The discussion above sets the stage for describing the composition ofthe prebiotic atmosphere. Obviously, given the uncertainties that have been mentioned, especially concerning Earth's bombardment history, it is not possible to provide definitive answers. However, we can still draw some tentative conclusions, which we list below in numbered form:
1. A conventional, highly reduced atmosphere composed principally of methane, ammonia and other reduced gases is unlikely. Ammonia, in particular, is unlikely to have been abundant because it is rapidly photolyzed and converted to N2 and H2 20 The hydrogen escapes to space, leaving stable, triply-bonded N2 as the major nitrogen-bearing gas. UV shielding by hydrocarbon haze appears unable to prevent this from happening.21 To be sure, this result depends on the distribution of particle sizes and so it could change as more sophisticated haze models are developed. (More small particles would cause better UV shielding.) But, for the time being, the models suggest that Miller-Urey type synthesis would not have been an efficient method ofmaking of prebiotic organic compounds.
2. Despite point 1, atmospheres rich in CH4, CO and CO2 remain plausible. CH4 has a potentially large abiotic source from impacts, although this thought should be tempered by the discussion of Earth's impact history in Box 8.1. CH4
also has a plausible abiotic source from the interaction of CO2-rich seawater with ultramafic (Fe- and Mg-rich) rocks. This process, termed serpentinization, occurs within mid-ocean ridge hydrothermal circulation systems. Currently, this source of CH4 is thought to be modest—enough to produce an atmospheric CH4 concentration of only a few ppm on an anoxic Earth.22 It could, however, have been larger in the past if more ultramafic rock was exposed as a consequence ofhigher geothermal heat flow and associated changes in plate tectonics. It is difficult to be quantitative about this because we do not really understand how plate tectonics operated in the distant past. An atmosphere containing CH4 and H2O would, of necessity, have also contained substantial amounts of CO and CO2, because the methane would have been oxidized by the by-products of water vapor photolysis. This process is quasi-irreversible because the hydrogen escapes to space and because CH4 is not reformed photochemically at any appreciable rate. Surface interactions, like the serpentinization reactions mentioned above, are required to regenerate CH4. Some CH4 could also have been photochemically converted into higher hydrocarbons, if the atmospheric CH4/CO 2 ratio exceeded unity.21 It is not clear whether this limit was ever achieved on the prebiotic Earth. On the postbiotic Earth, it is likely that this limit was reached and that hydrocarbon haze was indeed formed.21
3. As pointed out earlier, an atmosphere containing even a modest amount of CH4 (10-100 ppmv), which is entirely plausible on the prebiotic Earth, should have generated HCN and H2CO through photochemistry. These compounds are suitable starting points for the synthesis of amino acids, nucleic acids and sugars, although the details of how these complex compounds may have formed are in general not well understood. CO, which should also have been reasonably abundant (100 ppmv or more), is also a useful starting compound for prebiotic synthesis because its thermodynamic free energy is very high.23 So, the prebiotic atmosphere should have provided a generous source of small precursor molecules that may have contributed to organic synthesis and the origin of life.
4. Exactly when all of this happened makes a difference, as atmospheric composition almost certainly evolved during the first several hundred million years of Earth history. Both CH4 and CO2 and perhaps H2 as well, should have been more abundant at first, as a consequence of impact degassing during planetary formation. The concentrations of both gases should have subsequently declined, as CH4 was converted into CO2 (and perhaps into higher hydrocarbons) and as CO2 was converted into carbonate rocks. Surface temperatures may have declined as well during this period, as the atmosphere became thinner. We return to this topic in the next section.
5. In all ofthese scenarios, large impacts should have occurred as late as 3.8 Ga. In the Nice model (Box 8.1), they were clustered around this time; in other models, they were spread out between 4.4 Ga and 3.8 Ga. By "large", we mean impactors that were comparable in size to those that formed the lunar craters Imbrium and Orientale, both of which are about 1000 km across. Forming a crater of this size requires an impactor that is roughly 100 km in diameter.2 This is approximately 10 times the diameter, or 1000 times the mass, of the impactor that is thought to have killed off the dinosaurs at the end of the Cretaceous Period, 65 million years ago. A 100-km diameter impactor would evaporate the uppermost 100 m of the oceans, including the entire photic zone.2 Hence, if life did evolve prior to 3.8 Ga, which seems entirely possible, it would likely have experienced multiple global catastrophes that would have preferentially killed off organisms living in the near-surface environment. Organisms living in subsurface environments, such as the midocean ridge hydrothermal vent systems, would have been largely unaffected by such events. As has been discussed frequently at origin of life meetings, this provides a possible explanation for the predominance of hyperthermophiles (organisms with preferred growth temperatures >80°C) near the base of the ribosomal RNA (rRNA) evolutionary tree.24
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