Measurements of the rotation and orbital tilt of Titan show that the large Saturnian satellite may have a liquid water ocean beneath its icy surface. In recent years, the Cassini-Huygens mission has revealed much about the features and composition of this Earth-like planetary body, including liquid methane lakes and complex organic molecules. The latest data show that some of Titan’s surface features deviate from model predictions by as much as 30 kilometres.
Rose-Marie Baland and others at the Royal Observatory in Brussels looked at radar and gravity measurements from the Cassini spacecraft, and discovered that the orientation of Titan’s rotation axis is inconsistent with that expected of an entirely solid body occupying what is known as the “Cassini state”. Like Earth’s moon, Titan forever presents the same face toward its mother planet, and this synchronisation of the precession of Titan’s rotation axis and orbital nodes leads to a predictable degree of axial tilt. Baland and her colleagues measured that tilt at 0.32 degrees, which is too high for a all-solid body.
In an attempt to explain this orbital anomaly, the latest study, which will soon appear in the journal Astronomy and Astrophysics, proposes a new Cassini state model that assumes the presence of a liquid water ocean between an ice shell and an ice/rock core, and considers gravitational and pressure forces between the various layers of the satellite. While such a model cannot prove the existence of a water ocean, the results are indicative. Baland’s study is mathematically rigorous, with celestial mechanics calculations incorporating minor effects such as polar and equatorial flattening due to rotational forces and planetary tides. A few simplifying assumptions have been made, but the resulting model is in physical terms quite realistic.
Baland et al., “Titan’s Obliquity as evidence for a subsurface ocean?”, Astron. Astrophys. (2011, in press)