Boulder abundances and size-frequency distributions on Oxia Planum-Mars: Scientific implications for the 2020 ESA ExoMars rover

Abstract

This paper presents the abundances and the size-frequency distributions (SFD) of boulders identified on a sector of the prime landing site for the ExoMars 2020 rover, in Oxia Planum region. By means of a HiRISE image, boulders≥ 1.75m across have been identified and subdivided according to the two main Oxia Planum geological units: the Noachian clay-rich formation (Nc), and the Amazonian volcanic deposit (Av).The spatial density of boulders≥ 1.75m over the entire study area is 6.75 × 10−4/m2, with a size-frequency that is best fit both with power-law and exponential-law curves with indices of −4.9 + 0.1/−0.2 and −1.29 + 0.04/−0.06 respectively. Significant differences were found by analysing separately the Av and Nc geological units. The data collected in the Av unit are well-fitted with a power-law curve with an index equal to −4.8 +/−0.2 and with an exponential-law curve with an index of −1.24 + 0.05,−0.06, whilst in the Nc unit such indices are −5.5 + 0.3/−0.4 and −1.70 + 0.09/−0.12 (power-law and exponential-law curve, respectively).The spatial density of boulders in the Av unit is 7.0 times larger than in the Nc one. This may be due primarily to the distinct mechanical properties of the two units that may result in a different production rate or preservation of the boulders. Secondly, the Av unit overlies the Nc unit, possibly resulting in more impacts and/or different weathering processes throughout the ages.This study provides a quantitative evaluation of the abundances of boulders≥ 1.75m across on Oxia Planum: it is therefore a reference for the ExoMars 2020 mission, both during the landing phase and the rover traverse to specific areas of interest. The landing ellipse presents much higher abundances of boulders≥ 1.75m than all previous Martian rover landing areas. This is particularly evident when the rougher Av unit is taken into account. Contrarily, the Nc unit shows a much more comparable value, but still slightly higher, to the Mars Pathfinder landing site. We provide illustrative navigating scenarios for both the Nc and the Av units as well.