What was the nature of the pre-biotic terrestrial atmosphere?Submitted by Alberto E. Patio Douce, University of Georgia
Noble gas isotopic compositions show that the present-day terrestrial atmosphere is not a direct descendant of whatever atmosphere the Earth may have acquired during planetary accretion. Virtually all of that primordial atmosphere was lost to space (e.g., Pepin 2006, EPSL 252: 1-14) and was replaced at some point prior to 4.2 Ga by a secondary atmosphere that may have been a combination of late-accreting material and volcanic outgassing. The minimum age for Earth's secondary atmosphere is based on evidence for liquid water at the Earth's surface preserved in the oxygen isotopic composition of detrital zircons (e.g. Valley et al. 2005, Contrib. Min. Petrol. 150: 561-580). If liquid water was present at the surface at that point, then one can conclude that sufficient outgassing and/or late accretion of water had already taken place so a to build up atmospheric pressure above the triple point for water and, perhaps, to enable greenhouse heating to the point where liquid water could exist under "faint young sun" conditions. Pre-biotic evolution and the origin of life took place under that atmosphere. The composition and physical characteristics (pressure, temperature, optical depth, etc) of this "early secondary" atmosphere almost certainly changed with time, perhaps quite significantly. In particular, the oxidation state of this early atmosphere may have changed with time (I am referring to changes in oxidation state within an essentially oxygen-free atmosphere, and not to the "rise of oxygen" that occurred much later). Because atmospheric oxygen fugacity may have been one of the crucial limiting factors in origin-of-life processes, we want to know as much as possible about the nature of the pre-biotic atmosphere, and how it may have changed with time.
The oxidation state of carbon provides a convenient frame of reference to describe the chemical nature of the pre-biotic atmosphere. This could have ranged from a strongly reduced one in which methane was the dominant carbon species (e.g., present day Titan) to a mildly oxidized one in which carbon dioxide was the chief carbon species (e.g., present day Venus and Mars). The consensus appears to be that, although complex organic molecules may be preserved under the oxygen fugacity of a carbon dioxide-dominated atmosphere, pre-biotic evolution leading to the synthesis of complex organic molecules from simple inorganic C-H-O-N-S compounds (and to the eventual origin of life) may only be possible under more reducing conditions. Geochemical evidence from early Archaean lavas suggests that the oxidation state of the Earth's mantle has been close to its present day value (~QFM) since at least 3.9 Ga (Delano 2001, Or. Life Evol. Biosph. 31: 311-341). If the oxidation state of the pre-biotic atmosphere was controlled by the composition of volcanic gases and the date for the origin of life is no earlier than 3.9 Ga, then one must conclude that either (i) life originated in "reducing oases" isolated from the atmosphere (e.g., Russell & Arndt 2005, Biogeosciences 2: 97-111) or (ii) origin-of-life processes are possible under less reducing conditions than currently thought. But there are other possibilities. For example, the Earth's mantle and the volcanically outgassed atmosphere may have been more reducing prior to 3.9 Ga, and life may have originated during that earlier period of more favorable atmospheric conditions. Or, even if the Earth's mantle was at QFM at 3.9 Ga, perhaps the atmosphere was not in equilibrium with volcanic gases (it is hard to see how this could have been the case, however, as a methane-rich atmosphere unsustained by continuous methane recharge is quickly oxidized by photolysis and hydrogen escape (e.g., Catling et al. 2001 Science 293: 839-843). Alternatively, interpretation of the geochemical evidence (largely based on bulk Cr and V contents of mafic lavas) may be in error and volcanic gases at 3.9 Ga were significantly more reducing than QFM, but there are arguments based on the accretionary history of the Earth that support the idea that the current oxidation state of the mantle may be a primordial feature (e.g., Wade & Wood 2005, EPSL 236, 78-95). There may be other, less obvious, alternatives. Narrowing down the field of possible answers requires that we find a way of determining how the Earth's atmosphere evolved from the time when the primordial atmosphere was lost to the origin of life, no later than ~3.5 Ga and perhaps as early as 3.8 - 4.0 Ga.
In addition to its chemical composition we also want to know the mass (or density) of the pre-biotic atmosphere, as this parameter has a strong influence on planetary surface temperatures. Although astrophysical models still have some uncertainty regarding the absolute luminosity and luminosity-wavelength distribution of solar-mass stars shortly after arriving at the main sequence the consensus appears to be in favor of the "faint young sun" paradigm. If this is the case then greenhouse heating of the early Earth is probably required in order to account for the existence of liquid water at the Earth's surface. But depending on how faint the young sun really was, too thick an atmosphere may have led to a runaway greenhouse and a Venus-like environment, which, as best as we can tell, never occurred on Earth. The combination of uncertainties in solar luminosity and atmospheric density may result in a relatively narrow window within which both a snowball and a runaway hothouse can be avoided, or there may be a wide range of variable combinations within which clement conditions are possible. I am not aware of a comprehensive quantitative treatment of this question.