title: Large longitude libration of Mercury reveals a molten core.
abstract: 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.
authors.affiliation: Department of Astronomy, Cornell University, 304 Space Sciences Building, Ithaca, NY 14853, USA. jlm@astro.cornell.edu
authors.forename: J L
authors.initials: JL
authors.lastname: Margot
authors.forename: S J
authors.initials: SJ
authors.lastname: Peale
authors.forename: R F
authors.initials: RF
authors.lastname: Jurgens
authors.forename: M A
authors.initials: MA
authors.lastname: Slade
authors.forename: I V
authors.initials: IV
authors.lastname: Holin