Since landing on Mars, NASA's Curiosity rover has brought us plenty of stunning views of the Martian surface, but now the machine has stumbled across something possibly even more exciting — an ancient stream bed. Researchers were able to determine that the rocks in the bed were carried by water based on their size and shape, and NASA estimates that the stream was anywhere from hip- to ankle-deep, moving at a pace of around three feet per second. While evidence of water on the red planet already exists, this finding is the first of its kind. "This is the first time we're actually seeing water-transported gravel on Mars," says William Dietrich, from the University of California, Berkeley. "This is a transition from speculation about the size of stream bed material to direct observation of it."

"A long-flowing stream can be a habitable environment."

The research team may eventually use the information from Curiosity to attempt to determine the elemental composition of the gravel in order to learn more about what the stream was like. And while running water isn't proof, it does raise further questions about the possibility of life on the planet. "A long-flowing stream can be a habitable environment," admits Mars Science Laboratory Project Scientist John Grotzinger. Curiosity's next destination is Mount Sharp, where researchers hope that the area's clay surface and sulfate minerals may have preserved some of the carbon-based organic chemicals that can be an important ingredient for life.