Bombardment history of the Moon constrained by crustal porosity

Abstract

The formation and evolution of the terrestrial planets were shaped by a bombardment of large impactors in a cluttered early Solar System. However, various surface processes degrade impact craters, and the early impact history of the Moon and the ages of its ancient impact basins remain uncertain. Here we show that the porosity of the lunar crust, generated by the cumulative crustal processing of impacts, can be used to determine the Moon’s bombardment history. We use a numerical model constrained by gravity data to simulate the generation of porosity by basin-forming impacts and the subsequent removal by smaller impacts and overburden pressure. We find that, instead of steadily increasing over the history of the Moon, lunar crustal porosity was largely generated early in lunar evolution when most basins formed and, on average, has decreased after that time. Using the Moon as a proxy for the terrestrial planets, we find that the terrestrial planets experienced periods of high crustal porosity early in their evolution. Our modelled porosities also provide an independent constraint on the chronological sequence of basin-forming impacts. Our results suggest that the inner solar system was subject to double the number of smaller impacts producing craters exceeding 20 km in diameter than has been previously estimated from traditional crater-counting analyses, whereas the bombardment record for the lunar basins (>200 km in diameter) is complete. This implies a limited late delivery of volatiles and siderophile elements to the terrestrial planets by impactors.