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Striking new images of the Red Planet have raised hopes life could be found on Mars after all.
Scientists say they have photographic evidence that suggests liquid water may have been on the planet as little as five years ago.
Experts last night said Mars now appears more active than previously thought and the latest study shows why it is vital to continue to search for life on our planetary neighbour.
The first images of Mars' surface were taken in the 1960s and they suggested it was a dusty, cratered world rather like our Moon.
Now a new set of photographs has suggested that liquid water may have flowed on the planet a mere five years ago.
The discovery resulted from images taken by the Mars Global Surveyor which first recorded gullies and trenches that could have been made by fast-flowing water coursing down cliffs and crater walls.
Scientists in the USA decided to retake images of the gullies to search for any sign of recent activity.
Two of those originally photographed in 1999 and 2001 then photographed again in 2004 and 2005 showed changes consistent with water having flowed down the side of the crater. The discovery was made by scientists at the San Diego-based Malin Space Systems which operated a camera aboard the spacecraft.
Writing in the journal Science, the researchers led by Michael Malin said the properties and settings of the deposits in the gullies are consistent with water flow.
Expert Bruce Jakosky, of the University of Colorado at Boulder, said the study "underscores the importance of searching for life on Mars, either present or past.
"It's one more reason to think that life could be there," he added.
However Oded Aharonson of the California Institute of Technology urged caution over the findings.
He said the interpretation of the images as water activity was 'compelling' but only one explanation.
Until now the question of liquid water has mainly focused on ancient Mars and on the Martian north pole, where water ice has been detected.
Water along with a stable source of heat is essential for life to emerge.
Mars formed more than 4.5 billion years ago and scientists generally believe it went through an early wet and warm era that ended after 1.5 billion to 2.5 billion years, leaving the planet extremely dry and cold.
Water cannot remain a liquid for long because of subzero surface temperatures and low atmospheric pressure that would turn water into ice or gas.
But some studies have pointed to the possibility of liquid water flowing briefly on the surface through a possible underground water source that periodically shoots up.
