Nasa scientists have discovered what might be the most compelling evidence yet of rocks formed in water on Mars.
Scientists say the layered rocks could be sedimentary or volcanic
The Opportunity rover has sent back pictures of rock slabs that appear to contain thin layers, say researchers.
On Earth, this feature is suggestive of sedimentary rocks that are the product of material deposited by water or wind.
The layers are as thin as a centimetre in places which indicates they are unlikely to be old lava flows, but Nasa cautioned further analysis was needed.
Mission scientist Dr Andrew Knoll told a news conference that if the rocks were in fact sedimentary and not volcanic, they were probably formed by water and not by wind-borne sediments.
"When you look at the layers, these are really, really thin layers and that places some constraints on what this could be. These aren't lava flows, they're something we haven't seen before," said Steve Squyres, payload principal investigator.
"We are about to embark on what is arguably going to be the coolest geology field trip in history," he added.
But the rover will need to drive up to the rocks to examine them closely with its science instruments to rule out a volcanic origin.
The rover will be looking for several pieces of evidence. If the rocks are sedimentary, they may well display inclined surfaces. This "cross-bedding" is a characteristic feature of sedimentary rocks - but not exclusively so.
The rock would likely show evidence of chemical alteration and its constituent particles should be rounded and "cemented" in place, said Dr Knoll. The rover may be able to determine whether this is the case using its microscopic imaging equipment.
The rover may also find these rocks are simply the result of ash-fall from an ancient volcano. "If that's the case, so be it," said Dr Knoll.
Meanwhile, scientists said that Opportunity was losing some of its power.
Mission manager Jim Erickson said the probable cause was a thermostatically-controlled heater in the shoulder joint of the rover's arm, which is turning on automatically in response to drops in temperature.
"Normally, this is enabled by the ground and a thermostat on the side of the rover determines what the air temperature is, and if it is cold enough, it turns on the heater in this arm to keep it up to a temperature for operation," said Mr Erickson.
Opportunity's airbags made very clear imprints in the Martian soil
"Now, we don't normally always want it on because we aren't normally always operating the arm. Right now we're believing that it's going to be continuously on whenever it's cold enough."
On Wednesday, mission controllers will begin the process of lifting the rover up off its pad and getting it ready to drive on to Mars. After the rover has been raised, the front wheels will be turned outwards and checks carried out to make sure that all the components are securely in place.
Development manager Jennifer Trosper said Opportunity's twin rover Spirit was recovering well from its malfunction on Thursday.
She added that engineers had now effectively ruled out a fault with the rover's high gain antenna as the root of the problem. This means a problem with the file system on Spirit is now the most likely cause.