Hypothetical Hyperbolic Encounters between Venus and Proto-Mercury that Partially Stripped Away Proto-Mercury's Mantle

摘要

Compared to all other terrestrial planets in the solar system, Mercury has an unusually large metal core, comprising similar to 70% of its mass. Giant impacts could have removed a significant fraction of the silicate mantle of a chondritic proto-Mercury, forming the iron-rich present-day Mercury. However, such high-temperature giant impacts seem at odds with the retention of moderately volatile elements on present-day Mercury. We simulated a series of hyperbolic encounters between proto-Mercury and proto-Venus, which may have occurred in the chaotic early solar system. Tidal disruption of proto-Mercury always removes a portion of its silicate mantle, while its iron core remains intact. We find, in favorable cases, four close encounters with fast-spinning projectiles (resulting from previous encounters) that could lead to Mercury's present-day iron fraction. More encounters are needed when the spin and orbital angular momentum are not always aligned. These hyperbolic encounters have various outcomes, such as orbital decay, binary planets, and change of spin rates. These results suggest the importance of the proper treatment of close encounters in N-body simulations of planetary accretion.