Radar has proven to be an effective tool for probing beneath the surface sands of the Sahara Desert to delineate ancient drainage systems and provide clues to past climate conditions. Synthetic aperture radar (SAR) systems, operating at wavelengths up to 68 cm, permit broad swaths of the surface to be investigated to depths of 1-5 m. Surface-deployed ground penetrating radar (GPR), operating at a 75-cm wavelength, permits more localized studies at depths up to 12 m. In tandem, these radar systems enable definition of process-specific signatures diagnostic of past geologic processes by penetrating dry materials mantling the surface.

The 2003 Mars Express and 2005 Mars Reconnaissance Orbiter missions will carry sounding radars to probe the Martian crust in the 100 m to 5 km depth range. SAR and GPR systems have also been proposed for future orbital and landed missions. As a demonstration of the utility of such observations, Earth-based radar observations of Mars confirm that the surface is variably radar reflective, and suggest that radar penetration in many locations will be ~10 times the incident wavelength. In light of the extensive surficial deposits that blanket much of Mars, radar may hold the key to unraveling some of the planet's outstanding mysteries (e.g., evolution of the hydrologic cycle).

Radar studies of terrestrial analog environments can define expected radar properties for a variety of landforms, and form a guide to interpreting future SAR/GPR data for Mars. Ongoing studies of landforms in Hawaii, Arizona, the Mojave, and the Sahara are providing important clues regarding expected radar performance and likely results on Mars. Such studies highlight the ability of a Mars SAR to characterize morphology beneath the Martian dust layer, map the distribution of high-latitude ground-ice, and locate any relict coastal or fluvial features (e.g., channels or tributaries). A rover-deployed GPR could define stratigraphy at the decimeter-to-meter scale to depths of 10-15 m, thereby helping to establish geologic setting, evaluate the history of aqueous activity, and locate any accessible water. GPR can also provide context for other rover and orbital data sets, and enable 3-D mapping of local stratigraphy to guide subsurface sampling.