The Moon is a focal point of planetary research given its relative ease of access and extensively cratered surface. The Gravity Recovery and Interior Laboratory (GRAIL) mission revealed that the lunar crust is surprisingly porous. Large impacts are known to generate spaces between solid mineral grains, referred to as porosity. However, this porosity can be reduced by compaction, i.e., pushing the crystalline grains closer together if the surface is further impacted by smaller projectiles. Together, these factors shape a planetary surface.

A team from MIT led by Ya Huei Huang used GRAIL data to map crustal porosity across the Moon and constructed a model to simulate the evolution of lunar porosity. GRAIL revealed that areas surrounding large basins have porosities ranging between 10 and 18.5%, whereas regions with many smaller craters are less porous. This relationship suggests that basin-scale impacts generated high porosities reduced by impacts and crushing overburden pressure. To test this hypothesis, the team constructed models that add porosity through basin-scale impacts and remove porosity through small impact events and compaction. This model results in crustal porosity patterns that broadly match GRAIL-derived crustal densities, but the degree of observed compaction requires approximately double the number of small impacts compared to the number of small craters observed. This finding suggests that a more realistic cratering record could be attained through detailed porosity analysis, even in areas with crater saturation where it is impossible to determine the true number of craters formed.

The lunar impact record constrains rates of impact events throughout the inner solar system. These results indicate a need to re-examine terrestrial cratering rates and the effect of successive impacts on young planetary surfaces to fully understand the evolution of terrestrial planets and the solar system. READ MORE