulate in depressions and not get metamorphosed. In addition much of the planet is covered with basaltic lavas and some areas with old impact ejecta. Such rocks are likely to be porous. Liquid water reacts readily with basalt (or basaltic sediments). The water of hydration in fully hydrated marine basalts is around 3 percent by mass or 10 percent by volume. The amount of hydration is even higher if there are extensive ultramafic volcanics, as indicated by the analysis of meteorites attributed to Mars. Similar amounts of hydration might be expected on Mars. The heat flow and, hence, the geothermal gradient on Mars are likely to be less than those of inactive areas on the Earth. Metamorphism sufficient to release water of hydration on Mars is likely to be very local. The lower geothermal gradient, the lower gravity, and the likely presence of ice at shallow depths all aid the preservation of pore space on Mars. A few hundred meters of
74 THE SEARCH FOR LIFE'S ORIGINS water could easily be locked up in ice and groundwater if the layer of porous rocks is a few kilometers thick. A similar amount could be present in clays. The formation of channels probably ended when the bulk of the water became locked up and tectonic and metamorphic processes became too sluggish to recycle the water. The behavior of ice is also considerably different on the two planets. On Earth, large glaciers can exist only on land and usually at high latitudes. The thickness of ice is limited by its flow. In particular, ice flows more easily near the base of a glacier because the geothermal gradient means it is hotter there and because ice melts at lower temperatures under higher pres- sures. The lower gravity and geothermal gradient on Mars, combined with a much larger area of the planet on which ice can accumulate and a lower inventory of water, tend to preclude rapidly flowing continental glaciers on Mars. This allows water to be locked up as ice for considerable periods of time. Much can still be learned from theoretical models of Mars that include the physics and chemistry of the atmosphere, the groundwater, and ground ice, as well as metamorphic processes and weathering reactions. As noted above, brines may be detected by radar. Nearer-surface geophysical meth- ods are probably necessary to study the thickness of ice and the extent of porosity in sediments. Returning samples of Mars to Earth may be neces- sary to examine weathering in detail. Large Impacts on Mars Large impact basins are observed on Mars even though much of the surface is younger than these events. The hazard to life by the impacts would have been much less than the hazard on the Earth because the gravi- tational potential of Mars is much smaller. A low-velocity impact of an object with a similar orbit generates little rock vapor. A high-velocity impact generates rock vapor, but the vapor escapes into space rather than surrounding the planet. The effects of ejecta and tidal waves on the ocean on Earth are also not likely to be applicable to Mars. Thus, large impacts were probably lethal only to those Martian organisms (if any) that were obliterated in the impact crater or buried by thick ejecta. Prospects for Chemical Evolution and the Origin of Life on Mars Among all the scientific opportunities provided by NASA's space mis- sions, the return of samples from Mars for study in Earth laboratories (e.g., by the Mars Rover/Sample Return Mission) holds highest priority. It pro- vides a unique opportunity to address within the solar system the fundamen- tal issues of chemical evolution and the existence of life on another Earth-
EARLY PLANETARY ENVIRONMENTS 75 like planet, on the one hand, and the possible uniqueness of life on Earth on the other (cf. SSB, 1977, 19811. Not only does present knowledge of the ancient Martian surface indicate that early environments on Mars were similar to those at the same time on Earth, the environmental record of Mars' first billion years is potentially far better preserved than that on Earth, where continuing tectonic activity has destroyed almost all evidence. Samples of Mars can fill the gap in the Earth's geological record. In addition, the prospect of finding evidence of chemical evolution on Mars is as important as evidence of fossil life. Pri- mordial organic matter on Mars may be preserved at depth in the regolith or in ancient sedimentary deposits protected against destruction by oxygen, ozone, and other oxidants in the atmosphere and surface dust. Mounting evidence, some alluded to in the preceding section, indicates that Mars' surface environment 4.5 to 3.5 billion years ago was quite differ- ent from the Mars of today. Early Mars and the early Earth are compared in Table 3.2. The primary evidence suggesting that Mars' early history may have been conducive to chemical evolution and the origin of life is the presence of many geological features that can only be attributed to liquid water. Most persuasive of these is the evidence for extensive valley net- TABLE 3.2 Comparison of Early Earth and Early Mars Property Early Earth Early Mars Water Oceans Evidence for surface liquid water hydrological cycle (?) Temperature >273 K _273 K Atmosphere CO2 N2 H2O: CO2, N2 (?), H2O: >1 atm _1 atm Geochemical carbon cycle CO2 ~ carbonate rocks Reactions in water Carbonate rocks co2 Duration of thick atmosphere Preservation of rock record Biology Reactions in water Continued subduction, metamorphism, and volcanism 4.4 billion years (?) ~ present Highly altered and reworked Diverse life at 3.5 billion years Early volcanism and metamorphism only 4.4 billion years- 3.5 billion years (?) Ca. two-thirds surface is >3.8 billion years old
76 THE SEARCH FOR LIFE'S ORIGINS works believed to have been caused through erosion by running water. The valley networks are present over most of the ancient cratered terrain on Mars, implying that the period of water activity extended from before to somewhat after the decline in impact rates some 3.8 billion years ago. It is clear from the length and size of the valley networks that liquid water was stable at the surface This in turn implies that early Mars must have had a substantial atmosphere with correspondingly higher temperatures and pres- sures than at present. Current models of planetary accretion and atmospheric evolution suggest that this atmosphere was primarily composed of CO2 at a pressure of one to several atmospheres, with some N2 present. The amount of nitrogen in the atmosphere of early Mars may have been as small as it is today; if so, this may have had important implications for chemical evolution and the origin of life. Despite its present appearance of tectonic quiescence, the huge shield volcanoes and rift valleys attest to considerably more volcanism and outgassing, as well as higher heat flows, much earlier in the planet's his- tory. The duration and fate of Mars' early atmosphere are uncertain, but it is probable that the CO2, like water, was trapped chemically (as carbonate) and physically within the regolith over time. Possibly a thick CO2 atmo- sphere could have been maintained by volcanism for as long as a billion years. Without tectonic activity, which recycles Earth's sediments, it was almost inevitable that Mars would lose its atmosphere. Despite their differences today, the young Mars may have been similar to the young Earth in terms of several environmental features critical to chemi- cal evolution: moderate temperature active tectonism, the presence of liq- uid water, and the occurrence of biogenic elements in the atmosphere and surface rocks. It would have been within this environmental context that the process of chemical evolution might have occurred and left some relict evidence (see Chapters 4 and 5) in the geological record. Because well- developed microbial ecosystems evolved on Earth well within its first bil- lion years, the exciting possibility that life also arose on Mars provides a compelling reason for pursuing the active exobiological study of this planet. Because water plays such a critical role in biological and other surface processes on Earth, there is every expectation that Mars sites associated with water activity are the best places to look for evidence of chemical evolution and fossil life. These same sites are also the most suitable for studying the geochemical cycles and paleoclimates of the planet. Thus, there are compelling reasons for assigning high priority to the return of Martian samples that retain a record of liquid water activity, particularly from ancient water-laid sedimentary environments. To provide the scientific basis for the search for ancient life on Mars, the similarities and differences between the early surface environments of Earth and Mars must be considered in detail. (An example of key differences
EARLY PLANETARY ENVIRONMENTS 77 may be the atmospheric pressure of N2.) A comparative understanding of the planetary environments, coupled with a knowledge of the physiological and ecological requirements of putatively analogous ecosystems such as Antarctic microbial mat systems, would prove valuable in this effort. The model scenarios coupled with field techniques of micropaleontology will be essential for the development of experimental protocols for in situ investi- gations of Martian sediments. These issues are addressed in detail in Chap- ter 5. The committee wishes to emphasize that exobiological interest in Mars is not limited to a simple search for microfossils although such an investiga- tion would certainly be part of any sample analysis strategy. Exobiologists are interested in a comprehensive examination of Martian sediments and other samples, including isotopic geochemical analysis of hydrogen-, car- bon-, nitrogen-, oxygen-, sulfur-, and phosphorous-bearing materials in ig- neous rocks and the atmosphere; organic geochemical studies of any pre- served organic materials; and inorganic geochemical and mineralogical stud- ies of clays, carbonates, sulfates, phosphates, and other phases that are composed of the biogenic elements and are associated, at least on Earth, with water or biological activity. Data from such studies will reveal the history of the biogenic elements and water on Mars and factors in the development of the planet that may have influenced the progress of chemi- cal evolution toward living systems. Clearly, the suite of data germane to exobiology overlaps that sought by a broad range of other disciplines inter- ested in Mars. The committee notes that such an investigation will be scientifically meaningful whether or not evidence for extant or extinct life is found. Either result is important for understanding the origin and evolu- tion of life on Earth and other planets throughout the cosmos.