Nasa's Mars Reconnaissance Orbiter has finally spotted rocks on the Red Planet that bear carbonate minerals.
The ingredients needed to make the rocks are very evident, so their absence had been a major puzzle.
One theory to explain the omission is the idea that water on Mars has been too acidic to allow carbonates.
The rocks' identification now shows these harsh waters have not dominated all parts of Mars - and that is good news for the search for life.
"You want to get an environment that is basically as clement as possible, that's not difficult to live in," explained Bethany Ehlmann from Brown University in Providence, Rhode Island.
"It's difficult to live in a highly acidic environment; it's difficult to live in a very salty environment. If you have neutral waters then that presents a less difficult environment for microbial life," she told BBC News.
Ehlmann and colleagues have been detailing the discovery here at the American Geophysical Union's (AGU) Fall Meeting 2008. A paper explaining their findings is also being published in the journal Science.