Mounds in Chryse Planitia and Oxia Planum, Mars: an unrecognised archive of Noachian geology

McNeil, Joseph Douglas (2023). Mounds in Chryse Planitia and Oxia Planum, Mars: an unrecognised archive of Noachian geology. PhD thesis The Open University.



The southern margin of Chryse Planitia forms part of the martian global dichotomy boundary, dividing the ancient, rugged southern highlands from the younger plains of the northern lowlands. This region contains 14,386 isolated, positive relief, kilometre-scale landforms (‘mounds’) comparable to mesas, buttes, and inselbergs on Earth. The mounds are associated with several areas of geological and astrobiological significance including Oxia Planum, the future landing site of ESA’s ExoMars ‘Rosalind Franklin’ rover, and Mawrth Vallis, a type locality for martian phyllosilicate-bearing stratified rocks. Using high-resolution orbital remote sensing datasets, including HiRISE, CaSSIS, CTX, and CRISM, this thesis investigates the stratigraphy, age, and origins of the mounds for the first time.

I present a map that shows the extent of the mounds in the circum-Chryse region, defines their individual morphometries and morphologies, and establishes a relationship between these characteristics that reveals an erosional origin. Through investigation of their stratigraphy, composition, and geometry, I show that the mounds are phyllosilicate-bearing remnants of the circum-Chryse highland terrains that formed through stepwise backstepping and top-down aqueous alteration of highland material during the Noachian. Prior to this erosion, therefore, the dichotomy boundary lay hundreds of kilometres further north than at present.

The mounds are the last remnants of this ancient landscape, and are an accessible archive of substantial physical and chemical modification of the dichotomy. The mounds are key targets for astrobiological exploration and understanding Noachian Mars. They are the only known localities in Chryse Planitia where altered strata are interposed between unaltered strata, and thus provide the most complete record of Noachian aqueous conditions. Furthermore, I show that mound-forming material buried mission-relevant phyllosilicate-bearing plains at Oxia Planum, providing long-term protection from martian surface environments. The mounds are therefore ideal localities for ExoMars, or other future missions, to explore Mars’ aqueous history and search for biosignatures.

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