# The Free Librations of a Dissipative Moon [and Discussion]

C. F. Yoder , R. Hutchison

## Abstract

Dissipation in the Moon produces a small offset, ca. 0.23$^{\prime \prime}$, of the Moon's rotation axis from the plane defined by the eliptic 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 ca. 330 km to cause the signature observed by lunar laser ranging. Large (ca. 0.4-8.0$^{\prime \prime}$) 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.