Compositions of Mercuryʹs earliest crust from magma ocean models

The size of the Mercurian core and the low ferrous iron bearing silicate content of its crust offer constraints on formation models for the planet. Here we consider a bulk composition that allows endogenous formation of the planetʹs large core, and by processing the mantle through a magma ocean, would produce a low-iron oxide crust consistent with observations. More Earth-like bulk compositions require silicate removal, perhaps by a giant impact, to create the planetʹs large core fraction. We find that the endogenous model can produce a large core with either a plagioclase flotation crust or a low-iron oxide magmatic crust. Because a magma ocean creates a gradient in iron oxide content in the resulting planetary mantle, the parts of the mantle removed by a putative giant impact could result in either a high-iron oxide mantle in contradiction to current crustal measurements, or a low-iron oxide mantle consistent with the current understanding of Mercury. If a giant impact cannot preferentially remove shallow mantle material then the proto-Mercury must have had a bulk low iron-oxide composition. Thus a specific bulk composition is required to make Mercury endogenously, and either a specific process or a specific composition is required to make it exogenously through giant impact. Measurements taken by the MESSENGER mission, when compared to predictions given here, may help resolve Mercuryʹs formation process.