Large Longitude Libration of Mercury Reveals a Molten Core

• DOI: 10.1126/science.1140514
• Publication Date: 2007-05-03T21:48:12Z
• Authors: J. L. Margot; S. J. Peale; R. F. Jurgens; M. A. Slade; I. V. Holin
• Keywords: 0301 basic medicine; 03 medical and health sciences; 13. Climate action

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 C 22 , indicates that the mantle of Mercury is decoupled from a core that is at least partially molten.