Planum Boreum basal unit topography, Mars: Irregularities and insights from SHARAD

Abstract

Shallow Radar investigations of Planum Boreum, Mars' "basal unit" (BU) deposit have revealed multiple reentrants, morphologic irregularities, and thickness trends that differ from those of the overlying north polar layered deposits. We present detailed subsurface maps for these features and offer explanation for genesis of the deposit's morphologic asymmetry, expressed in different erosional characteristics between 0E-180E and 180E-360E. Additionally, this work revealed a depression in the basal unit that may have provided a site for spiral trough initiation. Interpretations of the findings suggest that antecedent BU topography has a marked impact on modern morphology and that aeolian forces have been the dominant driver of polar deposit accumulation since at least the end of rupes unit emplacement. We find no results requiring explanation beyond common Martian surface processes, including aeolian erosion and impact armoring. To add to the detailed morphologic study of the BU, we mapped the variability of the BU radar reflection character. Combining generalized katabatic wind flow with the radar mapping results suggests that rupes unit material sourced the younger cavi. We present clear evidence that, while compositionally distinct from the overlying layered deposits, the BU and its morphology are intimately linked to the morphology of the north polar layered deposits. Combining geologic evidence with paleoclimate modeling, the deposits contain evidence for a long history of aeolian emplacement and modification. Key Points As revealed by SHARAD, Planum Boreum's basal unit is morphologically complex No climate change evidence found from the upper basal unit to the modern regime Regional wind, local climate, and ice armoring dictate accumulation patterns