Mars rover Curiosity found rocks on the Martian surface that are too big to be carried by wind, so researchers suppose water must have existed on Mars at one time. Over the next two years, Curiosity will continue its search for signs of ancient microbial life on Mars.
The NASA rover Curiosity has beamed back pictures of bedrock that suggest a fast-moving stream, possibly waist-deep, once flowed on Mars — a find that the mission's chief scientist called exciting.
There have been previous signs that water existed on the red planet long ago, but the images released Thursday showing pebbles rounded off, likely by water, offered the most convincing evidence so far of an ancient streambed.
There was "a vigorous flow on the surface of Mars," said chief scientist John Grotzinger of the California Institute of Technology. "We're really excited about this."
The discovery did not come as a complete surprise. NASA decided to plunk Curiosity down inside Gale Crater near the Martian equator because photos from space hinted that the spot possessed a watery past. The six-wheeled rover safely landed Aug. 5 after a nail-biting plunge through the Martian atmosphere. It's on a two-year, $2.5 billion mission to study whether the Martian environment could have been favorable for microbial life.
Present day Mars is a frozen desert with no hint of water on its radiation-scarred surface, but geological studies of rocks by previous missions suggest the planet was warmer and wetter once upon a time.
The latest evidence came from photos that Curiosity took revealing rounded pebbles and gravel — a sign that the rocks were transported long distances by water and smoothed out.