Water Ice in the Subsurface and Polar Caps of Mars

Abstract Since the early days of Mars exploration it was evident that water played an important role in the geologic and climate evolution of the planet. In the decades following the conclusion of the Viking missions, determining the current amount, distribution and state of water, either liquid or solid, came to be considered one of the key investigations to understand Mars’ past. An orbiting ground penetrating radar was included in the payload of Mars Express: The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) was designed to probe the subsurface down to depths of a few kilometres to search for ice, water and dielectric interfaces outlining large-scale stratigraphy. MARSIS successfully probed both polar caps, providing unique insights on their structure and composition. In addition, MARSIS probed the widespread and unique Medusae Fossae Formation, revealing that it could contain a large amount of water ice. MARSIS provided the first evidence of a stable body of liquid brine beneath the South Polar Layered Deposits. In the coming years, new operative modes and improved processing methods hold the promise of providing new detailed information on the Martian subsurface at a vertical resolution of about 100 m and a horizontal resolution of a few to several kilometres.