Dielectric Mapping of Bulk Polar Ices on Mars With SHARAD Radar Data
Résumé
The SHAllow RADar (SHARAD) is a subsurface sounding instrument aboard the NASA's Mars Reconnaissance Orbiter (MRO) spacecraft. The main SHARAD scientific objectives are to map the underground distribution of water over the planet as well as to seek buried geological structures in order to understand the formation of the superficial Martian landscape. SHARAD is working at a 20 MHz central frequency with a 10 MHz bandwidth. Its penetration depth (i.e. the attenuation of the signal) depends of the dielectric properties of the sounded material; typically more than 1 km in water ice with ~7 m of vertical resolution. The Martian polar layered deposits (NPLD) are the largest reservoir of water on the surface. The physical properties of the polar ices is one the main unresolved questions in Martian polar science. In particular, accurate estimation of the dielectric properties is important since it is interrelated to the impurities contamination of the ice and consequently to its rheology; whereas the spatial distribution of the impurities is linked to the interaction of the NPLD with the Martian climate. Moreover, it is also a significant contribution for comparative planetology with terrestrial caps since the Martian ice has the same structure than on Earth (Ice Ih), but accumulated under an extreme planetary environment (mean surface temperature and pressure of 155 K and 0.008 bar respectively). We will present a study over the Gemina Lingula region where the bedrock of the NPLD is imaged by SHARAD. From this, it has been possible to map the dielectric properties of the bulk ice. A data set of 140,000 SHARAD pulses was used. The maps of the dielectric constant and the loss tangent of the ice will be presented. Both properties have a Gaussian distribution giving accurate results. The pick values are consistent with an extremely pure ice. The loss tangent map highlights the spatial distribution of the impurities. A drop of the dielectric constant along Chasma Boreale could be explained by a brutal 250-meter uplift of the base of the NPLD corresponding to an extent of the basal unit below Gemina Lingula.