Characterizing Rock Abundance At ExoMars Landing Site Candidates

Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyright owners. For more information on Open Research Online's data policy on reuse of materials please consult the policies page. Introduction: We present preliminary work to characterize surface rock abundance at ExoMars Rover landing site candidates. A challenge in quantifying the abundance of surface rocks is using the population of large (≳1 m) rocks that are resolved in orbital images to infer the size of the smaller, unresolved rock population. This is particularly relevant for the ExoMars Rov-er mission, where the Landing Module's clearance of 35 cm [1] makes it necessary to know the probability of encountering rocks where 0.35 < D < 1 m. 'Float rocks' are individual fragments of rock not associated with a continuous outcrop or body of rock —e.g. transported rocks or impact debris. These can be identified in Mars Reconnaissence Orbiter HiRISE [2] images, where the mid-afternoon local solar time, dictated by MROs' orbit, causes float rocks to appear as bright sunlit features adjacent to strong shadows. However , the smallest features resolvable in HiRISE images occupy around 3-4 pixels, corresponding to ~1-m sized rocks. This inherently limits the ability to directly identify from orbit the small, but potentially hazardous rock population. 'Outcrop' is defined as continuous expanses of bedrock or surficial deposits exposed at the surface [3]. Both float rocks and out-crop can contribute to slopes that may constitute a hazard for landed missions. We present rock counts at ExoMars Rover landing site candidates and assess approaches to constrain the morphological characteristics of Mars' surface that are relevant to rover and lander safety. Rock abundance model: Rock abundance (RA) is often quantified as the areal fraction, F, of a surface covered by rocks with diameter ≥D. The canonical rock distribution model [4] was established by fitting an exponential curve to the cumulative rock size frequency distribution (SFD) at the Viking landing sites. The small rock population, where D <1 m, was found to fit well to the relationship F k (D) = ke-q(k)D , where q(k) = 1.79 + 0.152/k, and k is the 'rock abundance factor'. By fitting a rock SFD to the model, a single value of k can be retrieved for a rock population. The model was derived using rocks observed in image mosaics of the near-and far-field in the immediate vicinity of …