The robot has drilled into a rock that contains clay minerals - an indication of formation in, or substantial alteration by, neutral water.

Scientists say the find is one more step towards showing conditions on the Red Planet in the distant past could have supported life.

Many rocks studied previously were probably deposited in acidic water.

While this would not have precluded the possibility of micro-organisms taking hold on Mars, it would have been more challenging, scientists believe.

Identifying clays shows there were at least some locations on the planet billions of years ago where environments would have been much more favourable.

"We have found a habitable environment that is so benign and supportive of life that probably if this water was around and you had been there, you would have been able to drink it," said John Grotzinger, Curiosity's project scientist.

The rover made the assessment after studying a powdered sample drilled from a fine-grained mudstone at its exploration site in Gale Crater, a deep impact bowl on Mars' equator.

I await the discovery of fossils with keen anticipation now. If they find simple organisms, that would be good. I’d really love it if they found evidence of a more complex creature, but I expect that’s just me wildly speculating. Current thought is that micro organisms would be the most prevalent form of life in the universe, so that is what is being looked for.

If the rover had landed on Earth today, it might never have found fossils. Even if they are there, it's going to take one hell of a stroke of luck to find them near the surface.

I await the discovery of fossils with keen anticipation now.

Fossils can be the impressions left behind by an organism - not just the actual organism itself. Leaf imprints, for example.