DISCUSSIONS

The statistical distribution of the inner wall mean slope highlighted a plausible power law trend in function of the diameter, at least for C1, C2 and C3 classes, whereas C4 class is affected by the maximum degree of degradation exhibiting a low topographic relevance. Craters with larger diameter (>4 km) in C1 and C2 show clear differences in terms of ejecta albedo and rim sharpness on the visual interpretation in the orthoimages, although the topographic analyses measure bland morphologic variations of the inner wall as enlighten in the plot of Fig.6. Hence in the first step of crater degradation the rim crest erosion probably prevails, whereas in the following steps the continuous infilling of the inner wall becomes topographically more important. However the gap among C1 and C2 improve when the crater diameter decrease under 3km, making topographic variations more evident. This trend may be related to the velocity of erosional processes on lunar impact craters, becoming quicker from large craters to small ones. Small impacts involve mainly the regolite and the fractured uppermost layers of lunar crust generating impact craters with blender morphologies. Hence the inner scarp of small impact craters interests materials with a lower strength than the basalts, being more affected to gravitative phenomena and erosional processes.

The inner wall mean slope of C1larger craters approximates the regolith angle of repose, about 31° [Nickerson et Al., 2011]. This is coherent with the relatively freshness crater morphology, shaped only by early post impact modifications. However <200m sized craters of C1, show very bright ejecta and radial rays typical of fresh craters, present a mean slope

value within the inner wall ranging from 18° to 20°, more typical of C3 mean values (14°- 18°) than C1 (28°-32°). This is probably related to the mechanics of small impacts that involve exclusively regolith layers, which have a lower strength if compared to basaltic layers and the resulting impact morphology is blander. Moreover in the post impact phase the regolith is probably much more prone to landslides and collapses of the inner scarp, as well as more susceptible to weathering processes. The distribution of impact craters revealed a diameter upper cut-off out 1 km for the most degraded crater class (C4). This cut-off can be explained probably with the relative young age of the considered units in mare Serenitatis [Hiesinger et al., 2011], that does not allow larger craters to be degraded up to C4 class. Indeed the diameter upper cut-off is present to all the degradation classes and increase to larger craters from C4 to C1. This cut-off will potentially allow a refinement of relative age discrimination among different geological units, with similar absolute model ages.

The mean concavity diagram (Fig.7) shows how the concavity decreases with growing diameter and increasing degradation status of impact craters. In general degradation processes reduce inner wall concavity, decreasing the slope gradient with the simultaneous crater infilling. Sediments from dry-flows, landslides and potential material ejected from

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surrounding impact events, tend to fill up the crater, supporting the increase and finally the prevalence of positive profile curvature values for the more degraded craters (C4). Craters also exhibit decreasing concavity with increasing diameters. These results can be explained under the light of the mechanic of impact that lead to more concave morphologies for small craters (bowl shaped floors and concave inner walls), to steep and sharp inner walls and flattened floors for larger craters, in particular at diameter closest to the transition with complex craters [Wilhelms, 1987; Melosh, 1989]. In addition larger craters are also interested by major and deeper gravitational events which might deform the carter floor at their foot leading to more irregular morphologies.

CONCLUSIONS

In this research we have applied morphometric analysis on high resolution topographic data in order to quantify the degradation of simple impact craters. We firstly derived and measured the morphological expression of the four different crater domains: the outer and inner wall, the rim and the floor. For this purpose we have chosen a particular fresh impact crater (Linné) as a reference for the quantification of the four homogeneous morphological domains. We applied the same analysis to other three simple impact craters, representative respectively of C2, C3 and C4 classes, in order to set up the characteristic thresholds of the four degradation classes. Afterwards we applied morphometric analysis on several simple craters on mare Serenitatis. Our results have provided a quantitative distinction among different crater classes and have shown common trends of degradation, going from larger to smaller craters within each class.

The statistical distribution of the inner wall mean slopes highlights a similar power law trend of the first three degradation classes, which seems independent from the degradation class.

We have also found a diameter upper cut-off at about 1 km of the most degraded crater class (C4), which can be explained with the relative young age of the considered unit in mare Serenitatis [Hiesinger et al., 2011], for which there were not enough time useful to degrade large craters up to C4 class. Indeed the diameter upper cut-off is present on all the degradation classes and increases from C4 to C1. This cut-off will potentially allow a refinement of relative age discrimination among different geological units, with similar and different absolute model ages.

The shape evolution of simple impact craters inner wall was suggested by the negative

profile curvature distribution, that shown a trend in function of diameter that enlightens a decreasing of mean concavity with both increasing diameters and raising degradation status: larger and more degraded craters have lower concavities and might eventually reach positive profile curvatures (convex shape).

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The development of morphometric variables classification described in this research, may be also useful for semi-automatic detection and characterization of the degradation classes of simple craters, potentially helpful for geological mapping and age determination of planetary surfaces.