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Science. 2007 May 4;316(5825):710-4.

Large longitude libration of Mercury reveals a molten core.

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Department of Astronomy, Cornell University, 304 Space Sciences Building, Ithaca, NY 14853, USA.


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.

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