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Review
, 2 (10), a004895

Earth's Earliest Atmospheres

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Review

Earth's Earliest Atmospheres

Kevin Zahnle et al. Cold Spring Harb Perspect Biol.

Abstract

Earth is the one known example of an inhabited planet and to current knowledge the likeliest site of the one known origin of life. Here we discuss the origin of Earth's atmosphere and ocean and some of the environmental conditions of the early Earth as they may relate to the origin of life. A key punctuating event in the narrative is the Moon-forming impact, partly because it made Earth for a short time absolutely uninhabitable, and partly because it sets the boundary conditions for Earth's subsequent evolution. If life began on Earth, as opposed to having migrated here, it would have done so after the Moon-forming impact. What took place before the Moon formed determined the bulk properties of the Earth and probably determined the overall compositions and sizes of its atmospheres and oceans. What took place afterward animated these materials. One interesting consequence of the Moon-forming impact is that the mantle is devolatized, so that the volatiles subsequently fell out in a kind of condensation sequence. This ensures that the volatiles were concentrated toward the surface so that, for example, the oceans were likely salty from the start. We also point out that an atmosphere generated by impact degassing would tend to have a composition reflective of the impacting bodies (rather than the mantle), and these are almost without exception strongly reducing and volatile-rich. A consequence is that, although CO- or methane-rich atmospheres are not necessarily stable as steady states, they are quite likely to have existed as long-lived transients, many times. With CO comes abundant chemical energy in a metastable package, and with methane comes hydrogen cyanide and ammonia as important albeit less abundant gases.

Figures

Figure 1.
Figure 1.
Gas compositions in equilibrium with ordinary H-type (high iron) chondrites at 100 bars (Schaefer and Fegley 2010). Abundances and oxygen fugacities are consistent with what is measured in ordinary H-type chondrites. Ordinary chondrites are often regarded as indicative of the bulk material of the Earth, although in detail the match is imperfect (Drake and Righter 2002). The gases are very reduced and at 100 bars pressure methane is strongly favored. The disappearance of CH4 at very low temperatures indicates that graphite has become the stable form of carbon. The temperature axis can also be interpreted as quench temperatures in the cooling impact ejecta plume or after some other cause of flash heating, for example by lightning or by impact. Generally similar results are obtained for other ordinary chondrites and enstatite chondrites (both EL and EH), because all contain significant amounts of metallic iron and iron sulfides (Schaefer and Fegley 2010).
Figure 2.
Figure 2.
Gas compositions in equilibrium with CI chondrites at 100 bars (Schaefer and Fegley 2010). Abundances and oxygen fugacities are consistent with what is measured in carbonaceous chondrites. Carbonaceous chondrites are the most highly oxidized meteorites, and thus the gases that result are relatively highly oxidized; the chief source of reducing power is the reduced carbon itself. The temperature axis can also be interpreted as quench temperatures. Hashimoto et al. (2007) obtained generally similar results for similar assumptions.
Figure 3.
Figure 3.
A schematic but energetically self-consistent history of temperature, water, and CO2 during the Hadean (after Zahnle et al. 2006). Steam and CO2 are in bars and read off the right hand axis; the pressure equivalent of the modern ocean is 270 bars. The Hadean began with a bang. (i) For 100–1000 yr after the Moon-forming impact Earth was enveloped in rock vapor. (ii) This was followed by a deep magma ocean that lasted for some 2 million years. The cooling rate was controlled by the thermal blanketing effect of water vapor and other greenhouse gases. (iii) Once the mantle solidified the steam in the atmosphere condensed to form a warm (∼500 K) liquid water ocean under ∼100 bars of CO2. This particular model assumes that CO2 was the major atmospheric gas apart from water. The warm early Earth would have lasted while Earth’s CO2 remained in the atmosphere. Here we assume that CO2 reacts with the seafloor and is subducted into the mantle on either a 20 Myr (solid curves) or a 100 Myr (dotted curves) time scale. (iv) The combination of the high chemical reactivity of ultramafc impact ejecta with CO2 and the faint early Sun suggests that, in the absence of another abundant greenhouse gas, the Earth should have become ice covered and very cold (Koster van Groos 1988; Sleep and Zahnle 2001). Occasional big impacts would have brought brief thaws (not shown).
Figure 4.
Figure 4.
Thermal response of Earth to an impact on the scale of the impact that created the lunar S. Pole-Aitken basin. Tens of these occurred on Earth during the Hadean. The total energy released is 1034 ergs; we assume that half promptly escapes to space or is deeply buried. The calculations assume an ocean of water (or ice) and either 1 or 10 bars of preexisting atmosphere. The method of calculation is described in Nisbet et al. (2006). The plot shows the temperatures of the atmosphere and ocean, the effect of evaporation on the depth of the ocean, and the pressures of steam and rock vapor. These atmospheres are thin enough to be heated to the temperature of rock vapor, which resets the chemical state of the entire atmosphere. The atmosphere will likely equilibrate with the rock vapor, which in many cases will be dominated by materials from the impacting body. An upper limit on the consequences for an ordinary chondritic impactor can be inferred from Figure 1. If the effective quench temperature is ∼1500 K, the result would be a CO-rich atmosphere. If the effective quench temperature were as low as ∼1000 K, the result would be methane. In practice these are upper limits because the atmosphere itself before the impact will have been more oxidized (either by hydrogen escape or by interaction with the mantle), and these pre-ëxisting oxidants would need to be accounted for.
Figure 5.
Figure 5.
Thermal response of Earth to an impact on the scale of the impact that created the lunar Orientale basin. Hundreds of these occurred on Earth during the Hadean. The total energy is 1033 ergs. The plot shows the temperatures of the atmosphere and ocean, the effect of evaporation on the depth of the ocean, and the steam pressure. The 1 bar atmosphere is thin enough that rock vapor should briefly raise the temperature of the atmosphere to ∼2500 K, but the event is brief and the system may not time enough to equilibrate thermally or chemically before the rock vapor precipitates. The 10-bar atmosphere is thick enough to effectively quench the impact, and a higher fraction of the impact’s energy goes into evaporating rather than into thermal radiation to space. The chemical consequences of such impacts might be considerable, especially in the latter case where methane would seem strongly favored.

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