Waves On Mars: Imaging Subsurface Sedimentary Structures In Utopia Planitia And Implications On The Retreat Of An Ancient Ocean

Abstract

A growing body of evidence supports the hypothesis that aqueous sedimentary processes were active on the Martian surface in the distant past. Constraints on the scale and persistence of these water bodies, however, remain limited by the lack of subsurface data beyond the weathered layer. Appropriating seismic migration algorithms to invert Ground-Penetrating Radar (GPR) data from martian rovers has proven a reliable and practical approach to imaging the structure of subsurface sedimentary deposits beyond the depth reasonable for, or attainable by, sample excavation and return. In study, we begin by implementing, from first principles, three of the most commonly employed migration algorithms, and rigorously assessing their reliability under increasingly non-ideal subsurface conditions. By benchmarking algorithm behavior against a range of conditions where ground truth is fully controlled by a known GPR model and synthetic data, we establish a basis for constraining the confidence in and qualitative interpretation of results obtained from real planetary data, where these conditions are largely unknown. We then apply these methods to data collected by RoPeR, the low-frequency GPR module of the Zhurong Rover along a 1.8 km traverse in Utopia Planitia—the site of a putative ancient coastline. Our interpretation of the migrated section reveals North–West-Northwest prograding clinoforms comprised of sigmoidal reflectors on a scale of tens of meters and with apparent dips of 5–20 degrees. The range of dips encompasses that of previous findings, but significantly more detail as to the distribution of these angles, and the spatial variation of these distributions along the traverse, is afforded with our multi-method approach. The lack of a common strike association between imaged reflectors suggests that coastal sedimentary processes were actively deforming the beach planform as a large body of water receded into Utopia basin. Moreover, diminished wave and current action in a large Martian ocean leaves these processes best explained by a glacio-marine setting, consistent with the cold, wet Hesperian hypothesis supported by other sites along and within the highland-lowland boundary.