The
European Space Agency's Mars Express Orbiter has spied evidence of a large
concentration of rust, revealed by erosion, on the planet's surface.
The finding,
detailed in a recent issue of the Journal of Geophysical Research, is
yet another clue that will help scientists piece together the picture of Mars'
past climate.
The bright
red dust covering most of the planet is known to be enriched in ferric oxides,
more commonly known on Earth as rust.
But Mars Express
observations show that the dark deposits of Aram Chaos, a crater about 170
miles (280 kilometers) in diameter lying almost directly on the Martian
equator, have a ferric oxide signature that is four times higher than elsewhere
on the surface.
Ferric
oxides are generally found with sulfates but, in this case, the lighter
sulfates have been blown away, leaving the ferric oxides exposed.
"They
have accumulated in dark deposits at the bottom of sulfate cliffs," said
Stephane Le Mouelic of the Universite de Nantes in France, and a member of the
team who performed the investigation with Express' OMEGA instrument, the
Visible and Infrared Mineralogical Mapping Spectrometer.
This
concentration suggests that the ferric oxides have been uncovered by erosion before
dropping to the base of the cliffs. The dunes in this region are also enriched
in ferric oxides.
This
phenomenon is not unique to the Aram Chaos region: NASA's Opportunity
rover discovered ferric oxide deposits in Meridiani Planum, about 600 miles
(1000 km away). The scientists called the deposits "blueberries,"
because of their spherical shape. Valles Marineris, about 1,850 miles (3,000 km)
away, also shows similar deposits. So Mars Express' detection of ferric oxides
in Aram Chaos links together widely separated areas of Mars.
There may
even be other regions that have witnessed the same accumulation process but now
lie hidden from Mars Express' view.
"OMEGA
is sensitive to the first hundreds of microns of the surface. So, a layer of
Martian dust just one millimeter thick will hide the signature from us,"
said lead author of the study Marion Masse, also of the Universite de Nantes.
Fortunately,
in many regions of Mars, such as Aram Chaos, wind erosion has blown the dust
cover away, leaving bulk rocks exposed.
The team is
now exploring possible explanations for how the sulfates and ferric oxides
might have accumulated in the first place. They're ruling nothing out: It could
be anything from atmospheric precipitation such as rain or snow, to volcanic
ashes or glacial deposits.
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