CRATER DENSITIES ON LOWER MOUNT SHARP AS A PROXY FOR MODERN GEOLOGIC EROSION

Estimating crater retention age from crater counts is an established technique for dating crater surfaces. However, such methods work best on areas greater than 10000 km² and with crater diameters greater than 250-500 m due to undersampling of large crater on small geologic units and erosion/burial of small craters over billion year timescales. Geologic units at tens of meters scale (i.e. HiRISE) are difficult if not impossible to date accurately at this time. We want to examine crater retention as a proxy to understand the current erosion rate of lower Mount Sharp morphologic units. We assume higher crater retention reflects more indurated sediments that in-turn are more difficult to erode, reflecting unique geologic units.

Our methodological assumptions are that craters with a diameter <=250 m this small are retained on small geologic units (<10000 km²) and the ability to erode craters this size takes less than one billion years. We calculate crater density by converting craters to points and measure that point density over a 200 m radius circle. Errors due to larger craters erasing smaller craters are ignored. Exhumed craters, of which we know there are a few, are also ignored, so our densities values may be overestimated.

Crater densities are several factors lower on ‘lower’ Mt. Sharp compared to Bradbury Rise. The Murray Formation from the Bradbury Rise edge up to Vera Rubin Ridge (VRR, i.e. ‘Hematite Ridge’) is almost devoid of craters. VRR has low to moderate crater density. The clay unit south of VRR is nearly craterless like the Murray. The ‘fan’ unit has similar crater density compared to the (VRR. The highest densities occur on some mounds/mesas farther up section as well as on a similar ‘fan’ unit west of VRR.

Decreases in crater density could be from increased eolian erosion as ‘lower’ Mt. Sharp is at minimum 100 m higher than Bradbury Rise, though the decreased density is not gradual and some of the highest values occur at higher elevations. Crater densities also mirror previously defined geologic units, thereby pointing to induration as the dominant factor in retention. From the crater density values, we conclude that the Murray and clay units are eroding faster than VRR and the upper sulfate units due to increased induration, which may reflect how they were deposited or post-depositional fluid flow emplacing cements.