Large Longitude Libration of Mercury Reveals a Molten Core

@article{Margot2007LargeLL,
  title={Large Longitude Libration of Mercury Reveals a Molten Core},
  author={Jean-Luc Margot and Stanton J. Peale and Raymond F. Jurgens and Martin A. Slade and Igor V. Holin},
  journal={Science},
  year={2007},
  volume={316},
  pages={710 - 714}
}
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… 
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