The free precession and libration of Mercury

Abstract

An analysis based on the direct torque equations including tidal dissipation and a viscous core–mantle coupling is used to determine the damping time scales of O ( 10 5 ) years for free precession of the spin about the Cassini state and free libration in longitude for Mercury. The core–mantle coupling dominates the damping over the tides by one to two orders of magnitude for the plausible parameters chosen. The short damping times compared with the age of the Solar System means we must find recent or on-going excitation mechanisms if such free motions are found by the current radar experiments or the future measurement by the MESSENGER and BepiColombo spacecraft that will orbit Mercury. We also show that the average precession rate is increased by about 30% over that obtained from the traditional precession constant because of a spin–orbit resonance induced contribution by the C 22 term in the expansion of the gravitational field. The C 22 contribution also causes the path of the spin during the precession to be slightly elliptical with a variation in the precession rate that is a maximum when the obliquity is a minimum. An observable free precession will compromise the determination of obliquity of the Cassini state and hence of C / M M R 2 for Mercury, but a detected free libration will not compromise the determination of the forced libration amplitude and thus the verification of a liquid core.