Rounded pebbles cemented together into telltale formations on Mars were created by a fast-flowing stream that may have been more than knee deep, scientists say.
An analysis of the rock formations, including the sizes and shapes of hundreds of pebbles, suggests that the stream that created them was between three and 90 centimetres deep and flowed at a rate of 25 to 75 centimetres per second, reported a paper published Thursday online in the journal Science.
The researchers believe the pebbles were carried at least several kilometres by flowing water.
"These rocks provide a record of past conditions at the site that contrasts with the modern Martian environment, whose atmospheric conditions make liquid water unstable," said Linda Kah, a planetary sciences researcher at the University of Tennessee in Knoxville, who co-authored the paper, in a statement.
"Finding ancient river deposits indicates sustained liquid water flows across the landscape, and raises prospects of once habitable conditions."
Images of the rock formations, known as conglomerates, were captured by the Mars Curiosity rover along the first 275 metres of the route it travelled from its landing site to its current location at an area called Yellowknife Bay.
NASA had previously released photos of the rock formations and suggested they were evidence of a streambed. The detailed analysis published Thursday confirms that hunch.
Conglomerates are formed when pebbles clump together with mud and sand, which are carried along with them by the water. Sometimes the clumps can harden into a concrete-like substance.
The shapes and sizes of hundreds of the pebbles in the images were analyzed by a large team led by Rebecca Williams at the Planetary Science Institute in Tucson, Ariz., in order to infer the water conditions that would have formed them.
The rock formations add to growing evidence that Mars was once had the conditions necessary to support life.
Minerals found on Mars have also provided evidence that the Red Planet was once far wetter than it is today, and the area being explored by the rover, Curiosity, contains geological features that appear to have been created by flowing water.