Exploring Organic Environments in the Solar System (2007) / Chapter Skim
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6 The Terrestrial Planets
6 The Terrestrial Planets
Pages 85-100
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From page 85... ...
. 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.
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From page 86... ...
, 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.
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From page 87... ...
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.
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From page 88... ...
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 coalbearing 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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From page 89... ...
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.
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From page 90... ...
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)
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From page 91... ...
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.
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From page 92... ...
. 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.
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From page 93... ...
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)
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From page 94... ...
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.
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From page 95... ...
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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From page 96... ...
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.
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From page 97... ...
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)
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From page 98... ...
G Flynn, "The Delivery of Organic Matter from Asteroids and Comets to the Early Surface of Mars," Earth, Moon, and Planets 72: 469-474, 1996.
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From page 99... ...
Appriou, and P.A. Rona, "Intense CH4 Plumes Generated by Serpentinization of Ultramafic Rocks at the Intersection of the 15°20'N Fracture Zone and the Mid-Atlantic Ridge," Geochimica et Cosmochimica Acta 62: 2323-2333, 1998.
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From page 100... ...
Lochmuller, "Evolved Gas Analysis of Inorganic Materials Using Thermochromatography: Model Inorganic Salts and Palagonite Martian Soil Simulants," Analytical Chemistry 69: 4586-4591, 1997.
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