In situ fragmentation of lunar blocks and implications for impacts and solar-induced thermal stresses

This study deals with an aspect of blocks observed on many rocky planetary surfaces: in situ fragmentation. Using LROC/NAC images, we characterized the morphology, morphometry and abundance of in-situ fractured blocks observed on the rim of six large impact craters of known emplacement age on the Moon. The relative number of disrupted blocks increases with crater-retention age of surfaces on which blocks are hosted, consistent with fragmentation post-emplacement due to impacts of small meteoroids. The type of break-up morphologies we observe appears to be independent of surface exposure age of the blocks. The inferred flux and size frequency distribution of projectiles responsible for disrupting blocks is consistent with expected lunar impact fluxes. Block fragmentation due to insolation-driven thermal stresses is subordinate to impacts. The possible effects of thermal stresses are evident as meridional cracks, which have preferred orientations in a young block population (~4 Ma), and as loose material (fillet) developing on top of surviving blocks in old populations (>800 Ma). •Using LROC/NAC images, we characterized the morphology, morphometry and abundance of in-situ fractured blocks on the Moon.•The relative number of disrupted blocks increases with the age of the surfaces on which blocks are hosted, consistent with fragmentation post-emplacement.•The inferred flux and size frequency distribution of projectiles responsible for fragmentation is consistent with meteoroid bombardment.•Block fragmentation due to insolation-driven thermal stresses is subordinate to impacts.