Rivers and Lakes in Western Arabia Terra: The Fluvial Catchment of the ExoMars 2022 Rover Landing Site

Oxia Planum, the landing site for the ExoMars rover mission, is a shallow basin on the southern margin of Chryse Planitia that hosts remnants of fan-shaped sedimentary deposits associated with the ancient channel system Coogoon Vallis. This indicates runoff from a catchment in Arabia Terra has transported sediment into the landing site. To explore this fluvial system we created a model catchment for Oxia Planum and, using 6 m/pixel ConTeXt camera orbital remote sensing image data, we digitized the fluvial and lacustrine landforms in Western Arabia Terra in and around this catchment. We find: (a) The catchment has a minimum area of ∼2.1 × 105 km2 and has been episodically deformed by tectonic activity; (b) There were at least two phases of fluvial activity. The first created a mature landscape associated with Coogoon Vallis, which may have deposited alluvial or deltaic deposits in the Oxia Basin. After a substantial hiatus, a second phase of activity incised U-section channels into the pre-existing landscape and channel systems; and (c) Evidence for numerous possible paleolake deposits within the catchment. These are not well connected to the fluvial system and were probably sustained by groundwater activity contemporaneous with both phases of fluvial activity. This groundwater might have modified the mineralogy of Oxia Planum. Oxia Planum probably experienced an alluvial or distal deltaic/lacustrine depositional environment during the mid Noachian, which was later overprinted by a younger phase of fluvial activity.

Plain Language Summary

Oxia Planum is the landing site for the ExoMars2022 rover, looking for evidence of ancient life on Mars. The rover will investigate regions where the rocks represent environments most like Earth's, and where there is evidence for past liquid water. We explore ancient rivers that drained into Oxia Planum because they could have transported evidence of life into the landing site. The oldest and largest river is Coogoon Vallis. Where this river ended helps us understand the formation of rocks in Oxia Planum. If the river ended where it does today, the rocks may have formed underwater or at a coastline. However, if the river extended further northeast, the rocks may have formed as flood plains. More recently younger rivers brought water through Oxia Planum. These were shorter lived events but the associated groundwater may have altered the chemistry of bedrock in Oxia Planum. We also find that groundwater may have supported lakes in the areas where the rivers came from, but most did not overflow into rivers that lead to Oxia Planum. This means that any ancient life that could have lived in these craters is less likely to have been transported to the Oxia Planum landing site.