Analysis of GRAIL data revealed a lunar crustal dichotomy that can be
described by a degree 1 spherical harmonic. A simple explanation of this
observation is a superposition of the Moon’s self-gravity and an
external and constant acceleration. I explored the possibility that the
Moon experienced a much greater prograde acceleration in the past (when
it was considerably closer to the Earth). I use a simple density balance
approximation to determine the approximate acceleration needed to
produce the observed asymmetry: ~0.009 m/s2 (or
~1/2% of the Moon’s self-gravity). In order to produce
this acceleration, the Moon would need to be within
~13,000 km of the Earth. This is well within the Roche
limit for completely fluid bodies (~18,350 km); for
completely rigid bodies, the Roche limit is ~9,480 km.
From this, it is extremely unlikely for tidally-induced acceleration to
explain the observed crustal asymmetry.