The Proto-Atmosphere of Terrestrial Planets

Abstract

The theoretically derived composition of the proto-atmosphere, which evolved during accretion, of terrestrial planets is composed primarily of CO₂, Ar and N₂ when the mass of a planet became greater than 3.30 x 10²⁶ g.  This is identical to the atmospheric compositions of Venus and Mars observed today.  Therefore, both Venus and Mars most likely retained their proto-atmospheres till now.  The Earth is situated between Venus and Mars, there is no reason whatsoever that the Earth’s proto-atmosphere should be any different from those of Venus and Mars.  Yet, today’s Earth atmospheric composition is drastically different from her proto-atmosphere.  Mercury is not massive enough to hold even CO₂ to form a proto-atmosphere.  Therefore, Mercury is virtually in vacuum. A “magma ocean” has been proposed to commence and cover the surface of the Earth when her radius grew over ~2550 km.  All H₂O in the primordial planetesi­mals would dissolve in the “magma ocean” during accretion. Thus, H₂O would never be one of the proto-atmospheric components even if the mass of terrestrial planets was over 8.06 x 10²⁶ g.  Any significant amount of H₂O on the surface of all terrestrial planets must be added or derived via other mechanisms after the completion of accretion.  The Earth’s hot and soda indigenous ocean of supercritical H₂O-CO₂ mixture was most likely derived from the giant impacting process that produced the Moon.  It is this ocean that removed all CO₂ from the Earth’s proto-atmosphere.