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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
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.
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