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 metres
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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