US space agency NASA’s Curiosity rover mission on Mars has found evidence a stream once ran across the area on the planet where the rover is driving, NASA said.

This evidence, images of rocks containing ancient streambed gravels, is the first of its kind, the space agency added.

Scientists are studying the images of stones cemented into a layer of conglomerate rock. The sizes and shapes of stones offer clues to the speed and distance of an ancient stream’s flow.

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

"Plenty of papers have been written about channels on Mars with many different hypotheses about the flows in them. 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 lies between the north rim of Gale Crater and the base of Mount Sharp, a mountain inside the crater.

The slope of Mount Sharp in Gale Crater remains the Curiosity‘s main destination. Clay and sulfate minerals detected there from orbit can be good preservers of carbon-based organic chemicals that are potential ingredients for life, NASA said.

"A long-flowing stream can be a habitable environment," said Mars Science Laboratory project scientist John Grotzinger of the California Institute of Technology in Pasadena, California. "It is not our top choice as an environment for preservation of organics, though. We’re still going to Mount Sharp, but this is insurance that we have already found our first potentially habitable environment."

During the two-year prime mission of the Mars Science Laboratory, researchers will use Curiosity‘s 10 instruments to examine whether areas in Gale Crater have ever offered environmental conditions favourable for microbial life.

Curiosity landed on Mars 41 days go.