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Science 4 May 2007:
Vol. 316. no. 5825, pp. 710 - 714
DOI: 10.1126/science.1140514
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Research Articles

Large Longitude Libration of Mercury Reveals a Molten Core

J. L. Margot,1* S. J. Peale,2 R. F. Jurgens,3 M. A. Slade,3 I. V. Holin4

Observations of radar speckle patterns tied to the rotation of Mercury establish that the planet occupies a Cassini state with obliquity of 2.11 ± 0.1 arc minutes. The measurements show that the planet exhibits librations in longitude that are forced at the 88-day orbital period, as predicted by theory. The large amplitude of the oscillations, 35.8 ± 2 arc seconds, together with the Mariner 10 determination of the gravitational harmonic coefficient C22, indicates that the mantle of Mercury is decoupled from a core that is at least partially molten.

1 Department of Astronomy, Cornell University, 304 Space Sciences Building, Ithaca, NY 14853, USA.
2 Department of Physics, University of California, Santa Barbara, CA 93106, USA.
3 Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA.
4 Space Research Institute, Moscow, Russian Federation.

* To whom correspondence should be addressed. E-mail: jlm{at}astro.cornell.edu

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THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
Return to Mercury: A Global Perspective on MESSENGER's First Mercury Flyby.
S. C. Solomon, R. L. McNutt Jr., T. R. Watters, D. J. Lawrence, W. C. Feldman, J. W. Head, S. M. Krimigis, S. L. Murchie, R. J. Phillips, J. A. Slavin, et al. (2008)
Science 321, 59-62
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Laser Altimeter Observations from MESSENGER's First Mercury Flyby.
M. T. Zuber, D. E. Smith, S. C. Solomon, R. J. Phillips, S. J. Peale, J. W. Head III, S. A. Hauck II, R. L. McNutt Jr., J. Oberst, G. A. Neumann, et al. (2008)
Science 321, 77-79
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Titan's Rotation Reveals an Internal Ocean and Changing Zonal Winds.
R. D. Lorenz, B. W. Stiles, R. L. Kirk, M. D. Allison, P. P. del Marmo, L. Iess, J. I. Lunine, S. J. Ostro, and S. Hensley (2008)
Science 319, 1649-1651
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Science. ISSN 0036-8075 (print), 1095-9203 (online)