By Jonathan Fildes

Science and technology reporter, BBC News, San Francisco



Water once flowed through fractures in layered rocks

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Shots of the deep valley Candor Chasma show light coloured areas of rock where water could have flowed.

These "haloes" surround fractures in the martian bedrock which provide a promising target in the search for evidence of past life on the planet.

The images, published in the journal Science, were taken by a camera aboard Nasa's Mars Reconnaissance Orbiter.

"It lends support to the idea that a substantial body of groundwater existed on Mars in the past and may still persist to the present day," said Professor Stephen Clifford of the Lunar and Planetary Institute in Houston, Texas.

"The fact that there is such persuasive evidence of joints and fractures in the crust also suggests that this groundwater had the ability to flow enormous distances."

Huge rift

The High Resolution Imaging Science Experiment (HiRise) camera started its surveys in November 2006, eight months after Mars Reconnaissance Orbiter (MRO) entered orbit around the Red Planet.

It is best known for capturing stunning images of Nasa's robot explorers Spirit and Opportunity as they surveyed the planet from the ground.

Clay-like minerals are found in the Red Planet's Mawrth Vallis region

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The newly analysed images were taken last year. They show a snapshot of conditions in Candor Chasma, an area of the great Martian rift valley Valles Marineris.

This deep gash, the length of the United States, is up to seven times deeper than the Grand Canyon in places.

The images show a hilly landscape composed of alternating bands of light and dark coloured rock, suggesting the layers were deposited by regular cycles of water, wind or volcanic activity.

MARS RECONNAISSANCE ORBITER Reached Mars in March 2006 Two-year primary mission HiRise images are the clearest pictures of Mars Two detectors on the camera have developed problems

Writing in Science, the University of Arizona team said that similar features on Earth were "a clear indication of chemical interactions between fluids circulating within the fracture and the host rock".

Likely fluids include water, liquid carbon dioxide or a combination of the two, and may have flowed up from deep underground reservoirs.

Protected habitats

Previously, Europe's Mars Express mission had detected the signature of minerals altered by water in the Candor Chasma area.

The washed-out features, which have been "cemented" by minerals contained in the fluids, now stand proud from the valley walls.

They have been exposed by millions of years of erosion.

However, the fact that they were once underground has important implications for their ability to support life.

"The overlying areas of rock would have acted as a buffer against any harsh environmental conditions on Mars at that time," said Dr Chris Okubu of the University of Arizon and one of the authors on the paper.

"These areas would be nice protected areas for any biological processes to occur."

Erosion has uncovered rocks that were altered by the action of fluids

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Last year, Nasa's Mars Global Surveyor (MGS) spacecraft spotted gullies and trenches thought to have been recently carved by outpourings of water or liquid carbon dioxide.

Other features, including dried-up lake beds, springs and river valleys, lend further weight to the theory that the planet was once awash with water and could therefore have supported life.

Nasa is examining all of these sites as possible targets for future ground-based robotic missions.

However, the space agency says that if it has a chance of finding evidence of life on the planet, it must broaden its search.

"Following the water is a central aspect of exploration but the habitability issue also requires addressing the source of energy for life," said Dr David des Marais of the Nasa Ames Research Center.

Possible future targets include areas of the planet where the building blocks of life, such as carbon, can be found.

"We not only need to follow the water but we also need to follow the energy," said Dr des Marais.

The MRO findings were presented at the American Association for the Advancement of Science (AAAS) annual meeting in San Francisco, US.