Large Longitude Libration of Mercury Reveals a Molten Core

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. Because Mercury is an end-member plan-et, the characterization of its interiorproperties is crucial to further our under-standing of the formation and evolution of habitable worlds. A question that has chal-lenged planetary scientists for more than three decades is the state of Mercury’s core (1). Determining the nature and extent of the core would enable progress in four fundamental areas. First, models of the internal structure of the planet have so far been constrained only b