Rock Abundance on the Lunar Mare on Surfaces of Different Age: Implications for Regolith Evolution and Thickness

The growth of lunar regolith over time affects surface rock abundance, because larger, less frequent impacts are needed to penetrate thicker regolith developed on older surfaces and excavate rocks. On younger surfaces with thinner regolith, smaller, more frequent impacts are sufficient to excavate rocks. We quantify the correlation between observed rock abundances and age on the lunar surface by comparing Diviner rock abundance data to the surface ages of intercrater parts on the maria. Our observations show the expected negative correlation between age and rock abundance. The commonality of nonzero rock abundance values on ancient surfaces, combined with a simple Monte Carlo model of the rock excavation process, suggest that rocks reexcavated from the regolith volume contribute to the presently observed rock population on the lunar surface. The half-life of meter-scale surface rocks most consistent with our observations is 80 +/- 20 Myr. Plain Language Summary The lunar surface is covered by regolith, which includes particles ranging in size from dust to boulders. Past work has suggested that regolith is meters to tens-of-meters thick, with average thickness increasing with terrain age. At least in the maria, beneath the regolith is fragmental bedrock. When impacts large enough to penetrate the regolith occur, boulders from this underlying bedrock are excavated onto the surface. Excavating boulders is expected to be more difficult where the regolith is thicker, because thicker regolith limits the ability of craters to eject material from the underlying fragmental bedrock. This implies larger and rarer impacts are needed to excavate rocks on older terrain. We tested this idea by comparing the rock abundance data from the Diviner Lunar Radiometer Experiment on the Lunar Reconnaissance Orbiter spacecraft with previously obtained crater statistics for part of the maria, excluding the area within and immediately adjacent to D >= 1 km craters. We show that older maria have, on average, lower fractional rock abundances, consistent with a thicker regolith on older surfaces influencing rock excavation. Rocks excavated from within the regolith are necessary to be consistent with the continued presence of nonzero rock abundance on old terrains.

Automated summary

This study analyzed rock abundance data on the lunar surface and found that the growth of lunar regolith affects the abundance of surface rocks, indicating that older surfaces have lower rock abundance, and younger surfaces have higher rock abundance. In addition, rocks reexcavated from within the regolith contribute significantly to the observed rock population on the lunar surface.