Earth's Earliest Atmosphere

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 fall out in a kind of condensation sequence. This ensures that the volatiles are 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 come hydrogen cyanide and ammonia as important albeit less abundant gases.

Massive impact-induced release of carbon and sulfur gases in the early Earth's atmosphere

“…Consequently the ocean-atmosphere system was likely to be poorly buffered, and subject to large swings in atmospheric pCO 2 (Ridgwell et al, 2003). In the Hadean, the specific buffering capacity in the oceans is unknown; as we discuss here (see also Zahnle, 2006), the oceans may even have been episodically frozen (effectively corresponding to zero buffering capacity). The extent of subaerial landmasses and the distribution of seafloor area with depth (important for abiogenic carbonate precipitation; Ridgwell et al, 2003) are similarly unknown.…”

Darwinian Behavior in a Cold, Sporadically Fed Pool of Ribonucleotides

“…After the Moon-forming impact at about 4.5 gigayears ago, a very hot planet probably cooled in a few million years (Sleep et al, 2001). Whereupon, given the lowered luminosity of the early Sun prior to its entry onto the stellar main sequence ( & 70% of present luminosity; Sagan and Mullen, 1972), Earth could readily have descended to subfreezing temperatures (Zahnle, 2006). An ancient icy Earth was locally liquefied by bombardment by intersecting planetesimals (Bada et al, 1994;Valley et al, 2002), and a ''cool early Earth'' was ultimately thawed by a CO 2 /H 2 O greenhouse effect (Sagan and Mullen, 1972;Bada et al, 1994;Zahnle, 2006).…”

“…Whereupon, given the lowered luminosity of the early Sun prior to its entry onto the stellar main sequence ( & 70% of present luminosity; Sagan and Mullen, 1972), Earth could readily have descended to subfreezing temperatures (Zahnle, 2006). An ancient icy Earth was locally liquefied by bombardment by intersecting planetesimals (Bada et al, 1994;Valley et al, 2002), and a ''cool early Earth'' was ultimately thawed by a CO 2 /H 2 O greenhouse effect (Sagan and Mullen, 1972;Bada et al, 1994;Zahnle, 2006). Recent zircon isotopic data are consistent with clement conditions through this era, suggesting continental crust in contact with liquid water back to at least 4.3-4.4 gigayears (Mojzsis et al, 2001;Harrison et al, 2005).…”

Water balance of Earth / Bilan hydrologique de la Terre

“…The Solar System is 4.567 billion years old (Valley, 2006). The Earth accreted most of its mass within 10 million years, and the Sun began to shine, entering the Main Sequence, within 50 million years (Zahnle, 2006), at about the time when the proto-Earth collided with a Mars-size object (Koeberl, 2006) code-named Theia, in an apocalyptic event that resulted in the creation of the double-planet system which the Earth now effectively forms with the Moon.…”

“…This leaves a wide field for reasoning and interpretation. As Zahnle (2006) explains, most water accreted by Earth was probably delivered in the form of hydrous silicates. There is general agreement that the Earth accreted "wet" and that the post-accretion influx of water to Earth from comets is limited to less than 20% (Dixon, 2003).…”