Several lines of evidence suggest that Saturn’s largest moon, Titan, has a global subsurface ocean beneath an outer ice shell 50 to 200 kilometres thick1,2,3,4. If convection5,6 is occurring, the rigid portion of the shell is expected to be thin; similarly, a weak, isostatically compensated shell has been proposed7,8 to explain the observed topography. Here we report a strong inverse correlation between gravity3 and topography9 at long wavelengths that are not dominated by tides and rotation. We argue that negative gravity anomalies (mass deficits) produced by crustal thickening at the base of the ice shell overwhelm positive gravity anomalies (mass excesses) produced by the small surface topography, giving rise to this inverse correlation. We show that this situation requires a substantially rigid ice shell with an elastic thickness exceeding 40 kilometres, and hundreds of metres of surface erosion and deposition, consistent with recent estimates from local features10,11. Our results are therefore not compatible with a geologically active, low-rigidity ice shell. After extrapolating to wavelengths that are controlled by tides and rotation, we suggest that Titan’s moment of inertia may be even higher (that is, Titan may be even less centrally condensed) than is currently thought12.
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We thank the Cassini radar science team, M. Manga, D. Stevenson, R. Pappalardo and W. McKinnon for their suggestions. Portions of this work were supported by NASA grants NNX13AG02G and NNX11AK44G.
The authors declare no competing financial interests.
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Hemingway, D., Nimmo, F., Zebker, H. et al. A rigid and weathered ice shell on Titan. Nature 500, 550–552 (2013). https://doi.org/10.1038/nature12400