A morphological evaluation of crater degradation on Mercury: Revisiting crater classification with MESSENGER data

Observations of impact crater morphology can be used to gain insight into the geological history and evolution of a planet's surface. Image data from the Mariner 10 mission revealed the diversity of impact crater morphologies and degradational states on Mercury, leading to early studies that sought to establish a stratigraphic column for the planet, despite only acquiring image data for ~45% of the surface. In 2011, the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft entered orbit around Mercury, returning a high-resolution global image dataset that enables a robust analysis of crater morphology and degradation to be completed for the entirety of Mercury's surface. In this study, we conducted a visual classification of crater degradation according to initial crater morphology, and assigned a degradation state to all craters on Mercury ≥40 km in diameter. In our scheme, Class 1 craters are those that are heavily degraded, and Class 5 craters are very fresh with bright ray systems. We discuss the processes involved in crater degradation and erasure, and the challenges associated with applying crater degradation to derive the timing of geological events. We found that, based on the global spatial density of craters in each class, there appears to be a dearth of Class 1 craters within the intercrater plains, likely due to several ancient basin-sized impacts effectively obliterating a considerable portion of craters ≥40 km in diameter in this region. The crater degradation database we present here will serve as a useful tool for future analyses of Mercury's geological evolution.