The lunar fossil bulge hypothesis revisited

The lunar gravity field can be satisfactorily explained by relatively-small wavelength, density anomalies located near the lunar surface. The exceptions to this are the second degree harmonics and we postulate a dual origin for these terms; they are partly a consequence of the same near-surface anomalies as the remainder of the gravity field and partly a measure of a fossil bulge. This latter contribution represents an equilibrium condition preserved from a time when the synchronously rotating Moon was approximately 25 Earth-radii from the Earth. If this shape was acquired at some time soon after 4.0 · 10⁹ years ago, then the rate of dissipation of tidal energy in the Earth has been much lower in the past then it is now. The implications of this on the evolution of the Earth-Moon system are briefly discussed.