Reconstructing the geologic history of Hebrus Valles and Hephaestus Fossae, Mars with SHARAD

Abstract:

Hebrus Valles (HV) and Hephaestus Fossae (HF) are well-preserved martian Early Amazonian outflow channel systems in SE Utopia Planitia (16-25 °N, 118-130 °E) carved into bedrock. They exhibit a diverse set of morphologies that indicate formation by one or more liquid water outflow events. These outflow events may be due to magmatic intrusion, causing melting and cracking of the cryosphere. However, little is known about their origin, timing, and the fate of water and sediments in the context of geologic evolution of Utopia Planitia during the Amazonian Period (3 Ga to present). Thanks to extensive coverage by recent datasets, it is now feasible to study the geologic history of these features with an integrated analysis of diverse and complementary data. Here we present the first results of subsurface mapping and statistical analysis of surface radar returns based on Shallow Radar (SHARAD) profiles integrated with classic geologic mapping based on multispectral and high-resolution visible imagery.

Radar subsurface mapping reveals numerous clusters of shallow reflectors, most of which are likely associated with volcanic deposits identified by geologic mapping. However, some reflectors occur very close to outflow channels, with a depth ranging from ~45 m assuming a basaltic subsurface composition to ~60 m assuming pure water ice. These depths are incompatible with the <30 m base depth of layers seen in nearby outcrops. These reflectors are also located within the topographic expression of a wrinkle ridge and pinch out or vary in depth considerably, suggesting a structural, rather than stratigraphic, origin. Further analysis of SHARAD profiles in the region combined with a statistical analysis of surface returns may reveal whether these reflectors correspond to the internal structure of the wrinkle ridge or an unrelated dielectric interface, such as the top of an ice table.

Statistical analysis of surface radar returns based on SHARAD raw data products reveals numerous anomalies at multiple mapping scales that cannot be explained by surface roughness alone, suggesting heterogeneity in the composition and structure of the shallow subsurface (up to ~10 m depth). Further analysis based on the decomposition of reflected power into its coherent and incoherent components will help to further interpret these anomalies.