Existence of Primary Solar-Type Atmosphere on the Earth Revisited

S. Sasaki (Univ. Tokyo)

Whether terrestrial planets were formed in the presence of the solar nebula or not is one of the most important questions in planetary formation as well as planetary evolution. When planetary formation proceeds in the presence of the primordial solar nebula, a planet should attract the surrounding solar-type gas to form the primary atmosphere, which would keep the planet hot and reduce iron oxide and silicate.

Recently presence of the dissolved component of the primary atmosphere is advocated again by Harper and Jacobsen based on the interior He and Ne. However, quantitative estimates of dissolution of all noble gases have not been performed. Here, we estimate quantitatively the dissolved noble gas amount from the surface pressure obtained from the numerical analysis of the primary atmosphere. Using experimentally-obtained Henry's constant of dissolution, dissolved abundances of all noble gases are calculated. Dissolved component can control He and Ne but may not affect gas heavier than Ar. The primary atmosphere should have existed at least by M = 0.3 tex2html_wrap_inline22 to supply enough tex2html_wrap_inline24 He. But the presence of primary atmosphere by the end of accretion is not plausible.

Recent studies show Ne isotopic ratios in the mantle possess two types of end members: MORB and Plume types. Both types can be explained by solar-type Ne with tex2html_wrap_inline26 Ne/ tex2html_wrap_inline28 Ne = 13.0 plus nucleogenic tex2html_wrap_inline30 Ne. The most simple model is that MORB component represents the present upper mantle whereas Plume component corresponds to the lower mantle. The difference of tex2html_wrap_inline30 Ne/ tex2html_wrap_inline28 Ne can be explained by the difference of remaining Ne concentration if both source regions have similar abundances of U and Th that supply alpha particles in the reaction tex2html_wrap_inline36 O( tex2html_wrap_inline38 , n) tex2html_wrap_inline30 Ne. The difference of tex2html_wrap_inline44 He/ tex2html_wrap_inline24 He and tex2html_wrap_inline48 Ar/ tex2html_wrap_inline50 Ar should also imply the relatively noble gas rich (or less-degassed) lower mantle.