A European spacecraft has used radar to probe one of the youngest and most mystifying deposits on Mars.
The European Space Agency's Mars Express orbiter studied the Medusae Fossae Formation (MFF), which straddles a divide between the highlands and lowlands near the Martian equator.
MFF deposits have remained an enigma partly because they are "stealth" regions that give no radar echo from certain wavelengths of Earth-based radar.
But while the deposits absorb radar wavelengths between 1.4 and 5 inches (3.5 to 12.6 centimeters), the Mars Express used its Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) that operates at wavelengths of 164 feet to 328 feet (50 to over 100 meters).
That means MARSIS radar waves can pass through the MFF deposits and bounce off the solid rock beneath, creating a subsurface echo image.
"We didn't know just how thick the MFF deposits really were," said Thomas Watters, the study's lead author at the Center for Earth and Planetary Studies for the Smithsonian Institution's National Air and Space Museum in Washington, D.C. "The new data show that the MFF are massive deposits over 2.5 km [1.5 miles] thick in some places where MARSIS orbits pass over them."
Scientists have proposed a variety of scenarios about the origin and composition of the MFF deposits.
They could be volcanic ash deposits, wind-blown materials eroded from other Martian rocks, or even ice-rich deposits that formed when the spin axis of the planet tilted over and made the equatorial region colder.
MARSIS revealed both the depth and electrical properties of the deposit layers, suggesting that the layers could be poorly packed, fluffy or dusty material.
But scientists are puzzled over how material from wind-blown dust could be kilometers thick and not have compacted under the weight of overlying material.
Although the electrical properties resemble those of water ice layers, there is no other strong evidence for ice remaining at the Mars equator.
The water vapor pressure on Mars is so low that any ice near the surface would quickly evaporate, leaving the mystery of the Medusae Fossae Formation for scientists to ponder.
"If there is water ice at the equator of Mars, it must be buried at least several meters below the surface," said Jeffrey Plaut, MARSIS co-principal investigator at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "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."
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