'Rock stripes' on Mars could suggest actions of waterWashington, Feb 18 (AZINS) NASA's Mars rover Opportunity, which reached its 5,000th Martian day, or sol, this week has observed possible "rock stripes" that could suggest actions of water, wind or other processes on the Red Planet.

The ground texture seen in recent images from the rover resembles a smudged version of very distinctive stone stripes on some mountain slopes on Earth that result from repeated cycles of freezing and thawing of wet soil, NASA said in a statement this week.

But it might also be due to wind, downhill transport, other processes or a combination, it added.

"It's mysterious. It's exciting. I think the set of observations we'll get will enable us to understand it," said Opportunity Deputy Principal Investigator Ray Arvidson of Washington University in St. Louis.

Opportunity, which landed on Mars in January 2004, is currently investigating a channel called "Perseverance Valley," which descends the inboard slope of the western rim of Endeavour Crater.

"Perseverance Valley is a special place, like having a new mission again after all these years," Arvidson said.

"We already knew it was unlike any place any Mars rover has seen before, even if we don't yet know how it formed, and now we're seeing surfaces that look like stone stripes," Arvidson added.

On some slopes within the valley, the soil and gravel particles appear to have become organised into narrow rows or corrugations, parallel to the slope, alternating between rows with more gravel and rows with less.

While the origin of the whole valley is uncertain, rover-team scientists are analysing various clues that suggest actions of water, wind or ice.

The prime mission of Opportunity was planned to last just 90 sols as NASA did not expect the rover to survive through a Martian winter.

But it did and the rover reached its 5,000th sol on February 16.

A Martian "sol" lasts about 40 minutes longer than an Earth day, and a Martian year lasts nearly two Earth years.

Opportunity has driven over 45 kilometres from its landing site to its current location and it has returned about 225,000 images so far, NASA said.