Nasa says it has found "compelling" evidence that liquid water flowed recently on the surface of Mars.
The finding adds further weight to the idea that Mars might harbour the right conditions for life.
The appearance of gullies, revealed in orbital images from a Nasa probe, suggests that water could have flowed on the surface in the last few years.
But some scientists think these fresh gullies could also have been cut by liquid carbon dioxide (CO2).
The latest research emerged when Nasa’s Mars Global Surveyor (MGS) spacecraft spotted gullies and trenches that scientists believed were geologically young and carved by fast-moving water coursing down cliffs and steep crater walls.
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