Titan's Interior Structure

Abstract

The goal of this chapter is to give a description of Titan's interior that is consistent with the new constraints provided by the Cassini mission. As the Cassini mission proceeds into its first extended phase, the data obtained during the nominal mission suggest that Titan is at least partially differentiated. An ocean would be present some tens of kilometers below the surface. By comparison with the Galilean icy satellites Ganymede and Callisto, Titan would be composed of a metal/silicate rich core and a H2O rich outer layer. These conclusions are drawn from the interpretation of the gravity data, the geological data and the presence of a Schumann resonance which has been inferred from the measurement of electric signals during the descent of the Huygens probe into Titan's atmosphere. Titan's high eccentricity implies that the interior has not been very dissipative, there is little tidal heating available for internal dynamics, and the ice layer is cold, which can be achieved if the ocean under the ice layer contains ammonia. This paper also describes observations and interpretations which seem difficult to reconcile with our present understanding of Titan's interior structure and evolution such as the shape of the planet or the obliquity. The last part of the chapter describes heat transfer models which suggest that the lower part of the ice crust could be convective. The NH3–H2o phase diagram indicates that the ocean is decoupled from the silicate-rich core by a layer of high-pressure ices. However, the interior model is largely uncertain because the interpretation of the data is still debated at present time. The additional information that will be acquired during the Cassini Solstice Mission should allow us to answer some of the questions.