Surface-based 3D measurements of small aeolian bedforms on Mars and implications for estimating ExoMars rover traversability hazards

Abstract

A B S T R A C TRecent aeolian bedforms comprising loose sand are common on the martian surface and provide a mobility hazard to Mars rovers. The ExoMars rover will launch in 2020 to one of two candidate sites: Mawrth Vallis or Oxia Planum. Both sites contain numerous aeolian bedforms with simple ripple-like morphologies. The larger examples are 'Transverse Aeolian Ridges' (TARs), which stereo imaging analyses have shown to be a few metres high and up to a few tens of metres across. Where they occur, TARs therefore present a serious, but recognized and avoidable, rover mobility hazard. There also exists a population of smaller bedforms of similar morphology, but it is unknown whether these bedforms will be traversable by the ExoMars rover. We informally refer to these bedforms as "mini-TARs", as they are about an order of magnitude smaller than most TARs observed to date. They are more abundant than TARs in the Oxia Planum site, and can be pervasive in areas. The aim of this paper is to estimate the heights of these features, which are too small to measured using High Resolution Imaging Science Experiment (HiRISE) Digital Elevation Models (DEMs), from orbital data alone. Thereby, we aim to increase our knowledge of the hazards in the proposed ExoMars landing sites. We propose a methodology to infer the height of these mini-TARs based on comparisons with similar features observed by previous Mars rovers. We use roverbased stereo imaging from the NASA Mars Exploration Rover (MER) Opportunity and PRo3D software, a 3D visualisation and analysis tool, to measure the size and height of mini-TARs in the Meridiani Planum region of Mars. These are good analogues for the smaller bedforms at the ExoMars rover candidate landing sites. We show that bedform height scales linearly with length (as measured across the bedform, perpendicular to the crest ridge) with a ratio of about 1:15. We also measured the lengths of many of the smaller aeolian bedforms in the ExoMars rover Oxia Planum candidate landing site, and find that they are similar to those of the Meridiani Planum miniTARs. Assuming that the Oxia Planum bedforms have the same length/height ratio as the MER Opportunity miniTARs, we combine these data to provide a probabilistic method of inferring the heights of bedforms at the Oxia Planum site. These data can then be used to explore the likely traversability of this site. For example, our method suggests that most of the bedforms studied in Oxia Planum have ridge crests higher than 15 cm, but lower than 25 cm. Hence, if the tallest bedforms the ExoMars rover will be able to safely cross are only 15 cm high, then the Oxia Planum sites studied here contain mostly impassable bedforms. However, if the rover can safely traverse 25 cm high bedforms, then most bedforms here will be smaller than this threshold. As an additional outcome, our results show that the mini-TARs have length/height ratios similar to TARs in general. Hence, these bedforms could probably be classified simply as "small TARs", rather than forming a discrete...

Distribution and Morphologies of Transverse Aeolian Ridges in ExoMars 2020 Rover Landing Site

“…While performing the morphological observations, we were able to identify most of the first-order crest-ridge plan view morphologies of TARs as explained by Balme et al [10], including the mini-TARs [31] in the study area ( Figure 6). We have not focused on several other reported morphologies such as "raked pattern TARs" [46] and "dust drifts" [16].…”

“…In addition to extensive stretches of TARs, Oxia Planum is also rich in so-called "mini-TARs", named so as these are about an order of magnitude smaller than most TARs; too small for their topography to be discerned using HiRISE digital elevation models (DEMs) [31]. However, these mini-TARs have length/height ratios similar to TARs suggesting them to be a possible variant of TARs [31]. In the present study, we performed the mapping at 1:2000 scale and did not consider these mini-TARs as they were not mappable at that scale.…”

“…The mini-TARs are abundant in two regions, the eastern margin as well as the south-west region, with usually NE-SW orientations. Figure 6f can be used for comparison purposes between TARs and mini-TARs, which are about an order of magnitude smaller [31]. Balme et al [31] have explained these mini-TARs to be potentially more hazardous for a rover as they are located in dense clusters.…”

“…For ensuring the best scientific returns from the planned ExoMars mission, it is necessary to scrutinize the terrain and landforms in Oxia Planum at high resolutions to better guide future traverses and mobility of the rover from the point of landing. As explained above, aeolian landforms, particularly TARs, are not only interesting science targets [28], but they also pose a threat to landing and rover mobility [29] in terms of sand/dust depositions, direct physical damage to the lander or rover chassis and wheels, sinkage into the loose unconsolidated aeolian material [30,31], enhanced slippage [30,32], and permanent entrapment [33]. Sullivan et al [28,29] provide detailed experimental background of Mars Exploration Rover (MER) Opportunity and MER Spirit where these rovers encountered TARs repeatedly in Meridiani Planum and Gusev Crater, respectively, because TARs were unavoidable during several traverse segments; but the scientific teams wisely used this as an opportunity to closely investigate TARs and other aeolian features.…”

The 2016 UK Space Agency Mars Utah Rover Field Investigation (MURFI)

“…The resultant 3D Ordered Point Clouds (OPCs; Traxler et al, 2018) were visualized in PRo3D; a software tool developed specifically for quantitative geological analysis of OPCs created from stereo rover-derived images (Barnes et al, 2018). PRo3D enabled immersive, real-time visualization of the 3D rendered image data for scientific purposes (e.g., Balme et al, 2017;Barnes et al, 2018), allowing for free roaming of a virtual representation of the rover's environment. Measurement tools built-in to the software allowed for the true scale and distances of objects to be measured, up to a distance of about 20 m from the Rover, beyond which the errors become higher.…”