Dichotomy retreat and aqueous alteration on Noachian Mars recorded in highland remnants.

The Mawrth Vallis region is a plateau situated on the highland side of Mars' hemispheric dichotomy boundary. It has a >200-m-thick phyllosilicate-bearing stratigraphic succession that indicates extensive aqueous alteration between 4.1 Ga and 3.7 Ga, during the Noachian Period. In addition, thousands of kilometre-scale isolated mounds in the lowlands north and west of Mawrth Vallis have been identified. Here we use geomorphological and spectroscopic analyses to show that the mounds are erosional remnants that formed through retreat of the highland plateau in the Noachian. Consequently, the escarpment that marks the surface expression of the dichotomy must have receded south-southeast by hundreds of kilometres in this area. Lateral and stratigraphic geochemical variation in the mounds show that widespread, multiphase aqueous alteration occurred in situ across this region in surface and subsurface environments. The mound succession is underlain by a pyroxene-rich unit that represents unaltered material below the regional phyllosilicate-bearing sequence and is unconformably overlain by a thin capping unit that marks the end of large-scale regional aqueous activity. Thus, the mounds contain a stratigraphic record of the onset, evolution and cessation of Noachian aqueous conditions in this region, detailing the environment and climate of Mars at its most habitable.