A close-up view of the eroded terrain in Nili Fossae using a CRISM infrared image to colorize a high-resolution HiRISE camera image. Beneath a rough-textured capping rock unit (purple) are banded olivine-bearing layers (yellow) which in some places have been partially altered to carbonate (bright green). The image is approximately 2.5 km across. Credit NASA/JPL/JHUAPL/University of Arizona/Brown University
Evidence of a key mineral on Mars has been found at several locations on the planet's surface, suggesting that any microbial life that might have been there back when the planet was wetter could have lived comfortably.
The findings offer up intriguing new sites for future missions to probe, researchers said.
Observations made by NASA's Mars Reconnaissance Orbiter (MRO), which just completed its primary mission and started a second two-year shift, found evidence of carbonates, which don't survive in conditions hostile to life, indicating that not all of the planet's ancient watery environments were as harsh as previously thought.
The findings are detailed in a study in the Dec. 19 issue of the journal Science and will be presented today at the annual meeting of the American Geophysical Union in San Francisco.
Escaping the acid bath
Over the past several years, evidence for water on Mars has been stacking up: Scientists have found gullies formed by running water, possible ancient lakes, and minerals formed by interaction with water.
But just how much water there was and how suitable it was for Martian microbes or other primitive life forms have been harder puzzles to solve.
Most evidence has pointed to a period when water on the planet's surface formed clay-rich minerals, followed by a time of drier conditions, when salt-rich, acidic water affected much of the planet. These later conditions would have proven difficult for any Martian life ? if it ever existed ? to endure or to leave any traces for scientists to find.
Because carbonates dissolve quickly in acid, finding them shows that at least some areas of the planet escaped the acid bath and were less hostile to life.
If primitive life sprang up in these pockets, it could have persisted and left clues of its existence.
"Primitive life would have liked it," said study author Bethany Ehlmann, a graduate student at Brown University in Providence, R.I. "It's not too hot or too cold. It's not too acidic. It's a 'just right' place."
The sites are dry now, however, so researchers are not expecting to find biology on the surface in those locations.
Pinning down sites
NASA's now-defunct Phoenix Mars Lander also found carbonate signatures in surface samples it analyzed earlier this year. Researchers have also found carbonates in Martian meteorites that fell to Earth, as well as in windblown Mars dust observed from orbit. But this dirt and dust could have been mixtures from many areas, so the origins of carbonates have been unclear.
Scientists had expected to find "extensive layers of carbonate" because they would be a natural consequence of the chemistry between Mars' carbon dioxide-rich atmosphere and the weathering products formed by water acting on volcanic rocks, said MRO project scientist Richard Zurek of NASA's Jet Propulsion Laboratory in Pasadena, Calif. But no carbonate deposits could ever be found.
But now, MRO's finer resolution has allowed scientists to finally locate carbonate. ?
"These carbonate exposures are small, which I think explains why we haven't seen them on the surface before," Ehlmann said during the AGU press conference.
The areas with carbonates found by MRO's CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) instrument were: Nili Fossae, which is 414 miles( 667 kilometers) long and lied at the edge of the Isidis impact basin; some sides of eroded mesas; sedimentary rocks within Jezero crater; and rocks exposed on the sides of valleys in the crater's watershed. Traces were also found in Terra Tyrrhena and Libya Montes.
These are locations where future rovers and landers could visit to search for life. (Nili Fossae was one of the final seven candidate sites for the Mars Science Laboratory, whose launch has now been delayed until 2011, but it didn?t make the MSL final-four list.)
"This is opening up a range of environments on Mars," said study co-author Jack Mustard, a professor at Brown.
CRISM has scoured Mars' surface looking for signs of carbonates, but this is the first time it has turned any up. This could indicate that this is "a local environment on Mars that is somewhat unique," Ehlmann said.
Still, there is the possibility that other regions will turn up carbonate signatures, or that carbonate layers could be hidden beneath the surface, she added.
The researchers have multiple theories for how the carbonates might have formed, including slightly heated groundwater percolating through olivine-rich rocks exposed at the surface and altered by running water or precipitation in small, shallow lakes.
The study was funded by NASA and the National Science Foundation.
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