USING DEGRADATION STATES OF IMPACT CRATERS TO DERIVE LUNAR SURFACE AGES

Impact craters throughout the Solar System are commonly used to determine surface ages. For the Moon, surface age estimates rely on linking crater populations with radiometric ages. The number and size of craters mapped on a surface vary between workers and methods, and all rely on linking certain crater densities to radiometric ages from Apollo samples. A complementary surface dating technique would help support or refute this dating system. Investigating crater degradation states, and understanding how impact crater shapes degrade and evolve over time, can be used to determine the ages of surfaces. Here we calculate ages of geologic units based on the degradation states of craters superposed on individual units.

To determine unit ages, we first measured the depth-to-diameter ratios for craters 1-5 km in diameter, between 60°S-60°N latitude, using the Kaguya Digital Terrain Model (DTM) and Lunar Orbiter Laser Altimeter (LOLA) data. Next, we utilized a topographic diffusion model and methods from Fassett and Thomson (2014) and Fassett et al. (2022) to determine the crater degradation state. Then, based on degradation, we assigned ages to craters. Finally, we assessed the distribution of modeled crater ages on individual units and assigned an age to the unit based on the 90th percentile crater age.

Results indicate that unit ages based on crater degradation for Apollo, Luna, and Chang'E landing sites, within lunar maria, are complementary to ages calculated from crater spatial densities and radiometric ages. For example, the Chang'E 5 landing site has a calculated radiometric age of 2.0 Ga (Li et al., 2021), and we calculated an age of 1.88 Ga (craters measured with the DTM) and 1.73 Ga (craters measured with LOLA) for the unit in which the landing site is located. Next, we calculated unit ages for far side lunar maria that do not have radiometric ages. Results indicate that volcanism on the far side of the Moon primarily falls within the predicted range of peak lunar volcanism (3.8-3.4 Ga). However, we calculate a unit age of 2.67 Ga within the Apollo Basin, which is the landing site of the Chang'E 6 mission. Our results show that calculating unit ages based on crater degradation is a reliable method where topographic diffusion is the dominate process in removing craters. This can be applied to units without radiometric ages and for units too small to be dated using crater spatial densities.