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Last Updated: Thursday, 19 August, 2004, 16:20 GMT 17:20 UK
Mars hill find hints at wet past
Spirit now has a scenic view of Gusev Crater from the Columbia Hills

The US space agency's robotic rover Spirit has found more evidence that water washed and altered the rocks it has been studying on the Red Planet.

The vehicle is examining the geology of an outcrop at Columbia Hills named Clovis, which shows chemical and physical signs of alteration by water.

Sprit's twin, Opportunity, has now completed its transect of rocks in a large crater on the other side of Mars.

Nasa says both rovers continue to work well as they move into Mars' winter.

Clearer picture

"We have evidence that interaction with liquid water changed the composition of this rock," said Professor Steve Squyres, of Cornell University in Ithaca, US, and principal investigator for the rover science payload.

Here, we have a more thorough, deeper alteration, suggesting much more water
Prof Steve Squyres, Cornell University
Clovis is part of the bedrock in Gusev Crater and sits on a part of the Columbia Hills about 9m (30 ft) above the plain.

Findings recently published by the rover team in the journal Science suggested a lava flow had covered up much of the evidence of past water on the plains of Gusev Crater - which is thought from other evidence to have once held a lake.

"This is different from the rocks out on the plain, where we saw coatings and veins apparently due to effects of a small amount of water. Here, we have a more thorough, deeper alteration, suggesting much more water," Professor Squyres added.

Clovis, Nasa
Clovis has a softer texture than the basalts on Gusev's plain
"To really understand the conditions that altered Clovis, we'd like to know what it was like before the alteration. We have the 'after', now we want the 'before'. If we're lucky, there may be rocks nearby that will give us that."

Analysis of Clovis' surface and interior with the Alpha Particle X-ray Spectrometer (APXS) instrument on the rover's robotic arm reveals relatively high levels of bromine, sulphur and chlorine in the rock - a chemical signature suggestive of alteration by water.

"This is also a very soft rock, not like the basaltic rocks seen back on the plains of Gusev Crater. It appears to be highly altered," said mission scientist Dr Doug Ming.

Rock bottom

On the other side of Mars, Opportunity has completed a transect through layers of rock exposed in the southern inner slope of the 130m-wide Endurance Crater.

The rocks examined range from outcrops near the rim down through progressively older layers to the lowest accessible outcrop, dubbed Axel Heiberg.

It was thought the rover might not be able to drive out of the crater if it passed beyond a gradient on the slope of 30 degrees. Examining the bedrock in Endurance Crater will fill in more of the history of liquid water at Meridiani Planum.

Popcorn, Nasa
The "popcorn" has a rougher texture than the blueberries
The concentration of the element chlorine in Axel Heiberg increased up to threefold in the rock's middle layers. Magnesium and sulphur declined nearly in parallel with each other in older layers, suggesting those two elements may have been dissolved and removed by water.

Tiny spheres called concretions, or "blueberries", made of the iron-bearing mineral grey haematite are also abundant at Meridiani and are also thought to have formed through the action of water.

But blueberries in Axel Heiberg have a rougher texture and are more variable in size than others previously seen by the rover. Some members of the rover team have dubbed them "popcorn".

The popcorn is also the same colour as the rock, rather than grey like the blueberries.

"We've noticed that in some cases where these are eroding, you can see a regular blueberry or a berry fragment inside," said Zoe Learner, a science team collaborator from Cornell University.

She said one possibility was that a water-related process had added a coarser outer layer to the blueberries, adding: "It's still really a mystery."

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