A new report in Nature Geoscience says there may be large deposits of carbonate sedimentary rocks a few miles beneath the surface of Mars.

If substantial carbonate minerals exist it might indicate a past surface environment with carbon dioxide, in contrast to its current acidic (and inhospitable) state.  

Researchers Joseph Michalski and Paul B. Niles found evidence for carbonate bedrock deep under the Martian crust and believe the ancient sediments were linked to a volcanic eruption by the Syrtis Major volcano.

If the volcano did bury the carbonates and a meteor exposed them, the researchers believe that it may help explain the origin of other carbonates on Mars - and may also be signs of more extensive carbonates, making it what the researchers call a 'high-priority' site for future exobiological exploration.

Citation: Joseph R. Michalski, Paul B. Niles, 'Deep crustal carbonate rocks exposed by meteor impact on Mars',  Nature Geoscience PUBLISHED ONLINE: 10 OCTOBER 2010 DOI: 10.1038/NGEO971