Rotational Motion of Ceres

Ceres is the most massive body of the asteroid belt and contains about 25 wt.% (weight percent) of water. Understanding its thermal evolution and assessing its current state are major goals of the Dawn Mission. Constraints on its internal structure can be inferred from various observations. Especially, detailed knowledge of the rotational motion can help constrain the mass distribution inside the body, which in turn can inform us on its geophysical history. Here, we compute the polar motion, precession-nutation, and length-of-day variations. We estimate the amplitudes of the rigid and non-rigid response for these various motions using a stratified model of Ceres interior constrained by recent shape data and surface properties. 1 Ceres’ Pole Position Ceres’s pole position in space has been inferred from adaptive optics and Hubble Space Telescope images [1,2,3] and the orbital pole of Ceres at J2000 can be obtained from the Horizons ephemerides [4]. The obliquity, defined as the angle between the normal to the orbital plane and the figure axis, brings information on the moment of inertia only if it has reached an equilibrium position [5], the present value from observations is 4.01 degrees for [1], 0.23 deg for [2], and 3.91 deg for [3]. That is far from the ∼ 0.01 deg expected for the equilibrium position. In addition, the required timescale to fully damped the obliquity appears to be very long [6]. Thus, it appears that the obliquity of Ceres has not yet relaxed in its Cassini state.