Constraining Ceres' interior from its rotational motion

Context. 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 types of observations. In particular, detailed knowledge of the rotational motion can help constrain the mass distribution inside the body, which in turn can lead to information about its geophysical history. Aims. We investigate the signature of internal processes on Ceres rotational motion and discuss future measurements that can possibly be performed by the spacecraft Dawn and will help to constrain Ceres' internal structure. Methods. We compute the polar motion, precession-nutation, and length-of-day variations. We estimate the amplitudes of the rigid and non-rigid responses for these various motions for models of Ceres' interior constrained by shape data and surface properties. Results. As a general result, the amplitudes of oscillations in the rotation appear to be small, and their determination from spaceborne techniques will be challenging. For example, the amplitudes of the semi-annual and annual nutations are around similar to 364 and similar to 140 milli-arcseconds, and they show little variation within the parametric space of interior models envisioned for Ceres. Conclusions. Owing to the small amplitudes of the nutation and the very long-period of the precession motion, the measurements of the rotational variations will be challenging to obtain. We also estimate the timescale for Ceres' orientation to relax to a generalized Cassini state, and find that the tidal dissipation within that object has probably been too small to drive any significant damping of its obliquity since formation. However, combining the shape and gravity observations of Dawn offers the prospect to identify departures of non-hydrostaticity on both global and regional scales, which will be instrumental in constraining Ceres' past and current thermal state. We also discuss the existence of a possible Chandler mode in the rotational motion of Ceres, whose potential excitation by endogenic and/or exogenic processes may help us to detect the presence of liquid reservoirs within the asteroid.