Secular obliquity variations of Ceres and Pallas

Abstract

We examine variations in the orientations of the orbit poles and spin poles of Ceres and Pallas, on time scales of a few million years. We consider these two bodies together because they have similar orbits, but very different present states of knowledge concerning internal mass distribution and spin pole orientation. For Ceres, the Dawn mission has recently provided accurate estimates of the current spin pole orientation, and the degree 2 spherical harmonics of the gravitational potential. The polar moment of inertia is not as well constrained, but plausible bounds are known. For Pallas, we have estimates of the shape of the body, and spin pole orientation and angular rate, all derived from optical light curves. Using those input parameters, and the readily computed secular variations in the orbit pole, we can compute long term variations in the spin pole orientation. This provides information concerning long term variations in insolation, which controls stability of surface volatiles.