The geologic origin of the ancient, phyllosilicate-bearing bedrock at Oxia Planum, Mars, the ExoMars rover landing site, is unknown. The phyllosilicates record ancient aqueous processes, but the processes that formed the host bedrock remain elusive. Here, we use high-resolution orbital and topographic datasets to investigate and characterize fluvial sinuous ridges (FSRs), found across the Oxia Planum region. The FSRs form segments up to 70 km long, with sub-horizontal layering common in ridge margins. Some FSRs comprise multi-story ridge systems; many are embedded within the phyllosilicate-bearing bedrock. We interpret the FSRs at Oxia Planum as deposits of ancient, episodically active, alluvial river systems (channel-belt and overbank deposits). Thus, the phyllosilicate-bearing bedrock was formed at least partly by ancient alluvial rivers, active across the wider region. Future exploration by the ExoMars rover can verify this interpretation and provides an opportunity to investigate some of the oldest river deposits in the Solar System.

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 sediment fans associated with the ancient channel system Coogoon Vallis. This indicates runoff from a catchment in Arabia Terra has transported material into the landing site. To explore this fluvial system we created a model catchment for Oxia Planum and, using 6 m/pixel ConTeXt camera (CTX) orbital remote sensing image data, we digitised the fluvial and lacustrine landforms in Western Arabia Terra in and around this catchment. We find: (1) The catchment has a minimum area of ~2.1×105 km2 and has been episodically deformed by tectonic activity; (2) 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 (3) Evidence for numerous possible paleolake deposits within the catchment. These are not well connected to the fluvial system and were probably sustained by ground water activity contemporaneous with both phases of fluvial activity. This groundwater might have modified the geochemistry 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.

There is conflicting evidence for an ancient ocean which occupied the northern hemispheric basin on Mars. Along different regions of the dichotomy boundary, sediment fans have been interpreted as either forming into a large water body or a series of smaller paleolake basins. Here, we investigate fluvial systems in the Memnonia Sucli region of Mars, set along the dichotomy, which comprise erosional valley networks, paleolake basins, inverted channel systems, and sediment fans. We focus our analysis on the evolution of the upslope catchment and characterizing the ancient environment of a large, downslope basin, bound by the topographic dichotomy and the Medusae Fossae Formation. The catchment fluvial systems comprise highly degraded valley networks and show a complex history of incision and filling, influenced by paleolake basin overflow, impact crater damming, aggradation, and possibly a downstream water body. The morphology of the sediment fans is consistent with either fluvial fans or deltas and they form at discrete elevations, rather than a common elevation plane. Our analysis is consistent with the sediment fans forming into a series of paleolake basins set along the dichotomy, rather than into a large inner sea or ocean-sized water body. The fluvial systems were likely active between the mid Noachian and early Hesperian periods. Our results demonstrate the complex, multi-phase evolution of fluvial systems on ancient Mars and highlight the importance of regional and local studies when characterising ancient regions of the dichotomy.