Earliest Planetary Crusts: Constraints on the Formation of Mercury and Implications for Bodies of Different

A volcanic crust may be produced as a result of magma ocean processes. Solidification of a magma ocean naturally produces a silicate mantle with denser layers near the surface, therefore unstable to gravitational overturn. The mantle is predicted to rapidly flow in the solid state to gravitational stability. Hot buoyant silicate mantle rising from depth will melt through depressurization and erupt lavas onto the planetary surface, as long as there is no flotation crust [1, 2]. These lavas solidify to form the planet’s earliest basaltic crust. The overturn model produces predictions for the compositions of cumulates that rise sufficiently to melt. Their exact melt composition cannot be calculated precisely as specific experimental studies would be required, but approximations can be made based on the source bulk composition. Flotation is only possible on bodies with gravity significant enough to drive crystal separation in magma, but not so large that the low-pressure stability of buoyant minerals is confined to only the final percentages of mantle solidification. Very small bodies and planetesimals present early in the Solar System will melt from the inside out and the crust would be the undifferentiated bulk composition. Internal dynamics may cause volcanism and so lavas may partially cover the primitive crust. Mercury’s large core and high density obscure the size and nature of the original accreting planet – did the planet form with enough metallic iron to create its large core (endogenous) or was silicate mantle later removed in a giant impact (exogenous)? Was the crust produced from overturn or from flotation of buoyant minerals? Its size is sufficiently small that flotation is possible, but large enough that magma ocean cumulate overturn would occur and would create an earliest volcanic crust in the absence of flotation. Here we consider both models and examine their predictions for the planet’s crustal composition, and we compare them to measurements of Mercury’s crust that show it has low iron oxide content [3, 4]