New crater calibrations for the lunar crater-age chronology

- New mapping and crater counts of Apollo and Luna chronology calibration sites.
- Most sites have a higher density of ≥1-km-diameter craters than previous work.
- Some ages dated via craters are up to 1.1 Gyr younger than previously thought.

Measuring the spatial density of craters emplaced on geologic units is the primary method used for remotely estimating ages of solar system surfaces. The calibration for this method, which enables conversion of crater density to absolute age, comes from Apollo and Luna lunar samples for which absolute radiometric ages have been determined. Researchers throughout the 1970s worked to establish the lunar chronology system based on that calibration, correlating crater densities with absolute ages. However, no uniform crater study has been conducted on all calibration terrains, a limitation that was previously unaddressed until this study. The latest lunar images from the Lunar Reconnaissance Orbiter were used here to re-map the eleven main sampling sites and new crater counts of those surfaces were conducted. These show significant differences for many sites' crater counts, in many cases having more craters than previously identified. These results, calibrated to the radiometric ages, show a revised lunar crater chronology that changes previously established crater-based ages by up about 1 billion years: Surfaces younger than ∼3.6Gyr and older than ∼3.9Gyr under the classic chronology are younger in this system, and those in-between are older in this new system.