Regional Geologic Mapping of the Oxia Planum Landing Site for the Exomars 2028 Rosalind Fanklin Mission

Introduction:  The ExoMars mission will deploy the Rosalind Franklin rover in Oxia Planum (OP), a region in western Arabia Terra at the transition between the heavily cratered highlands of Mars and the ancient and filled impact basin, Chryse Planitia [1]. This site was selected [2] because (i) landing is technically feasible and (ii) exposed phyllosilicate-bearing rocks offer access to the early history of the planet, when Mars is thought to have been most habitable. The primary science goal of ExoMars is to analyze the geology and geochemistry of the local environment with the aim of identifying potential biosignatures [1].

To characterize the geology of the landing ellipses, local HiRISE-scale mapping has been performed as a joint effort. A total number of 116 individually mapped 1×1 km boxes, covering an area of 8750 km² [3], has been assembled into a consistent map at a scale of 1:30,000 [4]. After the shift of the launch date from 2022 to 2028, however, the location of the landing ellipses has changed, so the HiRISE-scale map does not fully cover the new ellipses anymore. Regional mapping at ~CTX-scale fully covers the ellipses, and provides a more synoptic view of the wider landing site within OP, enabling the contextualization of the units within the stratigraphy of western Arabia Terra and Chryse Planitia. We also mapped a region in Xanthe Terra which is characterized by mineralogical and textural features that are very similar to Oxia Planum (e.g., presence of phyllosilicates, polygonised surface texture) [5]. This will enable a comparison of both sites at the same mapping scale, and will help to address the question whether Oxia Planum is a localized phenomenon, or is representative of regional or even global geologic processes on Mars [e.g., 6]. It is expected that the CTX-scale Oxia Planum map will serve as a reference throughout the mission and subsequent data analysis.

Mapping Area and Datasets:  The mapping area is located between 16.5°N and 19.5°N, and 334°E to 338°E. The geodetic reference is provided by the HRSC quadrangles MC11E and MC11W [9] which are tied to the global MOLA geodetic model. The data sets used for mapping include HRSC, THEMIS IR (day and night), CTX, and CaSSIS. Mapping scale in a GIS environment is 1:100,000, which will result in a final printable map at a scale of 1:1M. The mapping follows established and newly developed guidelines for planetary geologic mapping [10-13].

Preliminary Results:  The spatially most widespread units are the phyllosilicate-bearing plains that are the prime ExoMars target (with distinctly enhanced THEMIS nighttime temperatures when compared to its surroundings), a dark resistant unit of possibly volcanic or sedimentary origin, and a mantling unit that was likely emplaced by aeolian processes. Multiple channels of various morphology and degradation state as well as sedimentary fan-shaped deposits (with low nighttime temperatures) imply a diverse and possibly long-lived history of surface runoff, perhaps accompanied or replaced by groundwater processes such as sapping. Inverted landforms (channels, impact crater fills) are the result of intense erosion. Additional mapped features include tectonic structures such as wrinkle ridges and lobate scarps (delineating a basin-like depression in the central mapping area), remnant erosional buttes that are predominantly located in the northwestern portion of the mapping area (i.e., towards Chryse Planitia), craters and their ejecta blankets, and fields of aeolian bedforms and secondary craters.

Comparison with Similar Site(s):  The phyllosilicate-bearing rocks in Oxia Planum are part of a circum-Chryse »belt« of surface materials displaying spectral evidence of aqueous alteration [7]. If there is indeed such a geologically coherent »bathtub« ring, the phyllosilicates at OP may be characteristic for a basin-related setting of regional importance, rather than being unique to OP. In that case, hypotheses related to the origin of OP phyllosilicates may also be tested at other circum-Chryse locations. To enable such tests, we selected a »reference« site in northern Xanthe Terra (~11±2°N/316.5±1°E) that exhibits several key characteristics of OP: Light-toned and fractured bed-rock with a high nighttime IR brightness, nearby chan-nels and sedimentary deposits (the Hypanis Valles and their terminal fans exhibiting low nighttime IR brightnesses, and erosional remnant buttes towards Chryse. The combination and spatial pattern of these features is very similar to OP and suggests that a similar geologic evolution may have shaped both areas.

Preliminary Conclusions:  Overall, our mapping confirms previous geologic analyses. However, some features (e.g., contractional structures, channels, possible sapping landforms) need further attention as the may provide important constraints on the tectonic and aqueous evolution of the ExoMars landing area.

Acknowledgments: The data used in this study are available via the Planetary Data System (PDS) of NASA, the Planetary Science Archive (PSA) of ESA, and additional data repositories at DLR and the USGS.

References:

 [1] Vago, J. L. et al. (2017) Astrobiology, 17, 471–510. [2] Loizeau, D. et al. (2019) LPS L, Abstract #2378. [3] Fawdon, P. et al. (2021) Journal of Maps, 17(2), 621–637. [4] Fawdon, P., et al. (2024) Journal of Maps, 20(1), 2302361. [5] Früh, T. et al. (2023) 54^th LPSC, LPI Contribution No. 2806, id.1440. [6] Torres, I. et al. (2026] Icarus, 117113 (in press). [7] Carter, J. et al. (2023) Icarus, 389, 115164.