Nasa's Curiosity rover has only been on the surface of Mars seven weeks but it has already turned up evidence of past flowing water on the planet.
The robot has returned pictures of classic conglomerates - rocks that are made up of gravels and sand.
Scientists on the mission team say the size and rounded shape of the pebbles in the rock indicate they had been transported and eroded in water.
Researchers think the rover has found a network of ancient streams.
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