Some aspects of core formation in Mercury

Abstract

Evidence for a large metallic core in Mercury is all indirect; the internal magnetic field may imply a convective dynamo; the surface geology is suggestive of large-scale differentiation; and thermal history calculations based on cosmochemical models for Mercury predict core formation. The presence of a core will not be confirmed by an accurate measure of J 2, probably two orders of magnitude larger than the hydrostatic value. Core infall on Mercury would be accompanied by an increase in planetary radius of 17 km, an increase in the mean internal temperature of 700°K, and substantial melting of the mantle. If Mercury's core differentiated from an originally homogeneous planet, such an event must predate most of the present surface. Subsequent to core formation, Mercury's radius has decreased by about 2km due primarily to cooling of the lithosphere, in agreement with the estimate by Strom and others of the amount of planetary contraction based on photogeologic measurements of length, dip and throw on the global system of lobate scarps. A convective dynamo mechanism for Mercury's magnetic field is in apparent conflict with cosmochemical models that do not predict a substantial source of heat, most probably radiogenic, in Mercury's core. Without such a heat source, the core would solidify within about 1 b.y. after core infall, producing an unacceptably large contraction in Mercury's radius.