Influence of Lunar Mascons on its Dynamical Figure

Abstract

THE recent discovery of large mass concentrations (mascons) beneath the lunar circular maria from Lunar Orbiter tracking data¹ raises the question of whether mascons can influence the dynamical figure of the Moon. A convenient way to describe the lunar figure is in terms of the moments of inertia about the three principal axes, where C is about the axis of rotation, A is about an axis along the lines of centres of the Earth and the Moon, and B is about an axis tangent to the orbit. It is well known from observation that C>B>A and that their differences exceed by about an order of magnitude the predicted values from hydrostatic equilibrium and tidal distortion. There is a one-to-one correspondence between large mascons and circular maria¹. But the circular maria have a non-uniform distribution over the lunar surface, with a tendency for locations toward the equator and toward the sub-Earth point. These locations are in the correct sense to suggest that mascons are responsible for the non-equilibrium contribution to the inequality C>B>A.