¹Inès Torres Auré, ^2,3John Carter, ¹Cathy Quantin-Nataf, ^2,4Damien Loizeau, ¹Erwin Dehouck, ¹Matthieu Volat
Icarus (in Press) Open Access Link to Article [https://doi.org/10.1016/j.icarus.2026.117113]
¹Laboratoire de Géologie de Lyon : Terre, Planètes, Environnement (LGL-TPE), Université Claude Bernard Lyon 1, CNRS, Villeurbanne, France
²Institut d’Astrophysique Spatiale (IAS), Université Paris Saclay, CNRS, Orsay, France
³Laboratoire d’Astrophysique de Marseille (LAM), Université Aix-Marseille, CNRS, Marseille, France
⁴Qualisat, Bièvres, France
Copyright Elsevier

The Fe/Mg-phyllosilicate-bearing units at Oxia Planum and Mawrth Vallis, two key Noachian sites along the martian dichotomy, exhibit distinct compositions despite their proximity. Using novel spectral criteria developed in this study, we distinguish two clay types: Type-1 (Mg-rich smectites/Fe2+-bearing saponite/vermiculite) and Type-2 (Fe3+-rich nontronite). Hyperspectral (OMEGA/CRISM) and textural (HiRISE/CTX) analyses reveal a regionally extensive basal Type-1 unit continuous across both sites, overlain by a Type-2 unit limited to Mawrth Vallis and southeast of Oxia Planum (above main delta fan elevations). A cratered paleosurface, with a Type-1 spectral signature, marks their boundary, indicating a depositional hiatus. The Type-1 unit’s lateral extent (>600 km) and elevation range (>1300 m) suggest a large-scale aqueous process, while the Type-2 unit’s absence below Oxia Planum’s delta fan implies either post-depositional erosion or environmental controls during deposition at Oxia Planum. Our results constrain early Mars’ climate models, challenging localized deposition/alteration hypotheses and ocean scenarios. These findings reveal that Type-1 clays extend over a much broader area than previously assumed, indicating that the ExoMars Rosalind Franklin rover will not investigate a localized phenomenon but rather a process with significant regional–and potentially global–implications for the geological and climatic history of Mars.