Evidence for a Subsurface Ocean on Titan
25 March 2008
Based on Cassini radar observations of Titan's surface the moon's spin rate is now confirmed to be both non-synchronous and changing with time, providing evidence for the existence of a subsurface ocean.Identification of Titan surface features in radar images obtained by Cassini over a period of a few years shows the position of these features to deviate from their expected coordinates when assuming a constant spin rate for the moon of ~15.95 days (with Titan phase-locked in its ~15.95-day orbit around Saturn).
Two independent methods to derive Titan's spin rate from the Cassini radio observations have unambiguously proven the current spin rate to be faster than the synchronous spin rate by about 0.36° per year. The results are reported by Lorenz, R.D. et al. in the 21 March issue of Science.
Apart from the moon's deviation from synchronous rotation, Lorenz et al. also find an acceleration in Titan's spin rate. Both these results are explained by the authors as due to Titan not being a solid body rotator, but rather its crust is detached from the solid core by a subsurface ocean at a depth of at least 100 km.
In this scenario the affect of motions in Titan's massive atmosphere (~1.5 bar at the surface) on the crust is more profound. Changes in atmospheric circulation due to seasons (caused by Saturn's ~29.5-year orbit around the Sun) lead to changes in the exchange of angular momentum between the atmosphere and the surface.
Lorenz, R.D. et al. show that the observed value for Titan's spin rate could be reproduced by an existing model (Tokano, T. et al. [2005]) of the interaction between the crust and the atmospheric circulation patterns, provided that the crust is indeed detached from the moon's core.
The evolution of Titan's spin rate in this model - albeit with a necessary refining to match the observations - predicts a peak value between 2008 and 2010 after which the spin rate should decrease again as it oscillates around the synchronous spin rate, following seasonal changes in Titan's atmospheric circulation. Future Cassini observations could verify these predictions.
Related publications
Lorenz, R.D. et al., "Titan's Rotation Reveals an Internal Ocean and Changing Zonal Winds", Science, 319, 1649-1651, 21 Mar 2008
Tokano, T. et al., "Wind-induced seasonal angular momentum exchange at Titan's surface and its influence on Titan's length-of-day", Geophys. Res. Lett., 32, L24203, 22 Dec 2005