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Exploring Organic Environments in the Solar System 6 The Terrestrial Planets The terrestrial bodies of the inner solar system include Mercury, Venus, Earth, Earth’s Moon, and Mars. These bodies are composed primarily of rocky material, relatively devoid of carbon and other volatile elements compared to the outer planets of the Sun (Table 6.1). Despite the relative paucity of carbon on Earth, however, it is clear from geological evidence that rich organic environments existed early in Earth’s history and, by inference, perhaps on other inner solar system bodies as well. TABLE 6.1 Comparison of Cosmic Composition and Earth’s Crust for an Abridged List of the Lighter Elements Element Relative Cosmic Abundance Percentage of non-H Fraction Percentage of Earth’s Crust Percent Depletion Gas Phase: Diffuse Clouds Hydrogen 300,000 — 0.22 0 Carbon 100 24.7 0.19 37 Nitrogen 30.9 7.6 0.002 0 Oxygen 235 58.2 46.6 28 Fluorine 0.01 0.0025 — — Sodium 0.6 0.15 2.8 — Magnesium 10.6 2.6 2.1 76 Aluminum 0.8 0.19 8.1 — Silicon 9.9 2.4 27.1 91 Phosphorus 0.1 0.02 — — Sulfur 5.1 1.3 — 0 Chlorine 0.12 0.02 — — Argon 2 0.5 — — Potassium 0.03 0.007 2.6 — Calcium 0.8 0.19 3.6 100a Titanium 0.04 0.01 0.46 100a Chromium 0.18 0.04 — 99 Iron 8.9 2.2 5.0 100a Nickel 0.5 0.12 — 100a aLess than 1 percent of cosmic abundance observed in gas phase of diffuse clouds.
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Exploring Organic Environments in the Solar System The record of life in Earth’s early crust,1 the isotopic geochemical history,2 and inferences drawn from the lunar impact record3 all combine to constrain the time frame for the earliest emergence of organic environments and life to sometime between 3.5 billion and 3.9 billion years ago. Recently, evidence of isotopically light carbon, which may be indicative of biologically mediated processes, was measured in some highly metamorphosed rocks from the Isua and Akilia formations (West Greenland), suggesting that organic environments and life may already have existed 3.8 billion years ago.4,5 However, that evidence is compromised because thermal processes can also cause stable isotope fractionation, and those rocks have been deeply buried and heated at least once, and more likely, many times. If organic matter and life were indeed present some 3.8 billion years ago, then this would place the origins of life within the final stages of the late heavy bombardment of the inner solar system,6 thus narrowing the window of time needed for life to begin and providing a means both to destroy organic environments and to deliver extraterrestrial organic material to the surfaces of the inner planets. In situ synthesis of organic compounds on the terrestrial planets versus exogenous delivery of extraterrestrial organic material is discussed below in assessing the inventories of organic compounds and in a discussion of mechanisms of formation of organic compounds. INVENTORY OF ORGANIC COMPOUNDS ON THE TERRESTRIAL PLANETS Atmospheres As with the atmospheres of the outer solar system bodies, the organic molecules in the atmospheres of the terrestrial planets, apart from Earth, listed in Table 6.2 have been identified primarily by remote spectroscopic observations, mainly at infrared and ultraviolet wavelengths, from spacecraft missions and space- and ground-based telescopes. In situ and sounding measurements have been obtained for Venus (Mariner, Pioneer Venus, Venera), Mars (Mariner, Viking, martian meteorites), and, of course, Earth. Approximate mixing ratios for the carbon compounds are indicated in Table 6.2 in parentheses. Surfaces The surfaces of the inner solar system bodies provide a wide range of conditions, both environmental and geological, where organic compounds may be present. The following sections assess the likelihood of finding organics on the surfaces of the terrestrial planets. Potential inventories of organic materials on Earth’s Moon are of considerable scientific interest. At first sight, the Moon seems an unlikely location for organics. The Moon formed from the crystallization of high-temperature silica melts. Any organic carbon that may have been contained within the precursor material would be converted to simpler organics and H2 at temperatures as high as 1500 K. Indeed, samples returned from the Moon by the Apollo astronauts and the former Soviet Union’s robotic Luna missions are devoid of organic materials above and beyond that expected from the infall of carbonaceous meteorites and traces carbon implanted by the solar wind. Inorganic carbon is also found, for example, at concentrations of some 200 parts per million in lunar fines. Most of this carbon is in the form of carbon monoxide bubbles trapped in lunar glasses, consistent with the idea that the carbon was oxidized by mineral oxides at a high temperature.7 TABLE 6.2 Carbon Compounds Observed in Inner Solar System Atmospheres Class Planet Main Carbon Compound (mixing ratio) Trace Carbon Compound (mixing ratio) N2-dominated atmospheres Earth CO2 (0.00037) CH4 (10–6) CO (10−7 to 10−8) CO2-dominated atmospheres Venus CO2 (0.96) CO (10−6) COS (10−7) Mars CO2 (0.95) CO (10−4)
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Exploring Organic Environments in the Solar System The Moon is of interest to the study of organic environments for two very different reasons: The lunar surface as a witness plate. That is, it is a location that provides for long-term integration of collected material and thus might have sampled other carbonaceous asteroids that are not present in recent meteorite collections; and The lunar surface as the abode of special microenvironments. The lunar materials studied to date come from the Moon’s equatorial regions, and these areas are not typical of all lunar environments. The second possibility is of considerable potential interest, and the remainder of this section is devoted to its discussion. Permanently shadowed regions exist at both lunar poles. As long ago as 1961, Watson, Murray, and Brown suggested that the extremely low temperatures experienced in these locations, less than some 50 K, would act as cold traps for volatile material impacting the lunar surface.8 Thus, for example, water and other volatile materials—derived from comets, asteroids, meteorites, or interplanetary dust particles impacting the Moon’s surface or, alternatively, created during the reduction of lunar regolith by H- ions from the solar wind—could freeze out on grains in the polar regions and, in principle, persist for considerable periods of time.9 Such informed speculation has been supported by the subsequent detection of hydrogen concentrations in the lunar polar regions with the neutron spectrometer on the Lunar Prospector spacecraft.10 That is readily, but not definitively, explained as ice deposits. The possibility of water ice deposits at the lunar poles raises the issue of the presence of other volatiles, including organic volatiles, since the likely sources of the water, particularly from comets, may also be abundant sources of organic materials. Given a source of raw materials and the availability of likely energy sources (e.g., from cosmic rays and interstellar ultraviolet radiation), it is reasonable to ask if organic synthesis is actively occurring at the lunar poles. The irradiation of carbon-, hydrogen- and oxygen-bearing ices by ultraviolet radiation or cosmic rays can lead to the synthesis of organic compounds. Similarly, organics may be formed at the lunar poles by the action of the solar wind on the ice there in the same way that they are formed in ice on interstellar dust particles. The radicals formed by the radiation may react with the inorganic carbonaceous condensates to generate simple organic compounds (see in Chapter 2, in the section “The Interstellar Medium,” the subsection “The Synthesis of Interstellar Molecules”).11 Instruments on NASA’s forthcoming Lunar Reconnaissance Orbiter (LRO), scheduled for launch in 2008, will directly address questions relating to polar ices. These instruments include the following: The Lunar Exploration Neutron Detector (LEND), which will map the flux of neutrons from the lunar surface to create 5-km-resolution maps of the hydrogen distribution and characterize the surface distribution and column density of near-surface water ice deposits; The Diviner Lunar Radiometer Experiment, which will map the temperature of the entire lunar surface at 300-m horizontal scales to identify cold traps and potential near-surface and exposed ice deposits; and The Lyman-Alpha Mapping Project (LAMP), which will observe the entire lunar surface in the far ultraviolet to search for exposed surface ices and frosts in the polar regions and will provide subkilometer-resolution images of permanently shadowed regions at the lunar poles. None of these instruments, nor those on other planned lunar orbiters, such as India’s Chandrayaan 1, China’s Chang’e, or Japan’s Selene or Lunar-A, will directly address key questions surrounding the putative existence of organic materials at the lunar poles. Indeed, it is not clear that the definitive detection and study of lunar organics are possible within the current generation of remote-sensing instruments. It is possible that the secondary payload on LRO’s launch vehicle, the Lunar Crater Observation and Sensing Satellite (LCROSS), may return spectroscopic evidence of the presence of organic materials in the Moon’s polar regions, but it is likely that the study of lunar organics is more appropriately addressed by a lander mission.
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Exploring Organic Environments in the Solar System The NRC’s solar system exploration decadal survey gave very high priority to a mission designed to collect and return samples from the Moon’s South Pole-Aitken basin (SPA). This mission is, however, designed to address questions relating to the absolute chronology of the lunar surface, the timing of the late heavy bombardment and the impact frustration of the origin(s) of life on Earth, and the history of lunar differentiation.12 The search for and study of lunar organics, though intriguing, does not have the same scientific priority as the goals addressed by the proposed SPA mission. The study of volatiles and organics is, nevertheless, a major scientific theme identified in the solar system exploration decadal survey.13 Therefore, the inclusion of an in situ organic detection instrument should be regarded as an important adjunct to, but not necessarily a driver of, a future polar lander mission. Mercury The illuminated surface of Mercury is too hot (>700 K) to preserve complex organic carbon compounds. Possibly early in its history when the luminosity of the young Sun was approximately 30 percent less than its present value, there may have been a period when exogenous organic carbon delivered during the late heavy bombardment phase of planetary accretion might have accumulated on the surface. However, as the surface temperature rose to the current high values, this carbon would have been pyrolyzed, yielding various volatile molecules (e.g., CO2, CH4, C2H6) and a highly aromatic (if not graphitic) char. It appears likely that volatile products of such pyrolytic reactions would be lost to the minimal mercurian atmosphere. Pristine exogenous organic carbon might, however, survive in one environment, the bottoms of deep craters at the poles. Given that Mercury has essentially no atmosphere, any surface environment that is not directly illuminated (e.g., deep craters at the poles) experiences extremely cold ambient temperatures (e.g., the temperatures at night on Mercury may drop as low as 100 K). Therefore, it is likely, that complex organic carbon could persist and accumulate in the regolith at such locations. Note that a similar argument can be made for the Moon. Furthermore, the ongoing MESSENGER mission to Mercury does have a gamma-ray spectrometer to observe ice at the poles. Venus By virtue of its distance from the Sun, Venus had a larger inventory of organic carbon than Mercury. In fact, it was proposed that extensive radial mixing of protoplanetary material across a wide region of the accretionary disk may have resulted in an initial volatile content and composition of Venus similar to that of Mars and Earth.14 Whether that proposal is correct or not, subsequent processes of planetary evolution have given rise to Venus’s hot and corrosive atmosphere. Currently, the atmospheric pressure is approximately 90 bar. The atmosphere is dominated by CO2, where extensive green-house warming leads to temperatures on the order of 700 K near the surface. Given this extreme thermal boundary condition, the accumulation and preservation of significant organic carbon is unlikely. It is also unlikely that any exogenous organic carbon would be preserved on the surface due to these extreme conditions. In order for any endogenous organic matter to be detectable today it should be sequestered in the subsurface. Unfortunately, the extremely high surface temperatures create a thermal boundary condition that precludes the existence of cooler subsurface regions in which thermally labile organic carbon might be found. More thermally stable compounds such as methane and benzene might survive in subsurface reservoirs and/or fluid inclusions. Earth The current inventory of organic matter on Earth is dominated by biological sources, in particular the structural biopolymers of vascular plants, i.e., cellulose and lignin. While the total mass of the active biological component is estimated to be ~1013 kg,15 the majority of organic carbon lies preserved within sedimentary rocks. Recent estimates place the sedimentary carbon at ~1019 kg, distributed predominantly within oil shales and coal-bearing strata.16-18 The source of such organic matter originates from the selective preservation of biomolecular compounds derived predominantly from microbiota and vascular plants.
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Exploring Organic Environments in the Solar System Reentry of sedimentary organic carbon back into the global carbon cycle occurs in essentially two different ways. First, uplift of organic carbon-rich sedimentary rocks by tectonic processes may lead to surface exposure and erosion. The previously sequestered organic carbon is thus susceptible to oxidative, photochemical, or microbial degradation. Alternatively, the entire sedimentary section may be buried deeper, resulting in progressive thermal metamorphism, whereby the organic carbon entrained in the sediments is thermally converted initially to petroleum and ultimately to methane and various forms of inorganic carbon (e.g., CO2 and graphite). Sediment subducted at active ocean/continent margins (e.g., the west coast of the Americas) provides a means by which this residual carbon can reenter the atmosphere via volcanic exhalations. Note that at high temperatures, CO2, CO, and H2 are stable relative to methane. Subduction and volcanism associated with melting of igneous and sedimentary rocks provides a conduit through which organic carbon is recycled back into the atmosphere. Abiotic sources of organic carbon currently include the persistent rain of exogeneous organic carbon derived from carbonaceous chondritic meteorites, interplanetary dust particles, and the occasional comet. It has been estimated that early in Earth’s history (~4.5 billion years ago) up to ~109 kg/yr of organic carbon was delivered to Earth.19 Although this amount has tailed off considerably, current estimates for the influx of carbon-containing exogenous material is on the order of 2 × 108 kg/yr. Earth may also have received a portion of its volatiles from comets, potentially providing abiotic organic matter which some authors have argued is relevant to the origins and/ or evolution of life.20-22 Impacts may also have shock-synthesized organics in the atmosphere or as a result of the impact event (i.e., impact plume syntheses). Note that impacts would also destroy or modify organic matter. As has been observed in carbonaceous chondrites, the concentration of simple organic molecules under aqueous conditions would ultimately result in the formation of some of the more complex organic compounds (e.g., amino acids, nucleic acid bases, and sugars) typically found in modern cells.23 However, further studies of cosmogeochemical samples, coupled with laboratory experiments, are needed to probe the degree of chemical complexity that can be attained as a result of exogenous delivery of both intact and perhaps synthesized organic compounds to early Earth, early Mars, and other habitable zones. Endogenous abiotic production of simple organic compounds currently occurs in volcanic fumaroles (e.g., methane) and/or deep-sea hydrothermal vents (e.g., methane and formic acid).24,25 Traces of abiotically derived hydrocarbons have been identified in hard rock mines associated with ancient volcanogenic massive sulfide deposits, e.g., the Kidd Creek mine.26 Laboratory experiments have also demonstrated abiotic synthesis in aqueous media of more complex organic compounds from CO, e.g., long-chain fatty acids, fatty alcohols, and unsaturated hydrocarbons (i.e., lipids for primitive membranes),27 as well as di- and tricarboxylic acids.28 Furthermore, condensation reactions of fatty acids with alcohols and amines to form esters and amides also proceed in aqueous high-temperature fluids.29 This is important for prebiotic micelle formation. It is difficult to assess precisely how much abiogenic organic carbon could accumulate and contribute to Earth’s total inventory of organic carbon. One estimate proposes that, based on purely thermodynamic grounds, hydrothermal vent systems could provide up to 108 to 109 kg of organic carbon per year.30 Actual measurements of the concentration of methane in hydrothermal plumes emitted along the mid-Atlantic ridge yield methane at concentrations of up to 50 nmol/kg of fluid collected.31 Mars Mars could be the most interesting of the terrestrial planets besides Earth in terms of its potential inventory of organic carbon. By virtue of its distance from the Sun, Mars is expected to have formed from volatile-rich materials and also received volatile-rich exogenous complex organic matter after planetary accretion. Moreover, the current conditions of low temperature, no liquid surface water, low partial pressures of oxygen, and an apparently dormant tectonic state would be expected to provide a good environment for the preservation and accumulation of complex organic carbon absent the ubiquitous oxidizing materials found in the upper-most layers of the martian regolith. Mars has a lower atmospheric entry velocity for infalling debris because of its surface gravity is lower than that of Earth. The mass of meteoritic debris that survives martian atmospheric entry without melting has been estimated to be 8.6 × 106 kg/yr.32 Because Mars is less tectonically active than Earth, its surface may have accumulated debris over longer periods of time. Organic compounds contained within exogenous debris
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Exploring Organic Environments in the Solar System could have been protected from oxidation, especially once this material was incorporated into rocks and sediments. Although a small amount of organic matter is found in martian meteorites, the Viking lander experiments found no organic matter in the martian regolith. Similarly, the Alpha-Proton-X ray Spectrometer (APXS) experiment on Mars Pathfinder was unable to detect carbon in any form in the martian regolith. It has been proposed that oxidants in the martian regolith oxidize any exogenous or endogenous carbon contained within the near surface.33 If so, Mars may retain organic carbon deeper within its subsurface, i.e., below the level at which eolian and other “gardening” processes disturb the regolith. In the absence of an active biosphere, the most important endogenous source of organic matter on Mars is probably the abiotic production of organic compounds (e.g., hydrocarbons) derived from the catalytic reduction of magmatic CO2. Organosynthesis could have occurred both during volcanic exhalations and by hydrothermal alteration of basaltic crust early in martian history. Interest in all of these possibilities has greatly increased by recent claims of the spectroscopic detection of methane in the planet’s atmosphere by both ground-based telescopes,34,35 and spacecraft observations.36 MECHANISMS FOR FORMATION OF ORGANIC COMPOUNDS ON THE TERRESTRIAL PLANETS The Atmosphere of Prebiotic and Pre-photosynthetic Earth The composition of the earliest atmosphere on Earth and its evolution have not been precisely reconstructed. Competing theories exist for predictions of the ratio of CO2 to CH4 and the presence or significance of NH3. Actual levels probably depended on details of planetary accretion, interactions between the crust and mantle with out-gassed volatiles, the ultraviolet flux from the Sun (which could rapidly destroy CH4 and NH3), and rates of outgassing of chemical compounds into the atmosphere.37,38 Obviously, the atmospheric composition and the ultraviolet flux will determine the degree to which organic molecules could have been synthesized in situ in Earth’s early atmosphere.39 Early experiments to test whether there could have been a source of atmospherically derived organic carbon delivered to Earth’s surface were performed by Urey and Miller, who used an electric discharge to initiate chemical reactions in a gaseous mixture of CH4, H2O, H2, and NH3.40,41 These experiments showed that amino acids and other organic acids could be readily produced abiotically in such an atmosphere. If any of the terrestrial planets had such a reduced atmosphere, it appears likely that synthesis of organic compounds might have been a significant factor in generating a surface inventory of organic compounds. However, such a reduced composition for Earth’s early atmosphere is now considered unlikely due to the rapid photolysis of CH4 and NH3 in an atmosphere without an ultraviolet shield. Moreover, volcanic outgassing would most likely give the early Earth an atmosphere consisting of CO2 and N2 rather than CH4 and NH3. This, together with independent geochemical and cosmochemical constraints on CO2 and CH4 abundances from 2 billion to 4 billion years ago, suggests that the likely composition of Earth’s early atmosphere was predominantly N2 and CO2.42,43 Subsequent experiments performed on CO2/N2 atmospheres (with and without small amounts of CH4 and NH3) have shown that the yield of organic compounds via spark discharge is considerably less than in a highly reduced atmosphere containing mostly CH4 and NH3.44 Other possibilities for the generation of organic compounds via atmospheric chemistry have also been explored. An intriguing mechanism involves the synthesis of HCN via photochemical reactions between CH4 (of volcanic origin) and N2.45 If early in Earth’s history the mantle was much more reduced than it is currently, then the amount of CH4 emitted from volcanoes would have been greater. Rainout of substantial quantities of HCN would make the subsequent synthesis of purines, pyrimidines, and amino acids possible in the aqueous phase.46 Similarly, photolytic reactions initiated from CO2 and H2O at ultraviolet wavelengths have been postulated to produce rainout of formaldehyde (CH2O)47 with the possibility of subsequent condensation reactions yielding primitive sugars. This assumes that the concentration of formaldehyde rose high enough in aqueous solution on Earth’s surface.48
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Exploring Organic Environments in the Solar System Earth’s Current Atmosphere Earth’s atmosphere is unique in the solar system due to the large amount of O2 produced biotically by photosynthesis. Unlike Venus, most of Earth’s carbon inventory is sequestered in carbonate rocks. The abundance of O2 in Earth’s atmosphere (21 percent) results in the photochemical production of a very thin but important layer of ozone in the stratosphere. This layer absorbs ultraviolet photons in a region of the solar spectrum where there are no other absorbers (and where substantial damage to organic molecules can occur upon absorption). The Earth’s atmosphere is highly oxidizing due to the presence of both O2 and water vapor (which leads to the formation of highly reactive OH radicals). However, despite the oxidizing capacity of the atmosphere, significant amounts of CH4 (present at 1.7 parts per million by volume) are present in the atmosphere, principally as a result of metamorphic and biological processes. Given that the chemical lifetime of this CH4 in Earth’s oxidizing atmosphere is short (i.e., days to years), and the atmospheric composition precludes its synthesis by photochemistry, the continual, albeit trace, presence indicates a steady-state CH4 flux into the atmosphere. This input is predominantly from the biosphere (e.g., methanogens produce much of the CH4 present in the atmosphere). Thus, while CO and hydrocarbons in the outer planets’ atmospheres are abiotic in origin, the CO and CH4 in Earth’s atmosphere result from biological processes and/or human activities such as combustion of fossil biogenic material. Again, these inputs are not stable in an O2 atmosphere and require constant fluxes to make up for their rapid photochemical/oxidative degradation. In today’s atmosphere, the dominant chemical composition of 21 percent O2 and 78 percent N2 also prevents lightning-induced or ion-molecule reactions from producing organic compounds. Lightning produces significant amounts of nitrogen oxides, but no species with carbon-carbon bonds. The CO2-Dominated Atmospheres of Venus and Mars The atmospheres of Venus and Mars are dominated by CO2. Both are composed of approximately 95 percent CO2, with most of the remainder N2. Under those conditions, CO2 is easily photolyzed to form CO and O. Due to the presence of trace gases, particularly water vapor, the reformation of CO2 is catalyzed by the reactions of CO and O with radicals such as OH and HO2 (e.g., CO + OH → H + CO2), making CO2 highly stable in their atmospheres. As a result, more complex carbon-bearing species are not produced in the atmospheres of either Mars or Venus.49 Ion-molecule reactions and electrical discharges also do not initiate any further carbon chemistry in either planet’s atmosphere. Thus, the only carbon-bearing species observed in the martian atmosphere are CO2 and CO. On Venus, COS has been observed in addition to CO2 and CO, and is thought to be produced at the surface by equilibrium reactions between CO2, CO, and FeS2 at the high temperature and pressure there. (In contrast, COS is produced predominantly biotically on Earth by marine organisms, although COS is also detected in volcanic gaseous emissions.) Despite the lack of in situ production of organics in the atmospheres of Mars and Venus, some authors have suggested that chemical disequilibrium between trace constituents of Venus’s atmosphere is evidence for microbial life in the planet’s lower cloud layers.50,51 In particular, supporters of this conjecture point to the coexistence of chemical species not normally associated, such as H2 and O2 and H2S and SO2, and the existence of relatively benign atmospheric regions (i.e., with temperatures between 300 and 350 K, pressures of 1 bar, and water vapor concentrations of several hundred ppm).52 Such organisms presumably evolved when Venus’s climate was more Earth-like and then migrated to the clouds as the planet lost its surface water. Irrespective of such speculations, the evolution and present states of the atmospheres of Venus and Mars still bear on the history and evolution of both biotic and abiotic organic compounds in the solar system. For example, given the similar location in the solar nebula of Mars, Earth, and Venus, they should all have had similar bulk chemical compositions 4.5 billion years ago and would have been exposed to similar early radiation processes. The extent to which their atmospheres have evolved and diverged since that time yields information on the evolution of Earth’s atmosphere and the couplings of atmospheric composition with biology/life. Mars and Venus may also provide clues to the composition of past atmospheres on Earth that ultimately would have influenced the
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Exploring Organic Environments in the Solar System distribution of organic compounds on Earth (i.e., carbon reservoirs in the atmosphere compared with those at the surface, in the interior, in the ocean, and so on). The present atmospheric compositions of Venus and Mars also provide guidance as to the likelihood of finding organics produced abiotically or by ancient biota: Mars, for example, has no effective ultraviolet shield, whereas Earth and Venus do, the former via its significant ozone layer and the latter due to absorbance by sulfur compounds in the upper atmosphere. Therefore, ultraviolet-labile organics deposited on the surface of Mars as a result of either biotic or abiotic processes in the past will have been destroyed by ultraviolet light. Moreover, the photolysis of H2O by ultraviolet light reaching the martian surface would generate OH and HO2 radicals that would oxidize any organic compounds at the surface.53 A record of organics on Mars could conceivably be extant, however, below the surface. On Venus, despite the presence of an ultraviolet shield, it is unlikely that ancient organics, if they ever existed, could be recoverable from the surface since it is both very hot (743 K) and relatively young (<500 million years). Abiotic Organic Synthesis in the Interior of Earth There is a broad range of physical environments across the surfaces and in the interiors of the terrestrial planets; some of these may support conditions suitable for the abiotic synthesis of organic material. Notably, understanding of abiotic synthesis within the interior of Earth has grown considerably. Organic material has been synthesized abiotically throughout Earth’s history. For example, abiotic methane has been detected in fluids emitted in deep-sea hydrothermal vents,54 as fluid inclusions within recently formed ocean-crustal rocks (mid-oceanic ridge basalts),55 and in remarkably preserved ancient seafloor rocks some 3.2 billion years old.56 Locally significant quantities of hydrocarbon gases (methane, ethane, and propane) have been generated and reservoired in ancient volcanogenic massive sulfide deposits (e.g., the 2.7 billion-year-old Kidd Creek deposit in Ontario).57 There are two predominant sources for abiotic methane in the interior of Earth: The direct formation from carbon in volatile-rich fluids associated with the partial melting of rocks deep within Earth’s interior; and Formation in the vicinity of mid-oceanic ridges or spreading centers. These two possibilities are discussed in detail in the next two sections. Methane Formation in Earth’s Deep Interior Methane may form directly from carbon in volatile-rich fluids derived from partial melting of rocks deep within Earth’s interior. Carbon in such fluids is found in a number of forms/species, including CH4, CO2, CO, and graphite. At very high temperatures (e.g., 1300 K), thermodynamic calculations indicate that CO2 and CO should be the dominant, carbon-bearing phases for fluids, with CH4 essentially absent.58 Analyses of high-temperature volcanic gases (900 to 1500 K) support this conclusion, revealing no or only trace quantities of CH4.59 At lower temperatures, however, thermodynamic calculations predict that CH4 should be predominant.60 Shock proposed that if kinetic barriers suppressed the equilibrium shift from a predominance of CO2 and H2 to CH4 and H2O, reduction of CO2 to form other hydrocarbons would still be thermodynamically favorable (in the absence of appropriate catalysts, the methane-forming reaction is kinetically inhibited at low temperatures).61 Methane Formation at Mid-Ocean Ridges or Spreading Centers One of the primary mechanisms by which Earth loses heat is through the generation of new oceanic crust. This crust is basaltic and is created by the partial melting of mantle rocks deep within Earth, leading to melt migration through the mantle and its emergence along various mid-oceanic spreading centers deep under the oceans. While considerable heat is lost immediately at the spreading center, sufficient remains in the new seafloor to initiate
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Exploring Organic Environments in the Solar System hydrothermal circulation cells on the flanks of a spreading center whereby cold ocean waters are drawn down and circulate in freshly fractured hot seafloor.62 Once these fluids reach sufficient temperatures, they are capable of reacting with the new oceanic crust before they rise, completing the circulation pathway. If recirculating hydrothermal fluids contain dissolved CO2, then methane can be formed. This has been shown to occur experimentally.63 In natural, deep-ocean systems, methane plumes have been detected originating from deep-sea spreading centers as well as rift-valley regions in their vicinity.64 Whereas in some cases this CH4 is clearly biological, in many cases its relatively heavy carbon isotopic composition suggests abiological synthesis. The CH4 formation mechanism is as follows. The hydrothermal alteration (serpentinization) of basalt and exposed peridotite (in particular, transformation of the major mineral constituent, olivine) yields a more magnesium-rich hydrated silicate, serpentine. The excess ferrous iron reacts with water to produce magnetite (mixed ferrous and ferric iron) and hydrogen gas. It is likely that this newly formed magnetite provides a suitable catalyst for methane formation. Furthermore, in experiments designed to replicate this chemistry, ethane and propane also formed in proportions CH4 > C2H6 > C3H8. A proposed physical scenario for such chemistry in natural systems is as follows. Fluids containing CO2 and H2 originate at depth within the new oceanic crust via the previously described processes. The fluids migrate upward to the surface via a fracture-pore network. The fluids pass through rock containing catalytic mineral phases. These catalysts promote the dissociation of CO2 and drive catalytic hydrogenation of carbon, leading to the synthesis of CH4. However, in addition to methane, sequential insertion of CO followed by reduction leads to chain growth and the formation of ethane, propane, and higher homologs.65 These types of reactions are grouped under the umbrella of Fischer-Tropsch (FT) syntheses.66 These reactions have also been shown to occur experimentally, producing not only hydrocarbons but also fatty acids, fatty alcohols, and other compounds.67,68 The exact distribution in chain lengths and compounds is a complex function of fluid composition, temperature, pressure, the nature of the catalyst, and residence time. Given this reasonable scenario, it is not surprising that FT chemistry may occur naturally in hydrothermal vent systems as well as in volcanogenic massive sulfide deposits. As for the significance of such systems as global sources of abiotic organic carbon, it has been estimated that between 108 and 109 kg of organic material per year could be synthesized via hydrothermal systems.69 Placed in context with the major source of organic carbon synthesis on present-day Earth, primary productivity (i.e., the conversion of CO2 to biomass via photosynthesis) yields approximately 5 × 1013 kg of organic carbon per year.70 Thus, while hydrothermal sources of organic carbon may be significant, they are considerably less prolific on a planetary scale than biological organosynthesis, but may have been important as sources of prebiotic organic matter on the early Earth.71 Abiotic Organic Synthesis in the Interiors of Mars, Venus, and Mercury Given that organic materials are being synthesized abiotically on Earth, it is possible to speculate on the extent to which this might have occurred or is currently occurring on the other terrestrial planets. The likelihood of a thermal gradient across the accretionary disk72 suggests that the amount of hydrogen (predominantly in the form of water) and carbon (likely as CO2 or CO32−) remaining in the dust grains would decrease moving inward from Mars, past Earth and Venus, and ultimately to Mercury. However, this simple picture assumes that the protoplanetary dust particles and the progressively larger bodies that accreted from them all derived from the immediate neighborhood of the growing planets. Recent work has shown that considerable radial migration of source material due to collision and scattering of protoplanetary nuclei was likely, perhaps enhanced by the perturbative effects of an early-formed Jupiter. As a result, more-volatile-rich, cooler material might have contributed to the terrestrial planets.73 Taking these factors into account, one can speculate on the probability of abiotic organosynthesis on the terrestrial planets other than Earth. As stated earlier, Mars and Venus are likely to have formed from material that was compositionally similar to that of Earth. This similarity would suggest that all three planets started out with comparable inventories of water and carbon. Both Mars and Venus exhibit evidence of extensive volcanism. The estimated composition of their mantles suggest that, at least early in their respective histories, both planets had the potential for generation of at least methane and hydrocarbons, either via the thermal equilibra-
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Exploring Organic Environments in the Solar System tion of carbon-bearing volcanic exhalations or via hydrothermal production if extensive bodies of water once existed. The predominant gas species in the atmospheres of both Mars and Venus are CO2 and smaller amounts of N2 and CO. Such compositions are consistent with each planet’s atmosphere being formed from high-temperature volcanic exhalation derived from partial melting of mantle rocks. On Venus, the high surface temperatures set boundary conditions that might suppress the production of methane from sources other than direct volcanic exhalation. Lacking equilibration to form methane in the cooler regions of Venus’s upper atmosphere, methane generation may not therefore occur anywhere on Venus. On Mars, however, formation and trapping of methane might be possible. Indeed, interest in this possibility has been greatly enhanced by claims of the spectroscopic detection of methane in the planet’s atmosphere both by ground-based telescopes74,75 and by the Mars Express spacecraft.76 Although the results obtained from Mars Express are highly controversial, all three sets of observations indicate methane at concentrations of about 10 parts per billion. This is significant in that methane is unstable in the martian atmosphere and would disappear in ~300 years if not replenished. Although the origin of the methane has not yet been determined, possible sources include volcanic activity, chemical reactions between water and iron-bearing minerals in a hydrothermal system, and biological activity. Lower-temperature exhalations, e.g., on the flanks of the great martian shield volcanoes, could have equilibrated to form methane from CO2 and H2 in the relatively cool regolith and shallow lithosphere. If so, such methane might accumulate in the subsurface as stable methane hydrates.77 No surface water currently exists on Venus or Mars. Consequently, the generation of methane and higher-molecular-weight, organic matter via hydrothermal processes is currently not viable. In the case of Mars, however, there is strong evidence from the Opportunity and Spirit rovers that there was surface water early in martian history. Other missions have shown that there may have been lesser amounts of liquid water in recent history and that there is currently a large inventory of ground ice. This evidence, coupled with a history of active volcanism, perhaps as recently as ~10 Ma ago, suggests that extensive hydrothermal abiotic organic synthesis could have occurred. The ideal mode of preservation of such carbon would be as organic molecules trapped in fluid inclusions in rocks that constituted ancient martian seafloor or in silicified deposits in proximity to previously active hot springs. The preservation of organic carbon in shallow martian regolith may be problematic. On Mars, the ultraviolet flux is sufficiently high that photolysis of water entrained within the surface sediment may provide a source of OH radicals. These radicals may attack organic matter directly or recombine to form hydrogen peroxide that will oxidize organic carbon upon contact. Both these oxidants will be found predominantly in the photic zone.78 Organic carbon below the surface may also be threatened, not by direct reaction with OH radicals or peroxide but by reaction with strong inorganic oxidants (e.g., ferrate salts) that may mix down through regolith during wind-assisted sorting. Reaction of any of these strong oxidants in the martian soil with organic matter will either destroy the organic matter totally or oxidize it partially to form products such as polycarboxylated aromatic acids.79 Under high-soil-pH conditions, these molecules would reside as salts in the martian regolith. Thus, such acids, even if present in significant abundance, would not be volatile enough to allow detection by the instrumentation on board the Viking lander. If this scenario has merit, then preservation of organic matter in the martian shallow subsurface may be minimal, even though substantial reservoirs of abiotic organic matter might still exist deeper into the martian surface. There is the strong possibility of water ice a meter below the martian surface. Finally, there is the special case of Mercury. Several arguments suggest that abiotic organic synthesis within Mercury’s interior was and is insignificant. First, Mercury is considered a refractory planet in the sense that it presumably accreted from the most devolatilized dust grains. Thus, its carbon and hydrogen budget would have been low. Even if Mercury had started with carbon and H2O contents proportionally similar to those of Venus and Earth, the proposed, late enormous collision stripped off much of the outer, silicate-rich crust and, with it, the majority of volatile material.80 Second, although little is known regarding the tectonic history of Mercury, flyby imaging has not revealed any evidence of extensive volcanism on the surface of Mercury. Thus extensive exhalation of volatiles is not expected.
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Exploring Organic Environments in the Solar System TERRESTRIAL PLANETS: RECOMMENDATIONS Earth Abiotic sources of organic carbon on Earth are only now receiving attention. Over the past two decades, the pace of research focusing on the organic, inorganic, and biochemistry of deep-sea hydrothermal vents has been increasing. Notwithstanding these advances, little is known regarding the range of organic compounds synthesized abiologically in these deep vent systems. Much of the recent exploration has focused on marine biology in close proximity to “black smokers.” Although these black smokers are spectacular, the lower-temperature regimes on the flanks of oceanic spreading centers may constitute more important and prolific environments from the perspective of the abiological synthesis of organic carbon. Several projects are in progress. They include the following: The RIDGE (Ridge Interdisciplinary Global Experiments) program, a National Science Foundation initiative that lasted for 12 years starting in 1988, promoted interdisciplinary study, scientific communication, and outreach related to all aspects of the globe-encircling, mid-ocean ridge system. Research derived from this program has provided the vast majority of information regarding the biology, geology, and geophysics of these remarkable deep-sea environments. A continuation of the RIDGE program, RIDGE 2000, was created with the input of more than 200 U.S. scientists and is funded by the National Science Foundation. Compared to its predecessor, RIDGE 2000 has a more focused range of scientific priorities, with a greater emphasis on hydrothermal ecosystems and on understanding those systems in the context of regional and local volcanic and tectonic characteristics of specific sites. The North East Pacific Time-series Undersea Networked Experiments (Neptune) project is designed to create permanent undersea laboratories, initially along the Juan de Fuca ridge (a spreading center off the northwest coast of North America) and later at numerous other hydrothermal sites, to continuously monitor the chemistry, biology, and geology of these poorly understood terrains.81 The goal of the Neptune project is to establish a regional ocean observatory in the northeast Pacific Ocean. The project’s 3000-km network of fiber-optic/power cables will crisscross the Juan de Fuca region. Shore-based researchers will be able to interact with and obtain real-time data from instruments at and between these sites and thus monitor the physical, chemical, and biological phenomena taking place across several hundred thousand square kilometers of seafloor. Planning for Neptune is supported by the Keck Foundation and several federal agencies, including the National Science Foundation. Canadian participation in the Neptune project is funded by the Canadian Foundation for Innovation and the British Columbia Development Fund. The Archean Park program, an interdisciplinary characterization of the subseafloor biosphere, was initiated in 2000 and is led by scientists at the University of Tokyo. In 2001, the project drilled into the Suiyo Seamount (depth = 1380 m), a hydrothermally active volcano of the Izu-Bonin Arc (an active spreading center) in the Philippine Sea, and retrieved samples for biochemical and geochemical analyses. The goal of the program is to identify deep hydrothermal ecosystems where continental influence has been shown to be nonexistent. While intense interest exists within the scientific community regarding the potential role of deep-sea hydrothermal vents in the origins of life and Earth’s early atmosphere, the difficulty of studying the reaction zones in these remarkable natural sources of organic carbon precludes rapid development of a thorough physicochemical and biological understanding of such environments. Augmenting these studies are investigations examining the preserved vestiges of ancient spreading centers distributed throughout the geological record. Due to unusual and fortuitous aspects of deformation associated with plate collisions resulting from continental tectonics, sections of ancient seafloor now constitute coastal mountain ranges at numerous locations across the globe. Similarly, rocks associated with ancient spreading centers are preserved in ~3.3 billion-year old sediments of Western Australia and the 3.2 billion-year-old strata of South Africa. Studies of the inorganic chemistry of these rocks as well as analysis of any organic molecules trapped within fluid inclusions will go a long way toward improving understanding of the potential scope of abiological organosynthesis on early Earth and by comparison toward perhaps providing insight relevant to early Mars and Venus as well.
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Exploring Organic Environments in the Solar System Mars There are numerous reasons to expect that Mars should contain a rich inventory of organic carbon. However, past missions have shown that this carbon is not easily detected in the martian regolith and is, thus far, not detectable remotely. The principal problem may be that strong oxidants are formed in the shallow regions of the martian soil. Awareness of this potential problem arose from the results of the soil analyses made by the 1976 Viking landers. NASA’s next Mars mission, Phoenix, is instrumented for the analysis of volatile organics released on heating martian soils collected in the northern polar region of the planet. A more thorough search for martian organics will be undertaken by NASA’s Mars Science Laboratory, scheduled for launch in 2009. The recognition of the martian meteorites has significantly expanded knowledge of the chemistry, mineralogy, age, and isotopic composition of the environments in martian crust. However, most of the meteorites in hand are young, are igneous in nature, and appear to be from a similar location. As several studies have now indicated, none of these rock types appear to be conducive to the preservation and accumulation of organic matter. In formulating a strategy to search for organic environments on Mars and elsewhere in the solar system, researchers can draw some insight from Earth’s geologic record. On Earth, the most suitable lithologies for the preservation and accumulation of organic matter are sedimentary rocks that are typically fine-grained and are characterized by well-defined aqueously derived mineral assemblages. Thus, it may be possible to obtain additional information about the associated organic matter present in these martian mineral assemblages in a single measurement of the organic and inorganic material present. The detection and analysis of organic carbon on Mars will not be a simple task. There are likely to have been both exogenous and endogenous sources, the latter possibly carrying a biological signature. Furthermore, it is possible that life arose early in martian history, but may have not survived; thus the same difficult issues regarding identifying biomarkers arise for Mars as they do in studies of the most ancient rocks on Earth. Two aspects are then critical when prospecting for organic carbon on Mars. First, it is necessary to be able to distinguish between both endogenous and exogenous organics. Second, it is necessary to distinguish between organics derived from biotic and abiotic sources. From the perspective of identifying endogenous sources of organic carbon, therefore, it is critical that samples be obtained and analyzed from terrains where the formation history is well understood. In light of recent discoveries of surface water and volcanism in Mars’s relatively recent past, the existence of a small remnant martian biosphere cannot be excluded. Recommendation: Currently planned missions to Mars should seek to identify silicified martian terrains associated with ancient low-temperature hot springs in concert with a high probability of ground ice deposits to locate organic materials formed on Mars. Similarly, the identification of shallow marine and/or lacustrine sediments would provide another terrain well worth exploring in future missions as sites for martian endogenous organosynthesis. It has been proposed that any organic matter in the martian regolith, either endogenous or exogenous, will have been modified via reaction with strong oxidants present in the soil. Carefully designed laboratory experiments will allow an assessment of this problem and will point to the most effective strategies for direct analysis of organic materials by future Mars landers. For example, chemical derivatization schemes would be necessary to produce volatile organic compounds for gas chromatographic analysis if the only surviving organic molecules found in the regolith are polycarboxylated aromatics. Can a suitable simulated martian regolith be devised to support such studies? There is information on the elemental composition of regolith from the Viking landers, the Mars Pathfinder mission, and the Mars Exploration Rovers. In addition, there are analytical results from the SNC meteorites that,82 together with spectroscopic data, provide insights into the regolith’s composition. On the basis of these data, several analogs of martian regolith have been prepared.83 Although the oxidants in the soil have not been identified, it is possible that they are peroxides that were generated by short-wavelength ultraviolet light. Oxidation studies of organics using various regolith analogs and a variety of possible oxidants will provide insight into the likely oxidation products that will be observed on Mars. This information will be crucial to the design of the analytical capabilities of the lander that will perform the analyses of the oxidation products on Mars. Experiments designed to reproduce the chemical
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Exploring Organic Environments in the Solar System characteristics of martian regolith will lead to development of analytical strategies with the broadest bandwidth for detection of organic molecules. Assessment of the time scales for oxidative alteration of organic materials in the martian regolith would address issues related to optimal minimum drilling depths for future Mars lander missions. Recommendation: Laboratory models of Mars soil chemistry should be used to study plausible mechanisms for the oxidative alteration of organic materials in the martian regolith and to evaluate their integrated effects. Materials studied should include likely exogenous products (organic compounds like those found in meteorites) as well as conceivable martian pre- and postbiotic products. As instrument development continues for future robotic missions to Mars, it is important that such missions be capable of assessing as fully as possible the inventory of organic matter there. Clearly such development should be strongly guided by the information provided by the Spirit and Opportunity discoveries. Although such instruments will likely include mass spectrometers, considerable effort is required in the sample preparation stages prior to mass spectrometric analysis. Specifically, given what is now known about the regolith at the Spirit landing site and the outcrops at the Opportunity landing site, questions arise as to how to best isolate organic matter from the inorganic matrix and how to best introduce this organic matter into the source of a mass spectrometer. Even though future robotic missions will be equipped with instrumentation to analyze samples (e.g., the Mars Science Laboratory), these analyses will never be able to achieve the capabilities of Earth-bound laboratories. The discoveries by the rover Opportunity of what appears to be an unambiguously sedimentary outcrop greatly increases the impetus for martian sample return missions. Similarly, the discovery of the halogens bromine and chlorine in abundance at the location of the Spirit rover landing site strongly suggests the former presence of surface water. Samples from either location might very well contain organic matter derived from extinct (or perhaps even extant) life. The successes of Spirit and Opportunity further illustrate the importance of planning future missions to bring martian samples back to Earth. NOTES 1. W.J. Schopf and B.M. Packer, “Early Archean (3.3-Billion-to-3.5-Billion-Year-Old) Microfossils from the Warrawoona Group, Australia,” Science 237: 70-73, 1987. 2. M. Schidlowski, “A 3,800-Million Year Isotopic Record of Life from Carbon in Sedimentary Rocks,” Nature 333: 313-318, 1988. 3. K.A. Maher and D.J. Stevenson, “Impact Frustration of the Origin of Life,” Nature 331: 612-614, 1988. 4. M. Schidlowski, “A 3,800-Million Year Isotopic Record of Life from Carbon in Sedimentary Rocks,” Nature 333: 313-318, 1998. 5. S.J. Mojzisis, G. Arrhenius, K.D. McKeegan, T.M. Harrison, A.P. Nutman, and C.R.L. Friend, “Evidence for Life on Earth Before 3,800 Million Years Ago,” Nature 384: 55-59, 1996. 6. C. Chyba, “The Heavy Bombardment and the Origins of Life,” Astronomy 20(11): 28-35, 1992. 7. See, for example, A.L. Burlingame, M. Calvin, J. Han, W. Henderson, W. Reed, and B.R. Simoneit, “Lunar Organic Compounds: Search and Characterization,” Science 167: 751-752, 1970. 8. K. Watson, B. Murray, and H. Brown, “The Behavior of Volatiles on the Lunar Surface,” Journal of Geophysical Research 66: 3033-3045, 1961. 9. J.R. Arnold, “Ice in the Lunar Polar Regions,” Journal of Geophysical Research 84: 5659-5668, 1979. 10. W.C. Feldman, S. Maurice, A. Binder, B.L. Barraclough, R.C. Elphic, and D.J. Lawrence, “Fluxes of Fast and Epithermal Neutrons from Lunar Prospector: Evidence for Water Ice at the Lunar Poles,” Science 281: 1496-1500, 1998. 11. P.G. Lucey, “Potential for Prebiotic Chemistry at the Poles of the Moon,” pp. 84-88 in Instruments, Methods, and Missions for Astrobiology III (R.B. Hoover, ed.), Proceedings of SPIE, Vol. 4137, 2000. 12. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, 2003, pp. 4-6 and 194. 13. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, 2003, pp. 182-184. 14. G.W. Wetherhill, “Provenance of the Terrestrial Planets,” Geochimica et Cosmochemica Acta 58: 4513-4520, 1994. 15. R.H. Whittaker and G. Likens, “Carbon in the Biota,” Carbon in the Biosphere: Proceedings of the 24th Brookhaven Symposium in Biology (G.M. Woodwell and E.V. Pecan, eds.), United States Atomic Energy Commission, 1973. Available from National Technical Information Service, Springfield, Va. 16. D.H. Welte, “Organischer Kohlenstoff und die Entwicklung der Photosynthese auf der Erde,” Naturwissenschaften 57: 17-23, 1970.
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Exploring Organic Environments in the Solar System 79. S.A. Benner, K.G. Devine, L.N. Matveeva, and D.H. Powell, “The Missing Organic Molecules on Mars,” Proceedings of the National Academy of Sciences 97: 2425-2430, 2000. 80. G.W. Wetherhill, “Accumulation of Mercury from Planetismals,” pp. 671-691 in Mercury (F. Vilas, C.R. Chapman, and M.S. Matthews, eds.), University of Arizona Press, Tucson, Ariz., 1988. 81. For general background on Neptune and related projects see, for example, Ocean Studies Board, National Research Council, Illuminating the Hidden Planet: The Future of Seafloor Observatory Science, National Academy Press, Washington, D.C., 2000. 82. See, for example, S.P. Kounaves, S.R. Lukow, B.P. Comeau, M.H. Hecht, S.M. Grannan-Feldman, K. Manatt, S.J. West, X. Wen, M. Frant, and T. Gillette, “Mars Surveyor Program ’01 Mars Environmental Compatibility Assessment Wet Chemistry Lab: A Sensor Array for Chemical Analysis of the Martian Soil,” Journal of Geophysical Research 108(E7): 5077, 2003; and M. Koel, M. Kaljurand, and C.H. Lochmuller, “Evolved Gas Analysis of Inorganic Materials Using Thermochromatography: Model Inorganic Salts and Palagonite Martian Soil Simulants,” Analytical Chemistry 69: 4586-4591, 1997. 83. See, for example, T.L. Roush and J.B. Orenberg, “Estimated Detectability of Iron-Substituted Montmorillonite Clay on Mars from Thermal Emission Spectra of Clay-Palagonite Physical Mixtures,” Journal of Geophysical Research 101(E7): 26111-26118, 1996; and M. Koel, M. Kaljurand, and C.H. Lochmuller, “Evolved Gas Analysis of Inorganic Materials Using Thermochromatography: Model Inorganic Salts and Palagonite Martian Soil Simulants,” Analytical Chemistry 69: 4586-4591, 1997.
Representative terms from entire chapter: