The north polar basal unit of Mars: An Amazonian record of surface processes and climate events

The basal unit is a sedimentary deposit of water ice and lithic fines lying within the Planum Boreum of Mars. It can be divided into two subunits, rupēs and cavi, based on their stratigraphy and age. The basal unit lies between Late Hesperian and very recent (<4 Ma) units, and thus likely represents a record of polar geologic processes and climate events spanning large portions of the Amazonian Period. Despite the numerous recent studies, several key questions remain unanswered regarding its stratigraphic nature, age, and potential as a historical record of surface processes and climate. In this study, we combine the ability of the Shallow Radar (SHARAD) to reveal the large-scale stratigraphy and morphology of the basal unit within Planum Boreum with geologic mapping of visible exposures based on high-resolution imagery. This integrated study reveals that the basal unit has significantly larger extent but smaller volume than previously thought, and demonstrates that radar profiles acquired by SHARAD can provide the necessary stratigraphic correlation to comprehensively study isolated outcrops with high-resolution imagery that would otherwise lack important context. SHARAD also reveals portions of the contact between the rupēs and cavi unit in the innermost region of Planum Boreum which will allow us to better constrain the relative volume of the two units, and thus their water ice and lithic sediment content. New age constraints on the rupēs and cavi unit reveal a history of lithic fines and water ice accumulation and erosion spanning most of the Amazonian Period. Detailed mapping of the basal unit surface morphology underneath the North Polar Layered Deposits provides further insight on the erosional history of the rupēs and cavi units and suggest a more complex resurfacing history than previously thought, likely characterized by an intimate interplay with polar ice growth and retreat. SHARAD also reveals erosional forms in the cavi unit that provide new supporting observations of its hypothesized water ice and sand-sheet structure, confirming that it likely preserved former polar cap ice in its interior.