PROLONGED ROCK EXHUMATION AT THE RIMS OF KILOMETER-SCALE LUNAR CRATERS

Impact crater formation on the Moon results in the deposition of an ejecta blanket that contains a combination of impact melt, fine-grained regolith, and boulders. Over time, the boulders contained in lunar ejecta deposits break down due to subsequent impact bombardment and thermal cycling. Prior work has shown that the largest ejecta boulders on the lunar surface break down in ~300 Myr. However, other remote sensing-based studies indicate that boulders may take as long as ~1.0 Ga to break down completely. Those prior boulder breakdown studies differ in both the data and methods used to acquire boulder size-frequency measurements. In the work presented here, we use a combination of radar and thermal infrared data products to assess boulder breakdown rates at the rims and ejecta of 6,240 impact craters on the lunar maria between 0.8 and 2.0 km in diameter. The radar data used in this work include circular polarization ratio (CPR), same-sense (SC), and opposite-sense (OC) circular polarization products from the Lunar Reconnaissance Orbiter (LRO) Mini-RF instrument. The thermal-IR-derived Rock Abundance (RA) dataset used in this work is a product of LRO Diviner Thermal Radiometer measurements. Whereas the radar data used here are sensitive to surface and subsurface boulders on the scale of the S-band wavelength (12.6 cm), the RA dataset is sensitive to meter-scale surface rocks only. The impact craters that were measured using these data possess model age dates derived via diffusion modeling in prior work. A correlation between our rock population measurements and individual crater ages demonstrates that both the radar and rock abundance data associated with crater rims and ejecta decrease with time, but crater rim data values remain elevated above the ejecta at the oldest craters in our database, an inference confirmed by visual observations of boulders with LROC NAC images. These results indicate that the boulders represented in our data are present at the rims of the oldest craters in our database. We therefore infer that ejecta boulders are continually exhumed at impact crater rims for ~2–3 Gyr due to downslope creep of the overlying regolith. As a result of this continual exhumation, a population of exhumed boulders exists at impact crater rims for a prolonged period of time, making crater rims an ideal location for future lunar sampling initiatives [Nypaver et al., 2021 in press].