On water detection in the martian subsurface using sounding radar

Several radar experiments are planned to map the martian subsurface down to several kilometers, searching for subsurface liquid water reservoirs, using different concepts and techniques, all based on the penetration property of radio frequency waves in and soils. The penetration depth of low-frequency radar is mainly related to the electromagnetic properties of the investigated medium. Thus a good knowledge of the martian subsurface dielectric profile along the first few kilometers is necessary for future water identification and data interpretation. In this work we have investigated the electrical and magnetic properties of the martian surface and subsurface, using terrestrial laboratory analogues in the frequency range 1-500 MHz, covering the frequency domain of the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) experiment on board the Mars Express mission (ESA-2003), the NetLander ground-penetrating radar (GPR) (CNES-2007), and future sounding radar that may be updated to the Mars exploration program in the "follow the water" strategy. In our approach, we constructed experimentally the most common dielectric profile representative of the martian subsurface by measuring the electric permittivity and magnetic permeability of well defined mixtures of basaltic, volcanic, and sedimentary materials that have been reported for Mars. We also considered iron oxides (hematite and maghemite) and evaporites that may be present, such as gypsum, and their mixtures with representative amounts of the martian geological context under the most common petrophysical and geophysical conditions, along the subsurface profile. This led to synthetic representative samples of the martian subsurface materials under adequate conditions of porosity and temperature that should exist in the first 2.5 km of the upper crust. Dielectric measurements show that the first layers of the martian subsurface (a few hundred meters), which are mainly composed of volcanic iron-rich materials, could dramatically decrease the radar penetration depth initially foreseen, thus limiting deep subsurface exploration. We also investigated the constraints on subsurface water detectability in a radar lossy medium and its dielectric identification among surrounding geological materials. (C) 2001 Elsevier Science (USA).