The free librations of a dissipative Moon

Abstract Dissipation in the Moon produces a small offset, ca. 0.23", of the Moon's rotation axis from the plane defined by the ecliptic and lunar orbit normals. Both solid body tidal friction and viscous fluid friction at a core—mantle interface are plausible mechanisms. In this paper, I discuss the merits of each and find that solid friction requires a low lunar tidal Q,ca. 28, while turbulent fluid friction requires a core of radius 330 km to cause the signature observed by lunar laser ranging. Large (ca. 0.4—8.0") free librations of the lunar figure have also been detected by laser ranging. Both a very recent impact on the Moon and fluid turbulence in the lunar core are plausible mechanisms for generating these free librations.