Insight into the Subsurface Structure of Mid-Latitude Glaciers and Mantles on Mars from Impact Craters and Radar Sounding

The mid-latitudes (±30-50º) of Mars are host to landforms involving substantial quantities of ice. Among these features are lobate debris aprons (LDA), lineated valley fill (LVF), and concentric crater fill (CCF) interpreted to be debris-covered glaciers that are hundreds of meters in thickness and icy dust deposits, or "mantles," that are tens to over a hundred meters in thickness. Understanding their subsurface structure, including ice and debris content, is important for constraining past climate and volatile exchange on Mars and for assessing their accessibility for future robotic and human exploration. Combined with high-resolution imaging (e.g., HiRISE) and digital elevation models, impact craters can be used as natural probes to assess subsurface ice content, thicknesses, and density contrasts. Radar sounding by MRO's SHAllow RADar (SHARAD) enables constraints on the dielectric properties and thicknesses of the deposits.

We analyzed the subsurface structure of LDA, LVF, CCF, and mantle deposits within Deuteronilus Mensae (36-48.5ºN-13-36ºE) using HiRISE images and DEMs of impact craters, SHARAD radar data, and other remote sensing datasets. The shallow subsurface (<50 m) of LDA, LVF, and CCF consists of ice-dust mantle deposits on top of boulder-rich rockfall, with little internal strength variations. Radar data of LDA and LVF reveal low radar loss and basal reflectors with high power that imply nearly pure glacial ice throughout most of their depth. Adjacent mantle units have radar reflectors at average two-way time delays of 1.2 μs that are interpreted to represent the contact with underlying volcanic plains. Topography implies thicknesses of up to 100 m and real dielectric constants of 5.0, consistent with 50-70% debris and 30-50% ice/pore space. In other mantled areas, deeper radar reflectors (average two-way time delays of 4.2 μs) are continuous with LDA basal reflectors and suggest lower bulk dielectric constants of 4.5. The power and time delays of these reflectors imply a thick ( 150-200 m) mantle/debris layer on top of pure glacial ice, or a mixture of debris and ice throughout the 300 m depth. These results indicate that there are variations in ice content and structure of mantle units in the mid-latitudes of Mars that should be further assessed by future radar and geomorphic investigations.