The deformation of lobate debris aprons revealed by crater morphologies in Tempe Terra, Mars

Secondary impact craters (“secondaries”) are produced during the excavation stage of the cratering process, from material ejected from the primary crater. Assuming that secondaries can be associated with their primary crater, and that the age of the primary crater is known, secondary crater populations could be used as absolute stratigraphic markers. Using secondary craters to indirectly date distant features is not a new method – it was used during the Apollo missions, to determine the ages of both the Copernicus and Tycho impact events. In this work, we exploited the martian secondary crater population as absolute stratigraphic markers, to make new insights into the evolution of lobate debris aprons (LDAs).

LDAs are landforms found in the martian mid-latitudes and associated with the presence of past and present near-subsurface ice. It is suggested that these morphologies are the results of the flow of a mixture of ice and debris, which derived from the sensitivity of near-surface ice to fluctuations in climate conditions. LDAs are inferred to have formed in the Late Amazonian. However, age constraints of LDA formation are characterized by large uncertainties due to their complex history of modification by viscous deformation, and degradation by erosion, and ice sublimation. Currently, the LDA rate of deformation is considered extremely slow, if not zero, as there is no evidence for crater deformation.

In this work, we exploit the crater population at two LDAs in Tempe Terra and adjacent plain terrains, in the northern hemisphere of Mars. This region is affected by secondary impact craters derived from the primary Maricourt crater, which itself likely formed ~11 Ma. Therefore, LDAs and adjacent terrains in Tempe Terra constitute an ideal site where to extract a range of morphometric parameters through which we aim to assess the downslope deformation of the craters, distinguishing between primary and secondary craters, and discuss the results in terms of their meaning regarding sublimation-related changes and LDAs flow.

We show that 1) for most of the craters, the orientation of crater elongation is concordant with the LDA slope direction; 2) crater elongation is independent of the slope; however, 3) primary and secondary craters have distinctive depth-to-diameter ratios.