The MFF deposits intrigue scientists because they are associated with regions that absorb certain wavelengths of Earth-based radar. This had led to them being called ‘stealth’ regions because they give no radar echo. The affected wavelengths are 3.5 to 12.6 centimetres. MARSIS, however, works at wavelengths of 50 to over 100 metres. At these wavelengths, the radar waves mostly pass through the MFF deposits creating subsurface echoes when the radar signal reflects off the plains material beneath.
A variety of scenarios have been proposed for the origin and composition of these deposits. Firstly, they could be volcanic ash deposits from now-buried vents or other nearby volcanoes. Second, they could be deposits of wind-blown materials eroded from other martian rocks. Thirdly, they could be ice-rich deposits, somewhat similar to the layered ice deposits at the poles of the planet, but formed when the spin axis of Mars tilts over, making the equatorial region colder.
Deciding between these scenarios is not easy, even with the new data. 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. “It is still early in the game. We may get cleverer with our analysis and interpretation or we may only know when we go there with a drill and see for ourselves,” says Plaut.
Monica Talevi | EurekAlert!
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