Mars pebbles prove water history

The team only has pictures from the rover's main cameras. Attempts will be made to get close-up, high-resolution imagery of Gale's conglomerates in the weeks ahead using the Mahli 'hand lens'. Photo: BBC

Scientists now have definitive proof that many of the landscapes seen on Mars were indeed cut by flowing water.

The valleys, channels and deltas viewed from orbit have long been thought to be the work of water erosion, but it is NASA's latest rover, Curiosity, that has provided the "ground truth".

Researchers report its observations of rounded pebbles on the floor of the Red Planet's 150km-wide Gale Crater.

Their smooth appearance is identical to gravels found in rivers on Earth.

Rock fragments that bounce along the bottom of a stream of water will have their edges knocked off, and when these pebbles finally come to rest they will often align in a characteristic overlapping fashion.

Curiosity has pictured these features in a number of rock outcrops at the base of Gale Crater.

It is confirmation that water has played its part in sculpting not only this huge equatorial bowl but by implication many of the other landforms seen on the planet.

"For decades, we have speculated and hypothesised that the surface of Mars was carved by water, but this is the first time where you can see the remnants of stream flow with what are absolutely tell-tale signs," said Rebecca Williams from the Planetary Science Institute, US.

The American space agency first announced the discovery of the pebbles in September last year, barely seven weeks after Curiosity had landed in Gale.

Researchers have since been studying the robot's pictures in more detail and have now written up a report for Science magazine - the first scholarly paper from the surface mission to make it into print; and the study reinforces the initial interpretation.