Impact cratering on Mercury: consequences for the spin evolution

TL;DR

This study uses impact crater data on Mercury to infer its early spin states and demonstrates that collisional and secular evolution models can explain Mercury's current 3/2 spin-orbit resonance and primordial synchronous rotation.

Contribution

It introduces a collisional model consistent with impact crater distributions and shows how combined effects can reproduce Mercury's current spin state.

Findings

Impact crater distribution aligns with a primordial synchronous rotation.

The collisional model matches the impact size distribution with a power law index of 1.2.

The combined model reproduces Mercury's current 3/2 spin-orbit resonance.

Abstract

Impact basins identified by Mariner 10 and Messenger flyby images provide us a fossilized record of the impactor flux of asteroids on Mercury during the last stages of the early Solar System. The distribution of these basins is not uniform across the surface, and is consistent with a primordial synchronous rotation (Wieczorek et al. 2012). By analyzing the size of the impacts, we show that the distribution for asteroid diameters D < 110 km is compatible with an index power law of 1.2, a value that matches the predicted primordial distribution of the main-belt. We then derive a simple collisional model coherent with the observations, and when combining it with the secular evolution of the spin of Mercury, we are able to reproduce the present 3/2 spin-orbit resonance (about 50% of chances), as well as a primordial synchronous rotation. This result is robust with respect to variations in the dissipation and collisional models, or in the initial spin state of the planet.