The thermal consequences of giant impacts in the early solar system.

Abstract

Recent advances in understanding planetary formation indicate that mass and energy accumulation are dominated the impact of objects containing up to several percent of the protoplanet's mass. This work examines consequences of such "giant" impacts, including the initiation of core formation and the development and subsequent evolution of deep magma oceans. Large, high speed impacts on silicate planets with masses >∼10²³ kg result in formation of intact melt regions. Iron rapidly settles to the melt region's base, forming a negatively buoyant mass. If the differential stress generated by this mass too large, it overcomes the elastic strength of the mantle beneath and rapidly forms a core. The same process might also trigger differentiation of the largest known icy bodies. A melting model based on the Hugoniot equations, empirical particle velocity-distance, and linear shock-particle velocity relationships was developed. Impact-induced core formation occurs in silicate planets with masses between 1-6 x 10²³ kg. Thus, the largest planetesimals that accreted to form the Earth were already differentiated. Impact-induced core formation may occur in icy bodies as small as Triton, if an intact melt region forms on bodies that small. If the icy bodies accreted high speed, heliocentric particles, core formation can be triggered in somewhat smaller bodies. Giant impacts do not explain whole-body differentiation of asteroid sized bodies. Giant impacts on large terrestrial planets can form magma oceans 1000-2000 km deep. It is widely assumed that such a magma ocean necessarily differentiates by fractional crystallization. Scientists making this assumption have not accounted for the possibility that crystal suspension in a planetary-scale convecting system may prevent crystal-magma separation. It is shown that crystals <1-2 cm in diameter are suspended from the onset of cooling. Surprisingly, the ability of a magma ocean to suspend crystals is only weakly dependent on gravity and depth. The difference in magma ocean evolution between the Earth and Moon may be due to gravity's effect on the solidus, liquidus, and adiabatic temperature profiles.