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MARSIS fully deployed

Mars Express probes the Red Planet’s most unusual deposits

1 November 2007
The radar system on ESA’s Mars Express has uncovered new details about some of the most mysterious deposits on Mars: The Medusae Fossae Formation. It has given the first direct measurement of the depth and electrical properties of these materials, providing new clues about their origin.

The Medusae Fossae Formation (MFF) are unique deposits on Mars. They are also an enigma. Found near the equator, along the divide between the highlands and lowlands, they may represent some of the youngest deposits on the surface of the planet. This is inferred from the marked lack of impact craters dotting this terrain, unlike on older terrain.

Mars Express has been collecting data from this region using its Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS). Between March 2006 and April 2007, Mars Express orbited the region many times, taking radar soundings as it went.

For the first time, these radar soundings revealed the depth of the MFF layers, because of the time it took for the radar beam to pass through the top layers and bounce off the solid rock beneath. “We didn’t know just how thick the MFF deposits really were” says Thomas Watters, lead author of the results at the Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, USA.

Topographic divide between the Martian highlands and lowlands

The MFF deposits intrigue scientists because they are associated with regions that absorb certain wavelengths of Earth-based radar. MARSIS, however, works at wavelengths where the radar waves mostly pass through the MFF deposits creating subsurface echoes when the radar signal reflects off the plains material beneath.

The MARSIS data reveal the electrical properties of the layers. These suggest that the layers could be poorly packed, fluffy or dusty material. However it is difficult to understand how porous material from wind-blown dust can be kilometres thick and yet not be compacted under the weight of the overlying material. On the other hand, although the electrical properties are consistent with water ice layers, there is no other strong evidence for the presence of ice today in the equatorial regions of Mars.

“If there is water ice at the equator of Mars, it must be buried at least several metres below the surface,” says Jeffrey Plaut, MARSIS Co-Principal Investigator at the Jet Propulsion Laboratory, USA. This is because the water vapour pressure on Mars is so low that any ice near the surface would quickly evaporate.

So, the mystery of Mars’s Medusae Fossae Formation continues.

To find out more, read the full story on the ESA Science pages

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