ESA’s MARSIS “Digs” Below Mar’s Surface

ESA’s Mars Express radar sounder, MARSIS, has looked beneath the Martian surface and opened up the third dimension for planetary exploration. The technique’s success is prompting scientists to think of all the other places in the Solar System where they would like to use radar sounders. No matter how accurate a camera is, it can only map a planet’s surface. To retrieve information about the underground realm, planetary scientists in the past would have thought it necessary to land on the surface and start digging. But that would only be good for a single spot on a large planet and the first few decimetres of the surface. To get the global picture of the subsurface they need a radar sounder, such as the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS), to find the best spots for the future landers to go and dig. MARSIS was an experiment in every sense of the word. “It was a leap into the unknown,” says Ali Safaeinili, MARSIS co-investigator at the Jet Propulsion Laboratory (JPL), California. No one had ever used a radar sounder from orbit on another planet before. So the team could not even be sure whether it would work as planned. The subsurface of the planet might have been too opaque to the radar waves or the upper levels of martian atmosphere (ionosphere) might have distorted the signal too much to be useful. Thankfully, none of this happened. Funded by ESA‘s General Studies Programme, the new study is called Advanced Concept for RAdar Sounder (ACRAS). In the direction in which the satellite is moving, the returning radar has its frequency subtly altered by the Doppler effect. This is the same effect that makes an ambulance’s siren appear to change pitch as it passes. The Doppler effect in the radar beam can be analysed to determine where the reflected signals are coming from and reject the unwanted ones. In the ‘cross-track direction’, at right angles to the direction of travel, the study has determined that instead of sending out one radar beam, sending three with slightly different properties can be used to determine which reflected signals to keep. So, the ACRAS study may have now found the solution for 3D mapping.