Life on Mars If the surface life has been reanimated as recently as 450,000 years ago, then rovers on Mars could find dormant but still viable life at a depth of one meter below the surface, according to an estimate. In 1965, the Mariner 4 probe discovered that Mars had no global magnetic field that would protect the planet from potentially life-threatening cosmic radiation and solar radiation; observations made in the late 1990s by the Mars Global Surveyor confirmed this discovery. Scientists speculate that the lack of magnetic shielding helped the solar wind blow away much of Mars's atmosphere over the course of
Space-based radar and, thus, the unwanted echo from other surface objects. The first radar sounder flown was ALSE (Apollo Lunar Sounder Experiment) on board Apollo 17 in 1972. Other sounder instruments flown (in this case around Mars), are MARSIS (Mars Advanced Radar for SubSurface and Ionosphere Sounding) on board the European Space Agency's Mars Express probe, and SHARAD (mars SHAllow RADar sounder) on JPL's Mars Reconnaissance Orbiter (MRO). Both are currently operational. A radar sounder is also used on the Japanese Moon probe SELENE, launched September 14, 2007. A similar instrument (primarily devoted to ionospheric plasma probing) was embarked on the Japanese