By Jonathan Amos
Science reporter, BBC News, San Francisco
Nasa says its robot rover Spirit has made one of its most significant discoveries on the surface of Mars.
Scientists believe a patch of ground disturbed by the vehicle shows evidence of a past environment that would have been perfect for microbial life.
The deposits were probably produced when hot spring water or steam came into contact with volcanic rocks.
On Earth, these are locations that tend to teem with bacteria, said rover chief scientist Steve Squyres.
"We're really excited about this," he told a meeting of the American Geophysical Union (AGU).
It has been described as a fortuitous discovery. Spirit has been driving with a broken wheel that constantly digs a trough as the vehicle trundles across the Martian landscape.
In May this year, scientists working on the mission noticed the churned-up soil had a much brighter appearance than usual.
The rovers were originally designed to complete a 90-day mission
Further investigation revealed it to be extremely rich in silica - the main ingredient of window glass - and Spirit was commanded to examine the ground and nearby rocks in detail for extra clues.
The researchers have now concluded that the bright material must have been produced in one of two ways.
One hypothesis is that Spirit is seeing hot-spring deposits produced when water dissolves silica at one location and then dumps it in another.
The classic example is a geyser.
The other idea being pursued by the rover team is that they have stumbled across a fumarole, where acidic steam rises through cracks in rocks and strips them of all of their mineral components, apart from silica.
Search for life
"The important thing is that whether it is one hypothesis or the other, the implications for the former habitability of Mars are pretty much the same," Professor Squyres explained to BBC News.
"You can go to hot springs and you can go to fumaroles and at either place on Earth it is teeming with life - microbial life.
"So this is a representation of what was in the past a local habitable environment."
The rover is not equipped to look for the evidence of life itself.
It was designed to read the geological conditions in front of it and tell the scientists what the environmental conditions might once have been like.
However, future missions will search for signs of chemical traces that could have been left by life.
The US Mars Science Laboratory to be launched in 2009 will be capable of doing this, as will ExoMars, the European rover to be launched early in the next decade.
Spirit is now camouflaged by the dust resting on its solar panels
Professor Squyres suggested such missions ought to keep an eye out for silica-rich deposits.
And he said any future efforts to return material to Earth laboratories might also like to target these kinds of deposits.
He said: "One of the great things about these deposits is that not only does the hot water provide an environment in which the microbes can thrive, but the precipitation of that silica entombs and preserves them.
"So you can get wonderfully preserved microbial microfossils."
Both Spirit and its sister rover, Opportunity, have continued to work long past their designed mission lives of 90 days.
Spirit has been on station for 1,400 days; Opportunity, which is on the far side of the planet, has been operational for a slightly shorter period of 1,379 days.
The vehicles will soon have to reduce their activities to preserve power during the reduced sunlight of the Martian winter.
For Spirit, the coming months are likely to be its most challenging to date.
Dust that has settled on its solar panels has significantly reduced the available electricity.
The panels are currently operating at 42% of maximum performance.
"By the time we get to the depth of the Martian winter, extrapolating what the dust accumulation is, we may have only about 30% performance," said John Callas, Nasa's project manager for the rovers.
Surviving the winter could be "dicey", he added.