Why is the Moon synchronously rotating?

TL;DR

This paper investigates the Moon's current synchronous rotation by exploring its past spin evolution, resonance captures, and the influence of interior temperature and orbital eccentricity on its rotational state.

Contribution

It presents new models for the Moon's spin evolution considering different interior temperatures and orbital eccentricities, explaining the likelihood of capture into various spin-orbit resonances.

Findings

Higher eccentricity increases capture probability into 3:2 resonance.

Warmer, less viscous Moon more easily captured into supersynchronous states.

Current synchronous rotation likely resulted from specific initial conditions and orbital evolution.

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.