Review and Revision of the Lunar Cratering Chronology—Lunar Timescale Part 2

Abstract

Times and rates of planetary surface-modifying processes are crucial for the sequence and correlation of events on planetary bodies. For most planetary surfaces, superposition principles and crater densities are commonly used methods to collect relative age information. Lunar-based cratering-chronology models, which pair crater densities and sample ages from several lunar landing and sampling sites, calibrate the relative age information in absolute time. Here, we propose calibration pairs based on new crater statistics and spectrally supported sample-age assignments for the lunar cratering-chronology model. The resulting model reflects modern high-precision, radiometric ages, compositional and spectral information, and an up-to-date crater-production function. This revision supports a crater-forming projectile flux with monotonic decay, similar to previous standard models, but of distinctively lower flux. This originates from lower crater densities identified in spectrally and morphologically defined reference units, and from assigning more precise sample ages accounting for spectral resemblance between reference unit and sample. The observed maximal values for crater densities and ages provide the oldest and most densely cratered calibration pair. Because of the nature of highland samples, age constraints for the Luna 20 and Apollo 16 sampling sites remain challenging, which restricts the confidence in times for individual basin-formation events older than Orientale Basin. The new cratering-chronology model, when transferred to other planetary bodies, would cause aging of the surfaces, because of the lower overall flux.