Coupled dichotomy retreat and aqueous alteration on Noachian Mars

The Mawrth Vallis region is a cornerstone in our understanding of the role of liquid water on early Mars [e.g. 1], which is vital for our knowledge of the evolution of terrestrial planets, planetary habitability and the search for life elsewhere in the Solar System. Here, over 200 metres of phyllosilicate-bearing stratigraphy is exposed, recording extensive and prolonged aqueous alteration during the Noachian period [2]. In the Chryse Planitia lowlands north of the plateau, >14,000 kilometre-scale hills, mesas and buttes (‘mounds’) have been identified as remnants of a larger deposit [3]. However, the geological relationship between these mounds and the highlands remains unexplored.

Employing a novel “topography stacking” method and hyperspectral analyses in tandem with traditional stratigraphic remote sensing observations, we demonstrate that the mounds are erosional remnants of the Mawrth Vallis highland plateau, formed as the plateau receded over time. This finding reveals that the highland plateau extended hundreds of kilometres further north into Chryse Planitia in the Noachian, with the dichotomy escarpment retreating significantly over geological timescales.

There is substantial lateral and stratigraphic geochemical variation within the mounds. Plateau-distal mounds and deeper sections of the stratigraphy contain Mg-rich smectites (e.g. saponite), whereas plateau-proximal mounds and shallower sections are dominated by Fe-rich smectites (e.g. nontronite) that more closely resemble the Mawrth Vallis plateau. These compositional trends indicate differential alteration histories influenced by local environmental conditions.

The phyllosilicate geochemistry of the deepest altered mound strata resembles that of the clay-bearing plains [4] in nearby Oxia Planum—the future landing site of the ExoMars Rosalind Franklin rover [5]—suggesting that the alteration processes influencing Oxia Planum may have operated across a wider geographic area. Thus, by exploring these plains, Rosalind Franklin will also investigate aqueous environments that existed across broader regions of Noachian Mars than those preserved in the landing site.

Furthermore, the aqueous mound succession is interposed above a basal mafic unit (the first observation of unaltered material stratigraphically below the circum-Chryse phyllosilicate sequence [6]), and unconformably below a thin, spectrally bland capping layer. The mound sequence records a transition from early dry conditions through prolonged aqueous alteration to a final phase of non-hydrated deposition, documenting a near-complete stratigraphic history of aqueous conditions in the region.

Our findings highlight these mounds as an archive of early martian geologic history, chronicling the emplacement, alteration, and erosion of the circum-Chryse phyllosilicate deposit. The chemostratigraphical records preserved here provide new insight into the retreat of the dichotomy escarpment, the potential existence of a northern ocean in the Noachian, and the planet’s early habitability potential.

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