Radar techniques to study subsurfaces and interiors of the solar system objects

Over the past decades, a number of different Synthetic Aperture Radars (SAR) were developed for mapping the surface of the planets either from Earth or from orbiting spacecrafts. However, the idea to use radar to study the subsurface started to develop during the last 15 years. The ability of the radio waves to penetrate the ice, permafrost and arid surface was at the origins of the development of the Ground Penetrating Radars (GPR). GPRs have been widely applied on Earth with a large number of the scientific and industrial applications. The application of GPR to the space exploration relies on the same operation principle but requires the development of low power and low mass equipment. In this paper, we start by a short summary of the general electromagnetic behavior of the materials that determines the principal characteristics of ground penetrating instruments. Then we describe the GPR instrument that was developed for the Mars 1998 mission (unfortunately cancelled) and we discuss a general GPR design that can be implemented on future rover missions. The measurements from the surface cannot replace the global mapping from orbit using orbital radar sounders. MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Sounding) [1] and SHARAD (SHAllow subsurface RADar) [2] radars are examples of these orbital radar sounders that are now in Mars orbit on the Mars Express (ESA) and MRO (NASA) spacecrafts. The Lunar radar Sounder (LRS) on the Japanese mission Kaguya (Selene) (2007-2009) was the radar sounder on the Moon [9]. These radars work essentially in the altimeter mode even if some Doppler treatment is implemented and used in the data analysis. Another major milestone in planetary radar application will be once the radar tomography of the interior of the small objects is achieved. We discuss this new concept on the example of Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT) [3], the experiment on the mission ROSETTA (ESA).