Degradation of small mare surface features

Ring-moat structures are interpreted as remnants of flow topography associated with mare emplacement. Measurements of narrow moats characterizing these small mare features reveal a contradiction between their inferred preservation and the degree of surface degradation predicted by theoretical erosional models. Moat widths smaller than 50 m are recorded in areas where erosional models predict that walls of craters three to four times as large should be degraded to slopes of 1 deg. Crater statistics in such regions provide estimates of equilibrium conditions for impact crater formation that are consistent with the degree of preservation of the narrow moats. Although relative ages determined by observation of crater wall slopes near 20 deg, may be appropriate, it is concluded that current erosional models are not valid down to impact-degraded slopes of 1 deg. Relative age estimates based on the inferred degraded state of highly subdued craters are believed to be invalid, particularly for late Eratosthenian-age and Copernican-age mare units. In regions displaying ring-moat structures, highly subdued craters larger than the average moat widths are intepreted as either endogenic features associated with the emplacement of mare flow units or degraded secondary craters, which are typically subdued at the outset.