Water coursed through Martian hills

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Water coursed through Martian hills 15:57 08 October 04

NewScientist.com news service

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Polygonal cracks were carved on the "Escher" rock by a second wet

episode (image: NASA/JPL/Cornell)

 

The twin rovers on Mars - now reviving having survived the freezing

depths of Martian winter - have found new signs that water was once

plentiful on Mars.

 

The Mars rover Opportunity, which has nearly reached the bottom of

the 160-metre-diameter Endurance crater, has now found clear signs

that the area was drenched twice in the past, say NASA scientists.

 

Its twin, Spirit, which landed in what was thought to be an ancient

lakebed, had failed to find direct signs of water for months as it

crossed two kilometres of pure basaltic plains. The rovers landed on

the Red Planet in January 2004.

 

But since Spirit reached Mars' Columbia Hills in July, the rocks

have changed completely, showing evidence of abundant water. "We

have not seen a single fresh volcanic rock since we crossed the

line" from the plain to the hills, says rover chief scientist Steven

Squyres of Cornell University, US. "Every single one shows signs of

alteration by water."

 

The hills, Squyres explains, are like islands of much older rock

that stick up from the basaltic lava flows that later filled the

basin's depths, creating the smooth plains on which Spirit landed.

 

 

Soluble elements

 

 

By measuring the differences between the spectra of rocks on the

plains and those in the hills - using Spirit's Alpha Proton X-Ray

Spectrometer - the team discovered that the hillside rocks were all

significantly depleted in potassium, sulphur, bromine, and chlorine.

These are the most soluble elements and would be the first to go if

the rocks got wet.

 

Meanwhile, Opportunity has discovered one of the most interesting

rocks seen so far, says Squyres. While attempting to retreat from

the lower levels of the Endurance crater, the rover happened upon a

slab of rock - dubbed Escher by the team - which is fractured into

polygonal shapes (see image). It later found another rock, called

Earhart, showing similar shapes.

 

The rock itself is made up of fine layers that run across the

direction of the polygonal cracks, explains John Grotzinger, a

sedimentologist at MIT and a member of the science team.

 

 

Dry spells

The shapes suggest that the original sedimentary rock formed in a

lake or shallow sea - which then dried and solidified - before

undergoing a second wet episode and yet another drying out.

 

And large timescales may have been involved. The latter drying

episode must have come after the impact that formed the Endurance

crater, Grotzinger says.

 

Spectral data show that the concentration of chlorine, sodium and

other soluble elements is far heavier on the surface of the rock

than just beneath it, as it would be if the rock had been bathed in

water long after it formed, leaching out the salts.

 

Both rovers are now heading towards other stimulating targets.

Spirit is heading into the hills, towards what appear to be

extensive exposures of layered bedrock while Opportunity will

attempt to exit its deep crater by way of a steep, heavily layered

dark cliff of interesting looking rock.