The mineral, which forms in the presence of water, had previously been detected in trace amounts in Martian dust and soil. But its presence in bedrock indicates water there wasn't as acidic and thus more hospitable to life.
The region "would have really been a clement, benign environment for early Martian life," said mission scientist Bethany Ehlmann of Brown University.
The results were presented Thursday at an American Geophysical Union meeting in San Francisco and will appear Friday in the journal Science.
Scientists planning the next Mars landing - the Mars Science Laboratory - initially considered Nili Fossae as a potential landing site, but it did not make the final cut.
The launch of the lab is set for 2011.
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Mars Reconnaissance Orbiter mission:
