Constraining Dust Content in Individual Martian NPLD Layers Using SHARAD Data

Abstract:

The Martian North Polar Layered Deposits (NPLD) are thought to contain a record of the planet’s paleoclimate for the Late Amazonian period. Specifically, the variable dust content within the many sub-parallel layers of ice that make up the NPLD is thought to contain a climate signal related to the recent hydrologic cycle of Mars. Internal reflections in radar sounding data from the Shallow Radar (SHARAD) instrument on the Mars Reconnaissance Orbiter (MRO) are presumably caused by this variation in dust content. Using these reflections, it is possible to estimate the dielectric properties of NPLD layers, and thus bring us a step closer to determining the dust concentration as a function of depth (age) in the NPLD. This relationship would provide much needed constraints for climate models as well as models of NPLD construction.

Although SHARAD has previously helped constrain the bulk dielectric properties of the NPLD, leading to an estimate of 5% overall dust fraction, discrete layers have not yet been analyzed in detail. Using the simple plane wave model put forward by Lauro et al. (2012) it is possible to use the ratio of the surface reflection power to the subsurface reflection power to estimate the reflectivity of the subsurface interface. Combined with a thin layer reflection model, this reflectivity leads to estimates of reflector dielectric constants, and thus dust content. Using this technique we have mapped the reflectivity of multiple reflectors in the Saddle Region of the NPLD, and used these reflectivities to place constraints on the dust content at each reflector interface. Preliminary results favor the hypothesis that reflectors are caused by small fluctuations in dust fraction within an ice matrix, rather than by lag deposits of nearly pure dust material left behind during ablation.