Internal structure of Mercury: Constraints from MESSENGER

Following a six-and-a-half year cruise phase that included three flybys of Mercury, MESSENGER became the first spacecraft to orbit the planet closest to the Sun on March 18, 2011. One of the primary goals of the MESSENGER mission is to constrain the state and structure of Mercury's core. Mercury has a high bulk density that implies a larger mass fraction of iron in the planet's interior than in the other terrestrial planets. Tracking of radio-signal transmissions from MESSENGER's low- and high-gain antennas is yielding new information on the gravity field of Mercury. Importantly, MESSENGER is improving estimates of the second-degree spherical harmonic gravity-field coefficients C₂₀ and C₂₂. These coefficients, along with recent Earth-based observations of Mercury's orbital and spin dynamics, permit the 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 polar moment of inertia of the mantle and crust to the planet's polar moment of inertia (C_m/C). The C/MR² and C_m/C parameters provide complementary measures of the distribution of mass within the planet. We model the planet's interior as a four-layer structure consisting of a compressible solid inner core and liquid outer core and uniform-density mantle and crustal layers. With a Monte Carlo approach to account for parameter ranges, we calculate a large suite of models of Mercury's interior consistent with the planet's mean radius, bulk density, and a wide range of material and internal structural values. For each candidate structure we estimate the statistical likelihood that it is consistent with the observed moment of inertia parameters and their uncertainties to construct an appropriate sample of models consistent with Mercury's physical characteristics. The radius of Mercury's core, which constrains the mass fraction of metal and ultimately the bulk composition, is the most robustly determined parameter in this analysis.