Mars Clay "Layer Cake" Adds to Proof of Watery Past

Brian Handwerk
for National Geographic News
August 7, 2008
A Martian "layer cake" of clay minerals sliced open by an ancient channel adds to evidence of the red planet's watery past, according to a new study.

The clays could be key to determining which areas of Mars, if any, were habitable and how long life-sustaining conditions might have lasted.

(Related: "Mars Was Warm, Wet, May Have Hosted Life, Study Says" [July 16, 2008].)

"We see big, deep clay deposits, so there must have been water for a long time," said study co-author Janice Bishop of the SETI Institute and NASA Ames Research Center in Mountain View, California.

"And we see different types of clays, so there must have been some interesting chemistry going on."

Bishop and colleagues pored over images from NASA's Mars Reconnaissance Orbiter and mapped the distribution of clays known as phyllosilicates in the Mawrth Vallis channel.

A deposit of four-billion-year-old clays—some of the oldest exposed minerals on the planet—extends over a wide area in the western part of the channel, suggesting that a large body of water once covered the region.

"Clays really need standing water [to form]," Bishop said, "and we can see that this is a huge clay deposit."

Four-Billion-Year-Old Mystery

An instrument on board the Mars orbiter records the unique wavelengths of light given off by different minerals on the Martian surface.

Using these so-called spectral images, the team identified a layer of clays rich in iron and magnesium exposed in the Mawrth Vallis channel. The findings are detailed in this week's issue of the journal Science.

Based on the way clay minerals form on Earth, the scientists think the iron- and magnesium-rich layer was most likely created as water altered basalt, a volcanic rock that is common on Mars.

More recent layers above these minerals contain clays high in aluminum.

These may have been formed as water dissolved the iron and magnesium out of the soil or by changes to the chemistry of ancient Martian groundwater, the researchers say.

Other chemical changes in the clays, such as differences in the iron oxidation state in some layers, point to a major event that somehow heated up Martian water billions of years ago, Bishop said.

"Possibly a volcano erupted, or maybe some kind of impact made the water hot," she said.

"There are a lot of things that could have taken place, and the hard thing is that it happened four billion years ago."

Mawrth Vallis is one of the possible landing sites for the Mars Science Laboratory, which is slated to launch in late 2009. The roving lab is expected to better piece together the nature of Mars's soil chemistry.

Nicholas Tosca, an expert on Martian soils at Harvard University, called the new finding " a really interesting result."

But "it probably raises more questions than answers, I think," said Tosca, who was unaffiliated with the study.

"Unfortunately, we don't know much about the prevailing conditions at that time. I think this paper scratches the tip of the iceberg in getting more detailed information about what the clays are trying to tell us."

"Rocket Fuel" Salt in Martian Soil?

Meanwhile, NASA's Phoenix mission based near the planet's north pole recently found that soil samples taken in June and July appear to contain perchlorate salt.

Salts are minerals that, like clays, could be evidence of past water on Mars. Perchlorate, a common rocket-fuel component, occurs naturally in harsh Earth environments such as the Atacama Desert in Chile.

Some "extreme" microorganisms there have adapted to use the chemical as an energy source, but it is not considered a particularly auspicious molecule in the search for life on Mars.

The finding could be a setback from research released in June that Martian soil resembles the dirt in Earthly vegetable gardens.

Still, Phoenix scientists haven't ruled out the possibility that the Mars perchlorate is a contaminant that hitched a ride aboard the recently landed spacecraft.

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