Image: Candor Chasma
NASA / JPL / Univ. of Ariz. LPL
Ridges of light-toned bedrock known as "halos" remain behind as the surrounding bedrock erodes away around linear fractures in a Martian canyon known as Candor Chasma. Such features offer clues about past fluid flow and geochemical conditions within the subsurface. This image is about a half-mile (1 kilometer) across.
By Alan Boyle Science editor
updated 2/15/2007 5:45:07 PM ET 2007-02-15T22:45:07

Light-colored ridges that run through one of Mars' biggest canyons appear to serve as the fossil record for liquid water that seeped through the rock long ago — and may well point to places where water still exists in liquid form deep underground, scientists reported Thursday.

If life ever existed on Mars, such canyons would be a good place to look for the evidence, said Chris Okubo, a researcher at the University of Arizona's Lunar and Planetary Laboratory. "These areas would be nice protected areas for biological processes to go on," he told reporters here at the annual meeting of the American Association for the Advancement of Science.

The fresh findings from NASA's Mars Reconnaissance Orbiter, or MRO, add to discoveries made by the Opportunity and Spirit rovers and the now-defunct Mars Global Surveyor , as well as the still-operating Mars Odyssey and Mars Express orbiters — all hinting at the existence of liquid water on Mars billions of years ago, and perhaps the persistence of subsurface water even today.

Astrobiologists regard the presence of liquid water as one of the three requirements for life as we know it — along with nutrients and an energy source. For years, NASA has been using a "follow the water" strategy to look for evidence of Martian life, and now scientists say it's time to follow the food and energy as well. As a result, the search for life is likely to increasingly focus on the Martian subsurface, or on canyons and other places where the planet's geological depths are exposed.

The western Candor Chasma region, part of Mars' vast and deep Valles Marineris canyon system, served as the focus for observations by MRO's High Resolution Imaging Science Experiment, or HiRISE. A research paper about the observations — written by Okuba and the University of Arizona's Alfred McEwen, HiRISE's principal investigator — appears in Friday's issue of the journal Science.

Focus on Martian ‘halos’
When they studied Candor Chasma, Okubo and McEwen were intrigued to find a network of linear fractures, called joints, that criss-crossed the alternating layers of light and dark rocks. Dark sand dunes swept over most of the terrain, but some of the fractures were flanked by light-toned bedrock that appeared to rise above their wind-eroded surroundings. The researchers called those light-toned ridges "halos," and said they may hold the key to understanding the area's geological history.

They set out this scenario for the creation of the halos: Millions or even billions of years ago, the surface level was significantly higher than it is today, and covered with fluid — either water, or liquid carbon dioxide, or a combination of the two. The fluid seeped through the fractures in the bedrock, and minerals in the fluid served to bleach and strengthen the rock around the fractures.

As time went on, the fluid retreated deeper into the planet's interior. Martian winds eroded the walls and floors of the canyon — but the strengthened rock was more resistant to erosion, and thus the ridgelike halos now stand above their darker surroundings.

Okubo said this halo effect is seen on Earth — for example, on the Colorado Plateau.  "On Earth, bleaching of rock surrounding a fracture is a clear indication of chemical interactions between fluids circulating within the fracture and the host rock," he and McEwen wrote in the Science paper.

David Des Marais, an astrobiologist and Mars specialist at NASA's Ames Research Center, told that there could be alternate explanations for the halos. One example would be volcanic dikes, which are created on Earth when magma rises through fissures in the crust. But Des Marais said the explanation proposed in the Science paper was the "leading interpretation" of the orbiter data.

Following the water ... to life below?
Although the researchers acknowledged that the fluid flow might have involved some form of carbon dioxide, they favored the view that the halos were created by chemical reactions involving mineral-laden water from an underground reservoir, circulating through the fractures in the rock.

Okuda said such reactions would require "more than weeks or months," and most likely many years. That would support the view that water was present in a liquid state, at least beneath the Martian surface, long enough to support life.

Other researchers have cited evidence indicating that for most of its history, Mars was too cold and too dry to support life. But Stephen Clifford, a planetary scientist at the Lunar and Planetary Institute, said it was important to make a distinction between surface conditions and the environment deep beneath the surface.

"Liquid water could persist throughout Martian geologic history at depth," he said. "Early on, we think it may have existed at much shallower depths than it does today, simply because the temperature environment at Mars has changed."

The features seen by HiRISE appear to point to episodes when liquid water rose up through the fractures in the rock — but as time went on, the water "simply retreated deeper into the crust," Clifford said.

Far beneath the surface, Martian organisms couldn't use sunlight to power their biological processes — but on Earth, scientists have found microbes deep beneath the surface that use chemical reactions to produce energy. "If life is present on Mars now, it almost certainly has to be something that is not photosynthetic life," said Tori Hoehler, an astrobiologist at NASA's Ames Research Center.

But that's a big if, Des Marais said: "It's just belief at this point."

Next steps in the search
Okubo and his colleagues are looking for the halo effect in other regions of Mars, using the unprecedented capabilities of the HiRISE camera. Clifford said the camera's ability to resolve details down to the level of roughly 12 inches (25 centimeters) "is almost like being able to walk on the surface." And in fact, researchers already have found other regions like Candor Chasma, Okubo said.

Image: Becquerel Crater
This false-color image from NASA’s Mars Reconnaissance Orbiter shows light-toned layered rock in Mars’ Becquerel Crater. The layers reveal cyclic changes in thickness, which may be due to annual climate cycles, a cyclic variability in the source of the sediment, or both. The blue areas are extensive fields of sand dunes.
The high-resolution spectrometer aboard MRO, known as CRISM, could identify the chemical composition of the light-toned halos — which is an important step for verifying that liquid water was indeed involved in the process, Des Marais said.

NASA's Opportunity rover could well contribute to the investigation as well, Okubo said. Opportunity is now exploring layered deposits exposed in Victoria Crater, on an equatorial plain called Meridiani Planum, and Okubo said the crater's layered deposits may have been formed by fluid processes similar to those at work in the Candor Chasma region photographed by the HiRISE camera.

He said HiRISE images of Victoria Crater revealed structures along the eastern slopes of the crater that could have formed from fluid motion along fractures, just as in Candor Chasma. Right now, Opportunity is on the opposite side of the crater, making its way around the rim.

"One of the possible targets for Opportunity, if it can get that far, is to visit some of these potential cemented joints," Okubo said.

Speaking more broadly, Des Marais said future Mars probes shouldn't be limited merely to tracing the history of liquid water on Mars.

"We not only need to follow the water, we need to follow the energy. Where would be the places on Mars where you could obtain the energy as well as the water necessary for life? We should also follow the carbon and other nutrients — the building blocks of what appears to be so essential to construct cells as we know them," Des Marais said. "So we need a comprehensive study of Mars, to chart the history of the planet, to understand where and when a habitable environment might have arisen."

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