A fresh analysis by NASA's Curiosity rover confirms a stream once ran through Gale Crater on Mars.
During a pit stop last year, Curiosity came upon hundreds of smooth, round pebbles that look strikingly similar to deposits in river banks on Earth.
Scientists believe the rover rolled onto an ancient streambed, but needed to study the stones in more detail. So Curiosity snapped high-resolution pictures and fired its laser at several pebbles to analyze the chemical makeup.
Researchers say the roundness of the stones was shaped by a fast-flowing stream that probably was ankle to waist-deep. Curiosity landed in the crater near the equator last summer.
The analysis appears in Friday's issue of the journal Science.