Morphometric analysis of differently degraded simple craters on the moon

The main focus of this PhD research is the morphologic characterization of simple impact craters on lunar maria in order to find out a correlation between craters morphological degradation and absolute model ages of the surfaces where they were emplaced. Crater degradation can be indeed used to constrain the chronological evolution of planetary surfaces. The crater degradation is usually retrieved through visual inspection by subdividing craters into 4 classes: C1 represents the freshest ones, C2 are the ones with the first evidence of degradation (smoothed rim), C3 and C4 are related to morphologies ranging from heavily eroded to totally flattened respectively [Arthur, 1963]. We firstly conducted a morphometric analysis of craters representative of the four classes starting from the freshest one represented by the Linne crater. Craters were chosen on a homogeneous geological unit, the S28 unit in mare Serenitatis, with an absolute model age of 2.84 Gy [Hiesinger et al., 2011]. This analysis allowed us to establish the thresholds of mean slope from craters inner wall, in order to constrain the morphometric characterization of the four degradation classes. Successively we have extracted all impact craters (383) from a unique geological unit and we have defined the morphologic relationships among the degradation classes in function of the craters diameters. Finally, we expanded our analysis to six lunar maria, considering six lunar maria with different average absolute model ages, in order to perform this analysis with the wider range of ages. For each mare we considered a unique surface (dataset) derived from the merging of geological units with similar absolute model ages within the basin, in order to guarantee the most homogeneous possible surfaces, both in terms of impact rheology and absolute age. From the six surfaces we have extracted inner wall mean slopes from over 1000 impact craters. The mean slope values of the inner walls have shown a relation between crater morphology and the absolute model ages of the geological units where they are located. Older basins are characterized by craters with lower mean slope values, suggesting a dominance of older craters in their population, whereas the younger units have shown higher mean slope values of their simple craters, suggesting a population dominated by recent impacts. This tendency is the expression of the morphological alteration strictly connected to the lunar maria age. Since the geomorphometry of impact craters is influenced by the absolute age of the target area, we have constrained potential isochrones by fixing absolute age thresholds based on the morphological variations of impact craters.