Nasa has found evidence of an ancient flowing stream on the surface of Mars.
The $2.5bn Mars rover Curiosity has discovered ancient stream-bed gravels, which are the first of their kind discovered anywhere other than Earth.
Nasa said it was the first potentially habitable environment discovered on the planet.
The gravel was found between the north rim of Gale Crater and the base of Mount Sharp, a mountain inside the crater.
The rocks are between the size of a grain of sand and a golf ball, which means they could not have been carried in any other way than water flow.
Pictures taken from an orbiting craft above the surface show a fan of material 'washed' down from a channel above it.
The high number of channels in the fan "suggests flows continued or repeated over a long time, not just once or for a few years." Nasa said.
Mars Curiosity Pictures
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Open Image ModalIn this image released by NASA on Monday, Aug. 27, 2012, a chapter of the layered geological history of Mars is laid bare in this color image from NASA's Curiosity rover showing the base of Mount Sharp, the rover's eventual science destination. The image is a portion of a larger image taken by Curiosity's 100-millimeter Mast Camera on Aug. 23, 2012. Scientists enhanced the color in one version to show the Martian scene under the lighting conditions we have on Earth, which helps in analyzing the terrain. The pointy mound in the center of the image, looming above the rover-sized rock, is about 1,000 feet (300 meters) across and 300 feet (100 meters) high. (AP Photo/NASA/JPL-Caltech/MSSS) (credit:AP)
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Open Image ModalIn this image released by NASA on Monday, Aug. 27, 2012, an image taken by the Mast Camera (MastCam) highlights the geology of Mount Sharp, a mountain inside Gale Crater, where the rover landed. Prior to the rover's landing on Mars, observations from orbiting satellites indicated that the lower reaches of Mount Sharp, below the line of white dots, are composed of relatively flat-lying strata that bear hydrated minerals. Those orbiter observations did not reveal hydrated minerals in the higher, overlying strata. The MastCam data now reveal a strong discontinuity in the strata above and below the line of white dots, agreeing with the data from orbit. Strata overlying the line of white dots are highly inclined (dipping from left to right) relative to lower, underlying strata. The inclination of these strata above the line of white dots is not obvious from orbit. This provides independent evidence that the absence of hydrated minerals on the upper reaches of Mount Sharp may coincide with a very different formation environment than lower on the slopes. The train of white dots may represent an "unconformity," or an area where the process of sedimentation stopped. (AP Photo/NASA/JPL-Caltech/MSSS) (credit:AP)
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Open Image ModalIn this image released by NASA on Monday, Aug. 27, 2012, An image from a test series used to characterize the 100-millimeter Mast Camera on NASA's Curiosity rover taken on Aug. 23, 2012, looking south-southwest from the rover's landing site. The 100-millimeter Mastcam has three times better resolution than Curiosity's 34-millimeter Mastcam, though it has a narrower field of view. The gravelly area around Curiosity's landing site is visible in the foreground. Farther away, about a third of the way up from the bottom of the image, the terrain falls off into a depression (a swale). Beyond the swale, in the middle of the image, is the boulder-strewn, red-brown rim of a moderately-sized impact crater. Farther off in the distance, there are dark dunes and then the layered rock at the base of Mount Sharp. Some haze obscures the view, but the top ridge, depicted in this image, is 10 miles (16.2 kilometers) away. Scientists enhanced the color in one version to show the Martian scene under the lighting conditions we have on Earth, which helps in analyzing the terrain. (AP Photo/NASA/JPL-Caltech/MSSS) (credit:AP)
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Open Image ModalIn this image released by NASA on Monday, Aug. 27, 2012, a photo taken by the Mast Camera (MastCam) highlights the geology of Mount Sharp, a mountain inside Gale Crater, where the rover landed. Prior to the rover's landing on Mars, observations from orbiting satellites indicated that the lower reaches of Mount Sharp, below the line of white dots, are composed of relatively flat-lying strata that bear hydrated minerals. Those orbiter observations did not reveal hydrated minerals in the higher, overlying strata. The MastCam data now reveal a strong discontinuity in the strata above and below the line of white dots, agreeing with the data from orbit. Strata overlying the line of white dots are highly inclined (dipping from left to right) relative to lower, underlying strata. The inclination of these strata above the line of white dots is not obvious from orbit. This provides independent evidence that the absence of hydrated minerals on the upper reaches of Mount Sharp may coincide with a very different formation environment than lower on the slopes. The train of white dots may represent an "unconformity," or an area where the process of sedimentation stopped. (AP Photo/NASA/JPL-Caltech/MSSS) (credit:AP)
Mike Malin, John Grotzinger, Paul Mahaffy, Chad Edwards(05 of05)
Open Image ModalChad Edwards, chief telecommunications engineer for NASA's Mars Exploration Program, far right, explains how an international network of telecommunications relay orbiters bring data back from Mars during a briefing at NASA's Jet Propulsion Laboratory in Pasadena, Calif., Monday, Aug. 27, 2012. Curiosity already has returned more data from the Martian surface than have all of NASA's earlier rovers combined. Relay robotic spacecraft orbiting Mars seen on graphic: NASA's Mars Reconnaissance Orbiter, MRO, and Mars Odyssey ODY. Scientists from left: Mike Malin, imaging scientist for the Mars Science Laboratory, John Grotzinger, MSL project scientist, California Institute of Technology, Paul Mahaffy, NASA Goddard Space Flight Center, and Dr. Edwards. (AP Photo/Damian Dovarganes) (credit:AP)
The water was once at least ankle-deep in the area - and that could indicate one of the key conditions for life once existed on the planet.
"This is the first time we're actually seeing water-transported gravel on Mars," said Curiosity science co-investigator William Dietrich.
"From the size of gravels it carried, we can interpret the water was moving about 3 feet per second, with a depth somewhere between ankle and hip deep."
The rover is currently driving towards an area where many types of rocks and landscapes converge known as Glenelg, as part of its two-year mission to look for evidence of life on the planet.
"A long-flowing stream can be a habitable environment," said Mars Science Laboratory Project Scientist John Grotzinger. "It is not our top choice as an environment for preservation of organics, though. We're still going to Mount Sharp, but this is insurance that we have already found our first potentially habitable environment."