A pre-Caloris synchronous rotation for Mercury

TL;DR

This paper proposes that Mercury may have initially been in a retrograde rotation, leading to a high probability of synchronous rotation, which aligns with observed impact basin distributions and suggests a different evolutionary path than previously thought.

Contribution

It introduces a new hypothesis that Mercury's initial retrograde rotation could explain its current spin state and impact basin distribution, challenging prior assumptions.

Findings

Retrograde initial rotation leads to 68% chance of synchronous capture.

Impact events could have caused escape from synchronous resonance.

Distribution of impact basins supports the retrograde initial rotation hypothesis.

Abstract

The planet Mercury is locked in a spin-orbit resonance where it rotates three times about its spin axis for every two orbits about the Sun. The current explanation for this unique state assumes that the initial rotation of this planet was prograde and rapid, and that tidal torques decelerated the planetary spin to this resonance. When core-mantle boundary friction is accounted for, capture into the 3/2 resonance occurs with a 26% probability, but the most probable outcome is capture into one of the higher-order resonances. Here we show that if the initial rotation of Mercury were retrograde, this planet would be captured into synchronous rotation with a 68% probability. Strong spatial variations of the impact cratering rate would have existed at this time, and these are shown to be consistent with the distribution of pre-Calorian impact basins observed by Mariner 10 and MESSENGER. Escape from this highly stable resonance is made possible by the momentum imparted by large basin-forming impact events, and capture into the 3/2 resonance occurs subsequently under favourable conditions.