By Jonathan Amos

Science correspondent, BBC News

A channel connects two depressions in this MRO image

New images of Mars suggest the Red Planet had large lakes on its surface as recently as three billion years ago. The evidence comes from Nasa's Mars Reconnaissance Orbiter (MRO) which spied a series of depressions linked by what look like drainage channels. Scientists tell the journal Geology that the features bear the hallmarks of being produced by liquid water. But they appear to have formed much later in Mars' history than many thought possible, the researchers add. The team, from Imperial and University Colleges London, studied pictures of several flat-floored depressions located above Ares Vallis, a giant gorge running some 2,000km across Mars' equator. The hollows are about 20km in diameter. Scientists had previously ascribed their formation to the slumping of the ground as ice in the soil was lost to Mars' thin atmosphere almost four billion years ago in the process of sublimation (in which the ice turns directly from a solid into a vapour). But the detail in the MRO pictures has allowed the Imperial-UCL team to trace a series of channels that connect the depressions. The group says these channels could only be formed by running water, and not by ice turning directly into gas. Please turn on JavaScript. Media requires JavaScript to play. The scientists' ageing of the region, which on bodies like Mars is done by counting craters, suggests the features formed during the so-called Hesperian Epoch on the Red Planet. "The exciting thing is that this occurred at a time when Mars is thought to have been cold and dry and [liquid] water wasn't stable at the surface," Dr Sanjeev Gupta from Imperial College London told BBC News. The researchers propose that Mars may have experienced bouts of short-lived warming during this epoch that were caused perhaps by volcanic activity, meteorite impacts, or even shifts in the planet's orbit. This could have provided both the warmth to melt ice in the soil and the pressure needed in the atmosphere to maintain liquid water on the surface. "We don't really understand what caused this transient episode," Dr Gupta explained. "We have different hypotheses. Maybe local conditions generated an atmosphere creating a minor greenhouse effect that allowed these lakes to exist. We don't know how long they existed for, but it's exciting nonetheless that we see [evidence of] liquid water." The conditions would have made it possible for the depressions to fill with meltwater and even overflow, cutting channels as the liquid ran from a higher basin to a lower one. "This provides another environment - another place to go and look for microbial life," said Dr Gupta. "This would be fossil life. This is somewhere we hadn't perhaps considered as a place to go." Jonathan.Amos-INTERNET@bbc.co.uk



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