Martian ridges point to vanished water

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.

Science 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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