SHARAD detections of subsurface reflectors near RSL sites on the Tharsis Plateau immediately adjacent to the canyon rim of Valles Marineris

Abstract:

Recurring slope lineae (RSL) are very exciting features that exhibit evidence for water flow on the Martian surface. The number of RSL sites has risen to ~100 since their first detection in 2011. Those sites extend over a large portion of the Valles Marineris margin and nearby smaller canyons, but no source for this flowing water has been identified. Two possible sources exist for water near the Martian equator: the atmospheric and the subsurface. At low latitudes, atmospheric water vapor abundance is extremely low, and condensation of water from the air is unlikely. Furthermore, subsurface water ice is unlikely to remain stable in the long term, but scenarios for retaining ice on shorter timescales have not been fully tested.

The Shallow Radar (SHARAD) instrument on Mars Reconnaissance Orbiter has successfully probed the subsurface of Mars to locate dielectric interfaces that delineate volcanic boundaries, layers within the polar ice caps, and ice-rock boundaries. Using the same technique of searching for dielectric contrasts at lower latitudes, we have found several detections at the highest elevations of the Tharsis Plateau, near the cliffs that form Valles Marineris at depths ranging from 30 to 80 m, depending on the dielectric properties of the overlying material. These reflectors are located near the canyon rim and slope towards the canyon, potentially crossing geologic boundaries mapped from surface data.

Because of the proximity of the reflectors to RSL and the geometry of the reflections, we hypothesize that SHARAD may be detecting an ice or water reservoir that can act as a source for flowing water on the surface. We test this hypothesis by employing a one-dimensional thermal model to estimate the stability of ground ice over a wide range of durations at this latitude, including recent epochs of high obliquity, when ice would be more stable at low latitudes and for longer periods.