Mars rover Curiosity has discovered evidence that a stream once ran across the area where the rover is driving on the Red Planet, NASA reported.

Scientists are studying the images of rocks containing ancient streambed gravels, which are the first evidence of that kind for the presence of water on Mars. The stones’ sizes and shapes offer ideas as to what the speed and distance of an old stream's flow could have been.

"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," Curiosity science co-investigator William Dietrich of the University of California, Berkeley, said as quoted by NASA.

"Plenty of papers have been written about channels on Mars with many different hypotheses about the flows in them,” he said. “This is the first time we're actually seeing water-transported gravel on Mars. This is a transition from speculation about the size of streambed material to direct observation of it.”

The finding site is situated between the north rim of Gale Crater and the base of Mount Sharp, a mountain inside the crater. The slope of Mount Sharp remains the rover's main destination.

During the rover's two-year mission, researchers will use the its ten science tools to assess if the selected field site inside Gale Crater has ever had environmental conditions suitable for microbial life.