Clays Shed Light on History of Mars Water and Possibly Life

Claydeposits found in one of the oldest riverbed-like channels on Mars shows someunusual signatures that may shed light on the history of water ? and possiblylife ? on the red planet.

Observationsmade by an instrument onboard NASA's Mars Reconnaissance Orbiter (MRO),currently circling the planet, already have shown substantial clay depositsthat formed about 4 billion years ago in two regions of Mars, Mawrth Vallis andNili Fossae, that indicate that water was more widespread in those areas thanwas initially thought. Those findingswere detailed in the July 17 issue of the journal Nature.

Now, a newstudy, detailed in the Aug. 8 issue of the journal Science, took acloser look at the clays in the Mawrth Vallis region and found that they lie ina uniform sequence of layers that indicates that the chemistry of water therechanged over time.

""Wesee different clays, but the way we see them there, it's kind of like ? a layercake, where every place we get a glimpse of what's there,it's the same order," said study leader Janice Bishop of the SETIInstitute in Mountain View, Calif.

"Therewas a varied chemistry, and it was pervasive, because everywhere we look we seethis same trend," she added.

MRO's CompactReconnaissance Imaging Spectrometer for Mars (CRISM)detected the sequence, which features iron and magnesium smectites (clays richin those particular minerals) in the lowest layer, overlain by a layer enrichedin reduced iron (making it distinct from the iron in the first layer. Next is alayer of silica opal with a layer of aluminum-rich clays on top.

Bishop saysthat the iron and magnesium smectites were likely formed as the water in a hugelake transformed underlying basaltic ash or rock (formed by volcanism).

"They'repretty common, and we see those in a lot of areas on Mars," Bishop told"That's what happened first and that was probably pervasive; there wasprobably a lot of water for a long time and that happened in the whole area."

Thealuminum-rich top layer probably formed during a subsequent watery period wheresome type of acid-leaching removed the iron and magnesium, and aluminum was allthat was left, Bishop explained.

But thereally interesting middle layer, the one with the reduced iron, formed afterthe iron and magnesium-rich layer when "something kind of weird happened,"Bishop said.

Formingdeposits of reduced, or ferrous, iron "usually ? takes microorganisms,"she said. For instance, microbes on Earth can transform iron from its ferric toits ferrous state.

But thefinding doesn't prove that microbes once existed on Mars, as other processes couldaccount for the iron transformation, Bishop cautioned. Organic carbon, perhapsfrom an impacting comet, could have reduced the iron or some change in waterchemistry could also have done the job. Alternatively, the iron could have beendeposited and dried too quickly to oxidize. But which of those processes iscorrect is anybody's guess at this point.

"Rightnow we have more questions than answers," Bishop said.

But as moreCRISM images are analyzed and futurerobotic missions are sent to Mars, more information might be gleaned onthis unique geology which could help scientists "build a better story,"as Bishop put it.

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Andrea Thompson

Andrea Thompson is an associate editor at Scientific American, where she covers sustainability, energy and the environment. Prior to that, she was a senior writer covering climate science at Climate Central and a reporter and editor at Live Science, where she primarily covered Earth science and the environment. She holds a graduate degree in science health and environmental reporting from New York University, as well as a bachelor of science and and masters of science in atmospheric chemistry from the Georgia Institute of Technology.