By Katie Franklin and AgenciesNasa says its Mars rover Spirit has discovered "the best evidence yet" of a past habitable environment on the planet's surface.
Spirit has been exploring a plateau called Home Plate, where it discovered silica-rich soil in May.
Researchers are now trying to determine what produced the patch of nearly pure silica - the main ingredient of window glass.
They believe the deposits came from an ancient hot-spring environment or an environment called a fumarole, in which acidic steam rises through cracks.
On Earth, both of these types of settings teem with microbial life, said rover chief scientist Steve Squyres.
"Whichever of those conditions produced it, this concentration of silica is probably the most significant discovery by Spirit for revealing a habitable niche that existed on Mars in the past," he said.
"The evidence is pointing most strongly toward fumarolic conditions, like you might see in Hawaii and in Iceland.
"Compared with deposits formed at hot springs, we know less about how well fumarolic deposits can preserve microbial fossils. That's something needing more study here on Earth."
Spirit and its twin rover Opportunity have remained on Mars for much longer than originally planned.
Their mission has been extended five times since they landed on opposite sides of the planet in January 2004.
But the rovers faced their biggest challenge yet this summer, when a series of dust devils blanketed their solar panels and limited their movement.
Winds managed to clean off Opportunity, but Spirit is still covered in gunk and working at 42 per cent capacity.
Scientists said Spirit was now facing a face against time to reach a resting spot for winter.
"Spirit is going into the winter with much more dust on its solar panels than in previous years," said Nasa's John Callas, project manager for the rovers.
"The last Martian winter, we didn't move Spirit for about seven months. This time, the rover is likely to be stationary longer and with significantly lower available energy each Martian day."