By Jonathan Amos
Science reporter, BBC News, San Francisco

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.

Weathered rocks

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.

The carbonate minerals were detected in a mid-latitude region called the Nili Fossae, on the western edge of the Isidis impact basin.

The landscape viewed by Mars Reconnaissance Orbiter (MRO) is believed to have formed more than 3.6 billion years ago.

Carbonates are produced in the weathering process that sees water with dissolved carbon dioxide re-fashion the original chemistry of rocks. The carbonates - in this case, magnesium carbonate - precipitate out of solution.

On Earth, carbonates are usually associated with great marine sediments like limestone and chalk; although the scientists here stressed the Martian carbonates would look nothing like that.

Life hunt

Previous data from orbiting spacecraft and from the robot rovers on the surface of Mars has revealed salt-rich, acidic waters affected much of the planet in more modern times.

Given that carbonates dissolve quickly in low pH solutions, it is possible that many large carbonate formations created on early Mars may simply have disappeared; and this could explain why it has taken so long to find a carbonate signature.

But the MRO discovery shows that some areas of the Red Planet at least must have been untouched by these harsher conditions. That makes Nili Fossae an interesting place for future Mars missions to explore.

"If you preserve carbonates on the surface then you know carbon-bearing compounds can survive in some environments on the planet," said Richard Zurek, the project scientist on MRO.

"That means there are some places we can go and look for evidence for past life - if it ever existed."

Interestingly, Nili Fossae lost out in the site selection contest to choose the landing location of the next Nasa rover, called the Mars Science Laboratory (MSL).

The vehicle's launch recently slipped from 2009 to 2011 and the scientists at AGU said it was possible the contest outcome could now be reviewed. However, they also said there would be other opportunities to visit Nili Fossae.

"MSL is not the last lander that we intend to send to the planet. With this diversity of environments, there are many places to explore," said Dr Zurek.