Abstract
Recent 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 rover-based 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 mini-TARs. Assuming that the Oxia Planum bedforms have the same length/height ratio as the MER Opportunity mini-TARs, 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 population or sub-type of aeolian bedforms.
Highlights
The surface of Mars hosts various types of aeolian bedforms (Fig. 1), from small wind-ripples of centimetre-scale wavelength (e.g., Sharp and Malin, 1984; Sullivan et al, 2005), through larger decametre-scale “Transverse Aeolian Ridges” (TARs; e.g., Bourke et al, 2003; Balme et al, 2008; Berman et al, 2011) to kilometre-scale dunes (e.g., Cutts and Smith, 1973; Hayward et al, 2007)
The aim of this paper is to find a way to estimate the heights of aeolian bedforms that are too small to be measured using HiRISE (High Resolution Imaging Science Experiment; McEwen et al, 2007) Digital Elevation Models (DEMs), in order to increase our knowledge of the hazards they pose to rovers
Aeolian bedform height and length measured from the Mars Exploration Rover (MER) opportunity traverse
Summary
The surface of Mars hosts various types of aeolian bedforms (Fig. 1), from small wind-ripples of centimetre-scale wavelength (e.g., Sharp and Malin, 1984; Sullivan et al, 2005), through larger decametre-scale “Transverse Aeolian Ridges” (TARs; e.g., Bourke et al, 2003; Balme et al, 2008; Berman et al, 2011) to kilometre-scale dunes (e.g., Cutts and Smith, 1973; Hayward et al, 2007). Aeolian deposits consisting of loose unconsolidated material can constitute hazards to surface mobility of rovers: sinkage into the aeolian material and enhanced slippage can hamper traction and prevent forward progress, forcing the rover to backtrack (e.g., MER Opportunity, Arvidson et al, 2011) or, in the worst case, leading to permanent entrapment and end of mission (e.g., MER Spirit, Arvidson et al, 2010). Being able to estimate the depth of loose aeolian material (or the height of aeolian bedforms) before a rover drives over them is clearly of great advantage. The aim of this paper is to find a way to estimate the heights of aeolian bedforms that are too small to be measured using HiRISE (High Resolution Imaging Science Experiment; McEwen et al, 2007) DEMs, in order to increase our knowledge of the hazards they pose to rovers
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