By Dr David Whitehouse
BBC News Online science editor
Scientists are speculating that the Spirit rover on Mars may have already found evidence that water once flowed at its landing site, Gusev Crater.
It is a rock, but what caused the pitting?
One of its cameras may have detected small amounts of a mineral in the soil that suggests water was once present.
Spirit's mini-thermal emission spectrometer is sensitive to infrared light and is designed to analyse rocks.
Mars scientist Phil Christensen says the presence of the mineral - carbonate - suggests some rock formed in water.
"It might be that this carbonate actually does indeed have to do with the water that we came to look for," he told the Voice of America. But he remains cautious: Spirit must make more detailed measurements.
He also said there could be an alternative interpretation - the carbonate could have come from atmospheric dust.
When the six-wheeled robotic explorer starts its trek across the Martian surface its close-up views of the rocks may provide the definitive answer.
Cornell University scientist Steven Squyres says if the carbonate is found in wind-blown dust, it probably came from elsewhere on the planet and is not evidence of a watery past for Gusev Crater.
Spirit's infrared vision sorts the rocks out
But if the soil that contains carbonate is coarse, like water sediments, he adds, then it could be proof that the crater is an ancient lake bed.
"I think we're going to be chasing this carbonate story [for] weeks, months maybe," he said.
"What we can do as we start to head out across the countryside is we can look at different patches of soil and we can measure the carbonate abundance in different kinds of soil."
'It can't be mud'
Meanwhile, other scientists are scrutinising the sharpest pictures ever taken on Mars, showing a rock-strewn vista that stretches all the way to misty mesas and hills on the not-too-distant horizon.
According to mission scientist Jim Bell, also of Cornell University, the landing site is a scientific puzzle.
"As we zoom down to the foot of the lander we see one of the freshest rocks on the site; it's only a few tens of centimetres across, a relatively small rock, but it has some pits in it.
"Are they vesicles, are they volcanic features, is it a sedimentary rock, is it a metamorphic rock? These are the sort of questions we will be asking in the coming weeks."
Scientists are intrigued by the marks Spirit's airbags left on the surface. The soil shows an unusual cohesiveness, almost as if the soil grains were stuck together like mud.
Jim Bell says that they see "scratch marks from where the airbags were retracted and there are places where rocks were actually dragged through the soil and the soil was kind of stripped up and folded in some places in very interesting and quite alien textures".
Steve Squyres is also puzzled: "The way in which the surface has responded is bizarre. It looks like mud, but it can't be mud.
"We're going to have a real interesting time trying to figure this stuff out."