Cratering and penetration of the early lunar crust

Introduction: Giant impacts are expected at the late stages of planet formation [e.g., 1]. There is substantial evidence for the occurrence of such impacts in our own solar system, in particular our Moon is believed to have originated in such an event [2,3]. Recent work has shown that typically at least a few percent of the impacting mass will have sufficient speed to escape the system [e.g., 4]. For the Moon-forming impact, at least 10 kg (∼ 1.3 lunar masses) of debris is estimated to have escaped onto heliocentric orbits, a substantial fraction of which subsequently reimpacted the Earth and the Moon over a period of ∼100 Myr [5]. At the same time the Moon would have been cooling from a mostly molten state, having formed from a high-energy impact event. Without impact bombardment, the Moon is thought to have solidified fairly slowly over around 10 Myr due to the early lunar crust functioning as a thermal blanket [e.g., 6]. Reimpacting debris however may have had a large influence on the cooling rate. They could have punctured the early crust, increased the thermal heat flux and thus expedited cooling. At the same time reimpacting debris would have carried substantial kinetic energy, which may have resulted in heating of the Moon. As such recent work found that reimpacting debris may have temporally altered the early lunar thermal evolution [7].