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Science 21 March 2008:
Vol. 319. no. 5870, pp. 1649 - 1651
DOI: 10.1126/science.1151639
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Reports

Titan's Rotation Reveals an Internal Ocean and Changing Zonal Winds

Ralph D. Lorenz,1* Bryan W. Stiles,2 Randolph L. Kirk,3 Michael D. Allison,4 Paolo Persi del Marmo,5 Luciano Iess,5 Jonathan I. Lunine,6 Steven J. Ostro,2 Scott Hensley2

Cassini radar observations of Saturn's moon Titan over several years show that its rotational period is changing and is different from its orbital period. The present-day rotation period difference from synchronous spin leads to a shift of ~0.36° per year in apparent longitude and is consistent with seasonal exchange of angular momentum between the surface and Titan's dense superrotating atmosphere, but only if Titan's crust is decoupled from the core by an internal water ocean like that on Europa.

1 Space Department, Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA.
2 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA.
3 U.S. Geological Survey, Flagstaff, AZ 86001, USA.
4 NASA Goddard Institute for Space Studies, New York, NY 10025, USA.
5 Università La Sapienza, 00184 Rome, Italy.
6 Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA.

* To whom correspondence should be addressed. E-mail: ralph.lorenz{at}jhuapl.edu

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Science. ISSN 0036-8075 (print), 1095-9203 (online)