Why is the Moon synchronously rotating?

Abstract

If the Moon's spin evolved from faster prograde rates, it could have been captured into a higher spin-orbit resonance than the current 1:1 resonance. At the current value of orbital eccentricity, the probability of capture into the 3:2 resonance is as high as 0.6, but it strongly depends on the temperature and average viscosity of the Moon's interior. A warmer, less viscous Moon on a higher-eccentricity orbit is even more easily captured into supersynchronous resonances. We discuss two likely scenarios for the present spin-orbit state: a cold Moon on a low-eccentricity orbit and a retrograde initial rotation.Comment: accepted in MNRAS Letter