U.S. scientists say pictures sent back from the Curiosity probe on Mars show clear evidence that water — essential for life — once flowed on the planet.

The pictures of Curiosity's landing site in a Martian crater show pebbles and stones driven into the rock. The scientists say the stones are smooth and round, a sign that they were driven by flowing water — possibly a waist-deep river teeming with microorganisms.

The scientists say this area of Mars may have at one time been habitable.

The Curiosity probe landed on Mars last month for a two-year mission to explore the surface of the red planet.