Small-scale topographic irregularities on Phobos: Image and numerical analyses for MMX mission

Abstract

The mothership of the Martian Moons eXploration (MMX) will perform the first landing and sampling on the surface of Phobos. For the safe landing, the 2.1 m-wide mothership of the MMX should find a smooth surface with at most 40 cm topographic irregularity, however, whose abundance or even existence is not guaranteed based on current knowledge. We studied the highest resolution images of Phobos for possible topographic irregularities in terms of boulder (positive relief feature) and crater distributions. We find that the spatial number densities of positive relief features and craters can vary significantly; one region has 249/km 2 confirmed positive relief features (and 2,804/km 2 including candidates) and 342/km 2 confirmed craters (1,510/km 2 including candidates) as major negative features. These numbers contrast to another region, where only 46/km 2 positive relief features (260/km 2 including candidates) and 268/km 2 craters (526/km 2 includi ng candidates) exist, indicating that the surface irregularities vary significantly over the entire surface. We extrapolate the size-frequency distributions of positive relief features to evaluate the surface roughness below the image resolution limit. We find that the probabilities that topographic irregularities are <40 cm for the areas of 4 × 4 m and 20 × 20 m are > 65 % and <1 % for boulder-rich areas and >94 % and >41 % for boulder-poor areas, respectively, even for the worst-case estimates. The estimated probabilities largely increase when we reduce the assumed number of positive relief features, which are more realistic cases. These indicate high probabilities of finding a smooth enough place to land on Phobos' surface safely.