Lunar Cratering Chronology - Review and Revision.

The Apollo and Luna mission samples obtained on the lunar surface still provide most valuable temporal information to decipher lunar planetary surface evolution. Sequences of geological events can also be dated by crater statistics. Cratering chronology models have been derived using sample ages and crater statistics at the landing sites for calibrated absolute model ages. Since the initial models developed several decades ago, and despite increasing numbers of sample ages and improvements in crater statistics, the cratering flux is still debated to be mono-spike (terminal cataclysm or late heavy bombardment, sawtooth), multi-spike (spiky, sawteeth, picked fence) or monotonically decaying (tail end of planet formation).

Here, we present an in-depth evaluation of 1) newly mapped reference geologic units based on spectral and morphologic data along with 2) the cratering statistics of these units and 3) recalculated sample ages using recent monitor sample ages and updated K- and Rb-decay constants. Both sample ages and crater frequencies are new calibration input for the chronology models.

The recalculated sample ages tend to show increased age for already previously old ones, but less systematic changes for younger samples, which effectively slows the cratering flux decay. Compared to other landing site characterisations, the Chandrayaan M3 spectral data suggest a reduction of area comprising homogeneous units. It caused for the following crater counting evaluation a reduced crater size range for the interpretation of crater frequency. In the smaller crater-size range in old units, saturation and resurfacing events modify the crater record and result in apparently smaller crater frequencies.

Despite additional mineralogical data, linking sample ages to surface units has been the main challenge. We also provide a potential sequence of basin ejecta for each landing site, which characterises the landing sites for potential contamination by ejecta from other large basins, and we use the superposed crater record to define the amount of impact gardening. This way we get a better understanding of which sample ages are likely local and which may be ejecta from elsewhere. Jointly, these three puzzle pieces provide the base for new discussion on the evaluation of current lunar cratering chronology models.