Radar Fog in SHARAD Observations of the Martian South Polar Layered Deposits as a Result of Dust-Rich Layers

Abstract

The polar caps of Mars, primarily composed of water ice with minor fractions of Martian dust, serve as a record of the planet's past dynamics. Distinct dust-rich subsurface layers, observed in exposed outcrops of both the north and south polar layered deposits (NPLD and SPLD), have been detected by the orbital sounder SHAllow RADar (SHARAD) as sharp, bright, parallel reflectors. Some of the radargrams from high-latitude regions of the SPLD also include a pervasive elevated radar background either filling the spaces between layers or obscuring them entirely, usually referred to as “fog.” This phenomenon is typically attributed to off-nadir surface roughness or volume scattering based primarily on fog's appearance in radargrams. While a few studies have investigated its radar properties, none has used them to conclusively explain its origin. In this work, we measure fog backscattering and its decay with depth from a few SHARAD tracks and compare these to the signatures of plausible subsurface interfaces. While volume scattering is too weak to explain fog, specular reflections from dusty layers, when sufficiently thin or low in dust content, can account for fog's radar properties. This suggests that fog regions of the SPLD may host dust layers with lower reflectivity and more densely packed compared to the brighter reflectors detected in the NPLD.