Dynamics and evolution of Mercury's interior as constrained by MESSENGER observations

Mercury's long-term geological evolution is largely the product of the thermal history of its interior. Observations by MESSENGER, inserted into orbit about Mercury on March 18, 2011, are providing new constraints on this history with information on the record of volcanic and tectonic activity, the composition of surface materials, the state and structure of the interior, and the character of its internal magnetic field. Notably, MESSENGER is improving knowledge of Mercury's gravity field, which will allow (along with observations of Mercury's orbital and spin dynamics) determination of the planet's normalized polar moment of inertia (C/MR²), where M and R are Mercury's mass and radius, and the ratio of the moment of inertia of the mantle and crust to the planet's polar moment of inertia (C_m/C). These parameters ultimately permit estimation of the location of the core-mantle boundary (CMB), albeit with compositional assumptions. Mercury's bulk density indicates a large mass ratio of metal to silicate, and hence a large core and relatively thin mantle and crust. This comparatively thin-shelled mantle implies that the Rayleigh number, which describes the vigor of convection and depends on mantle heat production and heat flux across the CMB, may be quite low. Indeed, the Rayleigh number in Mercury's mantle could be sufficiently small to approach its critical value, leading to sluggish, or even unsustainable, convection. Mercury's widespread surface contractional features accommodate an average surface strain equivalent to a 1-2 km reduction in the planet's radius over the past 4 billion years. Such limited contraction requires only modest cooling over that time. In contrast, volcanism that was voluminous near the end of the late heavy bombardment has declined substantially since that time, consistent with a marked decline in heat production as inferred from MESSENGER Gamma-Ray Spectrometer measurements of U, Th, and K abundances at Mercury's surface. In order to understand the consequences of MESSENGER observations for Mercury's interior, we investigate the dynamics and evolution of the planet's interior with the axisymmetric, spherical-shell, finite-element code CITCOM2D to model convection in Mercury's mantle. The models employ an extended Boussinesq formulation with a temperature- and depth-dependent viscosity, a cooling core, declining heat production, and latitudinal variations in surface temperature consistent with Mercury's insolation. Results to date suggest that the pattern of surface temperatures can lead to notable variations in the planform of convection.