The Effects of Terrain Properties Upon the Small Crater Population Distribution at Giordano Bruno: Implications for Lunar Chronology

The distribution of impact craters on the ejecta of Giordano Bruno, a recent (<10 Ma) 22‐km diameter crater within the lunar highlands, exhibits substantial variations. We surveyed craters D≥ 10 m across a 1,323 km2area of Giordano Bruno's ejecta and compared the distribution of craters with variations in thermophysical properties derived from the Lunar Reconnaissance Orbiter Diviner instrument. We used Diviner‐derived rock abundance and nighttime regolith temperatures along with thermal model‐predicted surface temperatures for a diversity of terrains to identify and isolate areas of the ejecta based on thermophysical properties such as bulk density and thermal conductivity. We found that thermophysical properties of the ejecta vary considerably both laterally and vertically, and consistently differ from typical regolith, indicating the presence of higher thermal inertia materials. Crater‐size frequencies are significantly lower in areas with terrain properties exhibiting higher: rock abundance, nighttime temperatures, and/or modeled thermal inertia. This discrepancy in crater distribution increases for craters smaller than ∼25 m. These thermophysical variations indicate changes in the mechanical properties of the target materials. We suggest that these variations—specifically, terrain‐dependent crater scaling variations and impactor‐scale heterogeneities in material properties such as the presence or absence of large boulders—may influence crater diameters or inhibit crater production altogether in Giordano Bruno's ejecta; furthermore, these factors are size‐dependent. Our understanding of the lunar chronology relies heavily on comparing crater counts on the ejecta of impact craters with laboratory dating methods of samples returned by the Apollo and Luna missions. However, discrepancies in crater populations on crater ejecta blankets have been observed and may present a source of uncertainty in the modeling of the lunar chronology. In this study, we show that variations in the distribution of craters on the ejecta of crater Giordano Bruno correspond with variations in terrain properties. Nighttime temperatures are sensitive to variations in density and thermal conductivity. We identified the variations in these properties using Diviner nighttime temperature observations and show that these properties correlate with variations in crater frequencies. Our results strongly support the idea that target properties are likely influencing the production of craters and therefore must be accounted for when estimating crater count‐derived absolute ages. The distribution of craters on Giordano Bruno varies with thermophysical propertiesThermal modeling of the ejecta shows lateral and vertical variations in thermal propertiesVariations in target properties influence the production of craters on the ejecta, resulting in variable absolute model age estimates The distribution of craters on Giordano Bruno varies with thermophysical properties Thermal modeling of the ejecta shows lateral and vertical variations in thermal properties Variations in target properties influence the production of craters on the ejecta, resulting in variable absolute model age estimates