New Insights Into Subsurface Stratigraphy Northwest of Ascraeus Mons, Mars, Using the SHARAD and MARSIS Radar Sounders

The Tharsis Montes volcanoes on Mars are the source of laterally extensive lava flows and other volcanic deposits generating a complex stratigraphy throughout the Tharsis Volcanic Province. We use SHAllow RADar (SHARAD) and Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) observations in a region northwest of Ascraeus Mons to determine the composition, density, thickness, and spatial distribution of these emplaced volcanic materials. We identified subsurface reflectors along 43 SHARAD and five MARSIS observations. Reflectors in the volcanic plains are interpreted to be sequences of basaltic lava flows with interspersed pyroclastic material, dust, or regolith during a hiatus in activity. Others correspond to the base of three major flow fields. Several plain reflectors were detected by both MARSIS and SHARAD. Other notable reflectors were identified near Ascraeus' flank where lava buried glacially derived sediment. We derived thickness and other material properties for flows using their distinct topographic boundary visible in the radar images. Permittivity ranged from 7.0 to 11.2 corresponding to lava flow densities of 3.20-3.52 g/cm³. Flow thicknesses ranged from 19.8 to 60.2 m. Loss tangents were low for the flow fields ranging from 0.024 to 0.043. Loss tangents in the plains ranged from 0.010 to 0.076. Higher loss tangents correspond to lossier regions that may have higher concentrations of radar absorbing minerals like hematite. Surface roughness controls where reflectors are detected. SHARAD detects the base of three out of the four flow fields in this region with muted surface roughness from dust mantling and erosion.