Latest images from Nasa’s rover show trail of pebbles that were once dragged by water from crater rim to base of mountain

A shallow river once coursed through a great crater on Mars, according to the latest surface images, which suggest the dusty planet was more hospitable in ancient times.

Photographs from NASA’s Curiosity rover revealed clear signs of an ancient waterway winding from the northern edge of the Gale crater towards Mount Sharp, a mountain that rises 5km from the crater floor. The dried-up riverbed left a trail of pebbles and sand grains that over time became locked in rock. Their size and shape indicate a river that flowed at a metre per second at depths from ankle to waist deep.

The $2.5-billion mobile science laboratory began its work on Mars after a dramatic arrival last month in which the rover was winched to the surface from a spacecraft hovering overhead on rocket thrusters.

Curiosity is not searching for signs of past or present life, but for evidence that Mars was once habitable. Scores of earlier missions have found evidence of water on the red planet. Snapshots from spacecraft in orbit around Mars have beamed back images of lakes and gullies. The north and south poles are largely frozen water.

These pictures are the first to show stones and gravel that had been dragged along the Martian surface by a river. NASA geologists said the rounder shape of some of the pebbles suggests they had travelled long distances from above the crater rim.

The rover took the pictures with a telephoto camera on its central mast, downhill from a pattern of sediments called an alluvial fan created by several water streams perhaps billions of years ago. The stones vary from angular to smooth and range from golf ball-sized to grains of sand.

“The shapes tell you they were transported and the sizes tell you they couldn’t be transported by wind. They were transported by water flow,” said Rebecca Williams, who works on the Curiosity mission at the Planetary Science Institute in Arizona.

The rover’s destination is the slope of Mount Sharp, where clay and sulphate minerals have been spotted from orbit. These minerals can preserve the organic material that is crucial for life to thrive. “A long-flowing stream can be a habitable environment. It is not our top choice as an environment for preservation of organics, though,” said John Grotzinger, a project scientist and geologist on the mission at NASA’s Jet Propulsion Laboratory in California. — © Guardian Newspapers Limited, 2012