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.

Mercury’s surface is dominated by tectonic landforms formed by compression. Other than within basins, extensional landforms are not well known and have been presumed to be much rarer, with only a handful reported [1]. To date, two types of extensional grabens associated with lobate scarps have been described in literature: pristine back-scarp grabens associated with small lobate scarps (10s of kms in length and 10s of metres in relief) [2] and crestal grabens found on Calypso Rupes (381km in length and ~1km in relief) [3], [4]. This study identifies that such extensional grabens found on lobate scarps are much more widespread than previously recognised. These form when thrusting produces a hanging wall anticline, and local tensional stresses along the anticlinal axis cause antithetic faults to form in the folded strata, parallel or sub-parallel along the hinge zone, producing a down-dropped fault block. These small-scale features (often less than 1km in width, 10s of kms in length and likely 10s to 100s of metres in depth) are not expected survive 100s of millions of years because of regolith formation and impact gardening masking their signature [1], [2]. Our discovery and documentation of more extensional grabens may indicate that significant movement on many of Mercury’s large lobate scarps persisted until geologically recent times. [1] P. K. Byrne, C. Klimczak, and A. M. C. Sengör, “The Tectonic Character of Mercury,” in Mercury : The View After MESSENGER, 1st Editio., S. C. Solomon, L. R. Nittler, and B. J. Anderson, Eds. Cambridge: Cambridge University Press, 2018, pp. 249–286. [2] T. R. Watters, K. Daud, M. E. Banks, M. M. Selvans, C. R. Chapman, and C. M. Ernst, “Recent tectonic activity on Mercury revealed by small thrust fault scarps,” Nat. Geosci., vol. 9, no. 10, pp. 743–747, 2016. [3] C. Klimczak, P. K. Byrne, A. M. C. Şengör, and S. C. Solomon, “Principles of structural geology on rocky planets,” Can. J. Earth Sci., vol. 56, no. 12, pp. 1437–1457, Dec. 2019. [4] M. E. Banks et al., “Duration of activity on lobate-scarp thrust faults on Mercury,” J. Geophys. Res. E Planets, vol. 120, no. 11, pp. 1751–1762, 2015.

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.

Hundreds of ancient palaeolake basins have been identified and catalogued on Mars, indicating the distribution and availability of liquid water as well as sites of astrobiological potential. Palaeolakes are widely distributed across the Noachian aged terrains of the southern highlands, but Arabia Terra hosts few documented palaeolakes and even fewer examples of open-basin palaeolakes. Here we present a detailed topographic and geomorphological study of a previously unknown set of seven open-basin palaeolakes adjacent to the planetary dichotomy in western Arabia Terra. High resolution topographic data were used to aid identification and characterisation of palaeolakes within subtle and irregular basins, revealing two palaeolake systems terminating at the dichotomy including a ~160 km chain of six palaeolakes connected by short valley segments. Analysis and correlation of multiple, temporally distinct palaeolake fill levels within each palaeolake basin indicate a complex and prolonged hydrological history during the Noachian. Drainage catchments and collapse features place this system in the context of regional hydrology and the history of the planetary dichotomy, showing evidence for the both groundwater sources and surface accumulation. Furthermore, the arrangement of large palaeolakes fed by far smaller palaeolakes, indicates a consistent flow of water through the system, buffered by reservoirs, rather than a catastrophic overflow of lakes cascading down through the system.