A Proto-atmosphere and the Environment of the Earth During Accretion

Details A Proto-atmosphere and the Environment of the Earth During Accretion A Proto-atmosphere and the Environment of the Earth During Accretion

Dec 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.u51a..09a&link_type=abstract

American Geophysical Union, Fall Meeting 2001, abstract #U51A-09

Physics

1010 Chemical Evolution, 5407 Atmospheres: Evolution, 5420 Impact Phenomena (Includes Cratering), 5455 Origin And Evolution

Scientific paper

The earliest surface environment of the Earth is reconstructed in accordance with a recent planetary formation theory. The recent planetary formation theory suggests 2 stages of planetary formation; the stage of runaway growth followed by the stage of giant impacts. A Mars-sized planet forms in 106 years at the stage of the runaway growth. Since solar nebula likely exists at this stage, the proto-Earth at this stage should have attracted the nebula gas and have a distended solar-composition (H2He) atmosphere. Also, we expect impact degassing from Earth-forming planetesimals to form a degassed atmosphere, which sometimes called `a steam atmosphere.' Hence, a mixed proto-atmosphere of solar-type and degassed components would have formed. Though the structure of the mixed atmosphere embedded in the nebula gas is not well understood yet, it would be similar to that of degassed atmosphere with extended upper atmosphere while the planet is relatively small. Then, the surface temperature is controlled by that of the degassed atmosphere. Since the actual energy release occurs intermittently, the atmosphere may cool before the next planetesimal impact if the mean impact interval of planetesimals is longer than the cooling time of a hot atmosphere. However, judging from the frequency and thermal effects of planetesimals impacts, we consider that the proto-atmosphere has a strong thermal blanketing effect and a surface magma ocean is formed. Thus, dissolution of volatile components into the mantle is expected. Segregation of metallic iron from silicate also occurs at the super heated impact points. This will also lead to reaction of volatile components with metallic iron. Very large infrequent impacts are expected at the giant impacts stage. Though the solar nebula has likely been lost by this stage, the mixed proto-atmosphere would have survived the nebula dissipation, because the atmosphere is tightly bounded by the EarthOs gravity field. Each impact may drive off the existing atmosphere. However, it is not able to desiccate the interior of the Earth. Therefore, the atmosphere will soon recover through degassing or re-accretion of impact-generated circumterrestrial disk. It should also be noted that refractory Moon can be formed even from a volatile containing disk. Since a long impact interval is expected, the atmosphere may condense to form oceans between impacts at this stage. The interior of the Earth, at least at the upper mantle region, remains in a partially molten state during this stage and chemical differentiation is expected.

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