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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