Using Secondary Craters to Assess Strength Differences Between Neighboring Surface Units

An issue of Martian remote studies is the near inability to infer surface material properties. Through observations of surface features that are dependent on those properties however, we can work to constrain them. One underused feature type are secondary craters, which are often thought of as nuisances when finding accurate relative ages. However, an upside of secondary craters is that by forming from ejecta blocks moving at relatively slow speeds, they reside in a different crater scaling regime [1], one that is more sensitive to the crusts effective strength Y and its porosity. There are multiple parameters that influence the size of secondary craters, but we can measure craters in ways that isolate the dependence of crater size to Y. We focused on secondary clusters that have impacted into distinct but bordering geological units. These secondaries can be used to determine Y thanks to one important fact: all the secondaries are from the same distant primary, and as a result, the local ejecta velocity, block sizes, and densities are expected to be the same (at least to first order). Thus, variations between crater size on the two units are due to material property differences (i.e. Y). Thanks to steep the size frequency distributions of secondaries [2], secondary sizes are also tightly clustered, and we can use differences in median secondary sizes between two geologic units to constrain the ratio of Y. We have started using this technique on the secondary crater fields of Tooting Crater, Mars (TC; [3]). In the northeast section of TCs secondary field (found at -150.35 lon, 25.25 lat), there geologic unit contact between Amazonis Planitia (AP) and the Olympus Mons Aureole (OM). Our initial results show that at ~165 km away from TC the median secondary crater sizes are 96 m for AP and 59 m for OM, respectively. Following the scaling equations of [1] and assuming the units have similar porosity, this gives OM a value of Y that is about 8 to 20 times greater than AP. In the future, we plan on testing if differences in porosity could explain the signal, creating a network of Y ratios for Mars crust with more comparison sites, and expanding to the Moon and Mercury. [1] Holsapple and Schmidt (1987) JGR: Solid Earth, 92(B7), 63506376. [2] McEwen and Bierhaus (2006) Annl. Rev. EPS. 34:535-567. [3] Mouginis-Mark and Boyce (2012) Chemrie der Erde 72: 1-23.