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Last Updated: Tuesday, 23 March, 2004, 19:20 GMT
Mars rover sits on ancient beach
Mars rover impression, Nasa
Nasa has two rovers on opposite sides of Mars
The US space agency has announced that its robotic Mars rover Opportunity is parked on what was once the shore of a salty Martian sea.

There is multiple evidence that the surface of Mars was awash with liquid water at some time in its past.

But the latest findings from Nasa's robot explorers on the Red Planet are fleshing out a picture of what Mars must have been like when it was wet.

Opportunity has been studying the rocks in a small crater since January.

If you have an interest in searching for fossils on Mars, this is the first place you want to go
Ed Weiler, Nasa
Earlier in March, scientists announced that rocks at Opportunity's landing site at Meridiani Planum once had water seep slowly through them.

"What's happened since then is we have found what I believe to be strong evidence that the rocks themselves were sediments that were laid down in liquid water," Professor Steve Squyres told a told a news conference in Washington, US.

Wet environment

The earlier finding suggested a large quantity of liquid water had existed at Meridiani Planum. The latest announcement confirms that there was either a sea, or a series of pools on the surface.

LAID DOWN IN WATER
There is persuasive evidence for the rocks' watery past

"It's a fundamental distinction. It's like the difference between water you can draw from a well and water you can swim in," said Professor Squyres.

However, Professor Squyres said it was possible that the rocks were laid down by water under a sheet of ice.

Tuesday's announcement is significant because it lends greater weight to the hypothesis that Mars could have been a habitat for microbial life in the past.

"If you have an interest in searching for fossils on Mars, this is the first place you want to go," said Dr Ed Weiler, Nasa associate administrator.

But scientists agreed that only future sample return missions would be able to resolve the question of whether life was once present on the planet.

The evidence comes from analysis of 30cm of a rocky outcrop in Eagle Crater, Opportunity's landing site. Rocks in this outcrop are finely layered, one of the key characteristics of sedimentary rocks.

Water flow

Layered rocks can be formed by volcanic activity. But Nasa scientists said the presence of cross-bedding in the Martian outcrop, inclined discontinuities between different layers of the rock confirmed that the Martian rocks were sedimentary.

Professor Steve Squyres, principal scientific investigator for the rovers, added that high quantities of the element bromine in the rocks was one of the key pieces of evidence suggesting the outcrop was once a shore.

On Earth, this is characteristic of rocks that have had sea water evaporate from them. Salts in the rock pointed to evaporation. This suggested a past environment where water came and went.

Opportunity looks back at a job well done

However, it is not known for how long the region at Meridiani Planum was a wet environment.

Sedimentary rocks can be formed through the action of wind. But Dr Dave Rubin, of the US Geological Survey (USGS), told journalists that the form of the cross-bedding matched best with sediments laid down in water.

The researchers said the water could have been flowing at a speed of 10 to 50cm per second.

The announcement earlier this month that Opportunity's landing site at Meridiani Planum was once "drenched" in water came from the discovery of very high concentrations of sulphur salts in the outcrop.

On Earth, rocks with as much salt as these Mars rocks have either formed in water or, after formation, have been highly altered by long exposure to water.

Scientists do not know how long the water was present at Meridiani Planum. Neither do they know how deep the bodies of water were.




WATCH AND LISTEN
The BBC's David Shukman
"Since January Nasa has deployed two rovers on Mars"



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