Titan’s obliquity as evidence of a subsurface ocean?

R.-M. Baland^1,2, T. Van Hoolst¹, M. Yseboodt¹ and Ö. Karatekin¹

¹ Royal Observatory of Belgium, Brussels, Belgium
e-mail: balandrm@oma.be
² Université Catholique de Louvain, Earth and Life Institute, Georges Lemaître Centre for Earth and Climate Research, Belgium

Received: 25 January 2011
Accepted: 13 March 2011

Abstract

On the basis of gravity and radar observations with the Cassini spacecraft, the moment of inertia of Titan and the orientation of Titan’s rotation axis have been estimated in recent studies. According to the observed orientation, Titan is close to the Cassini state. However, the observed obliquity is inconsistent with the estimate of the moment of inertia for an entirely solid Titan occupying the Cassini state. We propose a new Cassini state model for Titan in which we assume the presence of a liquid water ocean beneath an ice shell and consider the gravitational and pressure torques arising between the different layers of the satellite. With the new model, we find a closer agreement between the moment of inertia and the rotation state than for the solid case, strengthening the possibility that Titan has a subsurface ocean.