The Mars Reconnaissance Orbiter has found 3.6 billion year old carbonates on the Martian surface, this finding suggests the red planet once was hospitable to life:
The MRO found carbonates, which is formed in alkaline water when water and carbon dioxide mix with calcium, magnesium or iron. Carbonates can be found on Earth in locations where life survives, which makes it a significant find on the Red Planet.
"It would have been a pretty clement, benign environment for early Martian life," Brown University graduate student Bethany Ehlmann said. "It preserved a record of a particular type of habitat, a neutral to alkaline water environment."
Because carbonates dissolve quickly in low pH solutions, it could have taken longer than expected to find carbonates on the Red Planet. MRO's findings are about the size of a football field and deposits are limited.