Geodetic Evidence That Mercury Has a Solid Inner Core

Geodetic analysis of radio tracking measurements of the MESSENGER spacecraft while in orbit about Mercury has yielded new estimates for the planet's gravity field, tidal Love number, and pole coordinates. The derived right ascension ( = 281.0082° ± 0.0009°; all uncertainties are 3 standard deviations) and declination ( =61.4164° ± 0.0003°) of the spin pole place Mercury in the Cassini state. Confirmation of the equilibrium state with an estimated mean (whole-planet) obliquity of 1.968 ± 0.027 arcmin enables the confident determination of the planet's normalized polar moment of inertia (0.333 ± 0.005), which indicates a high degree of internal differentiation. Internal structure models generated by a Markov-Chain Monte Carlo process and consistent with the geodetic constraints possess a solid inner core with a radius ( ) between 0.3 and 0.7 that of the outer core ( ).