Impact-induced atmospheres and oceans on Earth and Venus

Abstract

High-velocity impacts of planetesimals onto a growing planet result in the impact-degassing of volatiles and the formation of an impact-induced atmosphere. Because of the blanketing effect of such an atmosphere, it is likely that the surface of the proto-Venus melted once the radius exceeded ∼40% of the final radius. The final mass of H₂O in the impact-generated atmosphere, predicted to be ∼10²¹ kg on the basis of thermal evolution models of the growing proto-Venus, does not depend on the initial water content of the Venus-forming planetesimals and is almost identical to the present mass of the Earth's oceans. We show here that an impact-induced H₂O atmosphere of ∼10²¹ kg mass probably formed on both Venus and Earth during accretion, but that whereas H₂O in the proto-atmosphere of the Earth could condense to form a hot (∼600 K) ocean, such condensation probably did not occur on Venus.