Constraining the Dielectric Properties of the Phyllosilicate Deposits of Eastern Valles Marineris Mars

Abstract:

The phyllosilicate deposits of Mars represent the outcome of aqueous conditions during the early periods of the planet's history. To study these conditions and the subsequent history, we characterize geologic evidence of surface and subsurface water in the Noachian and the bound water content of relict soils. Our study area is the plateau south of Ganges Chasma (GC), an ideal location for radar analysis. In most regions of Mars with clay deposits, surface relief produces an abundance of side echoes, confusing interpretations. Further, clays with high bound water content should attenuate radar signals too strongly for useful observations (Stillman et al. 2011), but SHARAD can penetrate the deposits in this region to obtain a basal reflection, making this the first time that clays' bulk properties can be studied on Mars with radar.

We hypothesize that after formation, the formerly wet plateau underwent water table retreat, dehydrating the clays over time, leaving mineral deposits that attenuate radar waves less than predicted, allowing deeper penetration and observation of a subsurface reflector >10 m beneath the surface. This is supported by the presence of dehydration from erosional channels and desiccation polygons in the region that align with spectral and radar detection of these deposits.

We use SHARAD to map the extent of subsurface reflections and then cross-reference them with CRISM and HiRISE data to expand past mapping of the deposits. The depth of the detections will be measured using digital elevation models from stereo imagery in order to calculate the dielectric constant of the surface material.

Finally, we will perform dielectric measurements on terrestrially derived materials of the same composition as the Fe/Mg and Al clays located around GC. For these measurements, we use methods developed by Boivin et al. 2018 to measure the signal transmitted through and reflected by the sample in a coaxial airline from 300 kHz to 8.5 GHz. Then we calculate the dielectric permittivity and loss tangent of the sample. In order to determine the bound water content of the deposits on Mars, we will repeatedly dehydrate samples until a signal is measured that compares to that measured on Mars. This will allow us to constrain a water weight percentage of those phyllosilicates necessary to allow SHARAD to observe the basal reflections.