The geological history of Oxia Planum, landing site of the ExoMars Rosalind Franklin rover

Oxia Planum, the selected landing site for ESA’s ExoMars Rosalind Franklin Mission (RFM), is a window to deposits from the period of martian history most conducive to biosignature formation and preservation where the RFM aims to identify signs of life and characterize the geochemical environment in the subsurface as a function of depth. In preparation for this mission ESA, a program of high resolution morphostratigraphic mapping and analysis has been conducted to provide context for in-situ sample analysis and to serve as an input into strategic planning for rover operations.

We present work that combines high-resolution mapping of the ~50 km landing zone (Fawdon et., al 2024) with regional (~500 km) remote sensing observation using HiRISE (McEwen et al., 2007) and CaSSIS (Thomas et al., 2017) data to interpret stratigraphy and propose our best working explanations for the geological history recorded and paleoenvironmental scenarios thus identifying key hypotheses for the mission to test and discus how future RFM observations will impact these questions and our wider understanding of Mars.

Our investigations suggest: The oldest units, the phyllosilicate-rich Lower Bedrock Group, represent altered basement or transported material from western Arabia Terra. The phyllosilicates could be both authigenic and detrital clays in different areas and in the wettest scenarios related to a northern ocean. After infilled impact structures mark an erosional phase.

The Upper Bedrock Group is lighter-toned and unconformable, likely formed through rapid deposition (e.g., volcaniclastic?) with reworking in the upper part of the group by aggrading fluvial systems.

Above this lies a remnant regional layer (McNeil et al., 2022), preserved as mound tops, though its origin and erosional processes remain uncertain. A second erosional phase reactivated fluvial networks, forming U-shaped channels and depositing an overlying Dark Material, associated with inverted sediment fans and hydrated silica horizons east of the landing site. The final, wind-dominated erosional phase, continuing today, produced inverted crater fills, reduced mound size, and exposed the Lower Bedrock Group.

These results provide essential context about the geological evolution of ancient Mars for RFM investigations into: Formation of clay-bearing terrain during the Noachian and possible northern ocean; Volcanic origins of regional layered terrains; Evolution of martian fluvial systems on intercrater plains; a Possible role for hydrothermal groundwater activity forming silica deposits and in preserving the enigmatic mounds. This work is a framework that will guide interpretation of RFM mission result providing context for the mission’s astrobiological goals and advance understanding of early martian environments.