The density structure of Titan’s outer ice shell

Abstract

We use the recent models of Titan’s shape and gravity to estimate the large-scale density structure of its outer ice shell. We assume that the bottom boundary of the ice shell is an equipotential surface, in agreement with the decrease in ice viscosity expected near the ice/water interface, and the topography is supported by density variations within the ice shell. The density model shows strong degree 2 and 4 zonal components, indicating an important role of atmospheric and/or oceanic processes. We discuss three mechanisms that may explain the derived density anomalies: ethane precipitation, temperature anomalies and variations in porosity. While ethane precipitation provides a plausible explanation for positive density anomalies associated with polar depressions, large-scale variations in temperature are likely to play a role at mid- and low latitudes. These variations can be associated with a laterally varying thickness of the methane clathrate crust, which controls the heat transport in the ice shell, and/or with variations in the heat flux from the ocean. The density of the ice shell can also be affected by variations in porosity of the near-surface material, but these variations are unlikely to be the main cause of the large-scale density anomalies identified from the shape and gravity data.