WASHINGTON, May 30 (Xinhua) -- Observations by U.S. Mars rover Curiosity have revealed areas with rounded pebbles which indicate a stream once flowed on the red planet, a study said Thursday.
The finding, published in the journal Science, represents the first on-site evidence of sustained water flows on the Mars landscape, and supports prospects that the planet could once have been able to host life, researchers from the Curiosity rover mission said.
The accomplishment comes from Curiosity's exploration during its first 100 "sols," or Martian days, the equivalent of about 103 days on the Earth, on the Mars surface last year.
During that time, Curiosity traveled about 400 meters from its landing site, examining multiple outcrops of pebble-rich slabs. The rover took high-resolution images of these pebbles at three locations known as Goulburn, Link and Hottah and then sent them back to the Earth for further analysis.
Researchers now said the pebbles' grain size, roundness and other characteristics suggest that they had been transported by water over long distances.
When rocks are worn by wind, they become angular and rough, but when they are moving in a mixture of flowing water and sand, the corners and edges of the rocks eventually become smooth and rounded, they explained.
"We could see that almost all of the 515 pebbles we analyzed were worn flat, smooth and round," said Asmus Koefoed, a research assistant in the Mars Group at the University of Copenhagen in a statement.
There are both light and dark rocks in various shades and colors, much like the original rocks on Earth and Mars, the researchers said.
Knowledge of the pebbles' size and shape led the researchers to make a more direct estimate of how fast water there moved and how deep it was.
"In order to have moved and formed these rounded pebbles, there must have been flowing water with a depth of between 10 cm and 1 meter and a flow rate of about 1 meter per second -- or 3.6 kilometers per hour -- slightly faster than a typical natural Danish stream," said Morten Bo Madsen, head of the Mars research group at the University of Copenhagen.
Apart from running water, investigations with Curiosity since its landing last August have shown that there were a pH-neutral environment and minerals that microbial life could use for nourishment.
Curiosity has thus achieved one of its objectives, namely to investigate whether there are areas on Mars that could have been habitable for microbial life, the researchers said.