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In recent years several small concentrations of carbonate minerals have been found on the surface of Mars. These minerals are of special interest because their formation is connected to the presence of liquid water. The number and size of carbonate deposits so far identified on Mars, however, are far too small to suggest that the planet was once covered by oceans.
But a recent study, by researchers at NASA's Ames Research Center (amongst them Janice Bishop and Chris McKay) published in the International Journal of Astrobiology, suggests that the quantity of carbonates on Mars could be far higher, and that their lack of detection could be due to the fact that they are covered by the rusty deposit that covers virtually the entire surface of the red planet (actually called "red" because of the colour of the iron oxide).
Bishop and colleagues reached their conclusions after a series of tests on carbonate-rich rocks from Little Red Hill, in the Mojave desert in California. Although not actually Mars, this region has many similar surface characteristics, above all a rusty deposit, that mixed with the carbonates makes them very difficult to identify spectroscopically. Essentially, in order to detect the carbonates, the surface rust has to be scraped off, as in fact has been done by the martian rovers Spirit and Opportunity.
The rust-covered carbonates nonetheless have a specific spectroscopic signature, that appears to be similar to that found by the Mars Reconnaissance Orbiter in an ancient region of the martian surface called Nili Fossae, already noted for its carbonate deposits.
Perhaps even more interesting, is the fact that in the Mojave desert samples, the researchers found dehydration-resistant algae that are protected from the solar ultraviolet radiation by the covering of rust. The same thing may have happened on Mars.
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