Mapping and Characterizing Subsurface Ice in Arcadia Planitia using Mars Reconnaissance Orbiter Shallow Radar Instrument (SHARAD)

There is great interest in better understanding mid latitude subsurface ice on Mars. Constraining the thickness, concentration, and extent of the ice is invaluable to understanding its links to Mars' climate and in the identification of potential resources for future human landing sites. Bramson et al. [2015; GRL 42] mapped a shallow subsurface interface with data from the Mars Reconnaissance Orbiter Shallow Radar (SHARAD) sounder in Arcadia Planitia and used terraced craters to calculate the real dielectric permittivity (ɛ') of the overlying layer. They found an average ɛ' of 2.5 and concluded that the layer consists predominantly of excess water ice.

As a part of a NASA-funded study, we mapped subsurface reflectors in a 600-km-wide swath between 0°N to 60°N and centered at 198°E that spans Arcadia and Amazonis Planitia. We extended the mapping of the Bramson et al. shallow reflector, with delay times of 0-1.75 µs from the surface. Primarily due to the inclusion of new coverage, we greatly increased the density and extent of the region where the reflector is mapped, now found to extend from 34.5°N to 55°N across the whole swath with only minor gaps.

Following our mapping effort, we reassessed the real dielectric permittivity of the putative ice layer. At 19 locations with 5 different types of surface features we used topographic information to constrain the dielectric permittivity. At all 19 locations, a rapid surface elevation change is associated with a decrease in the mapped subsurface reflector delay time such that the subsurface and surface reflectors become indistinguishable. We found an average ɛ' of 4.68 with values ranging from 3.08 to 5.99. Our average value was higher than that of pure water ice (ɛ'=3.15), suggesting higher regolith contents than concluded by Bramson et al. [2015], either distributed throughout or due to layering of regolith and ice. Nunes et al. [2011; JGR 116] examined pedestal craters globally and calculated ɛ' from elevation of the pedestals and a subsurface reflector only present below the pedestals, arriving at similar values with an average of ɛ'=4. These results further support the interpretation of widespread mid-latitude subsurface water ice in Arcadia Planitia, likely with a substantial lithic component. Additional work is needed to quantitatively constrain the fraction of ice present.