Mounds in Oxia Planum: The Burial and Exhumation of the ExoMars Rover Landing Site

Oxia Planum, the planned landing site of the ExoMars "Rosalind Franklin" rover, is a low relief clay-bearing plain, of which approximately 1% is covered by 396 upstanding isolated landforms ("mounds"). The mounds are continuous with a circum-Chryse mound population representing the remnants of a regionally significant Noachian-aged deposit. This detailed study suggests that the Oxia Planum mounds are also erosional remnants of this deposit, with little evidence to suggest they are constructional landforms such as sedimentary volcanoes. We calculate that up to 130 m of mound-forming material has been removed from the landing site through erosion. The mound-forming layer lies unconformably on the clay-bearing plains with the upper surface severely truncated by significant erosion resulting in the topography we see today. The mounds themselves comprise at least three members, distinct in color and texture, separated stratigraphically by further unconformities. Calculated minimum erosion/deposition rates of the removed mound material are comparable to previous Noachian estimates, suggesting a more erosive (and probably therefore warmer and/or wetter) environment than today. The clay-bearing materials which remain buried directly under the mounds have been continually protected from the Martian environment since the Noachian, and are likely to represent some of the most pristine clay-rich materials in the landing site. By inference, the plains directly adjacent to the mounds are most likely to have been exposed for less time than areas further from the mounds. These are therefore amongst the most likely locations where Rosalind Franklin could sample recently exposed materials and hence detect biosignatures.