Ancient clay deposits on Mars may not indicate that the Red Planet was originally a warm, wet place, as scientists have thought. Instead of needing liquid water to form, many of Mars’ 4-billion-year-old clays could have originated from cooling lava, researchers report online September 9 in Nature Geoscience.
Clays are widely scattered across Mars’ oldest terrain, dating to the Noachian period 4.1 billion to 3.7 billion years ago. When the Mars Express and Mars Reconnaissance Orbiter discovered these minerals from orbit several years ago, geologists assumed the clays were a result of large bodies of water weathering and altering Mars’ basalt surface. But last year, some researchers suggested that underground hydrothermal activity provided the water that is necessary to form the clays (SN: 12/3/11, p. 5).
Now there’s another suggestion: Crystallizing lava may have contained tiny pockets where water could react with other chemicals to make small amounts of iron- and magnesium-rich clay. No additional water flowing on the surface or belowground would be needed. So early Mars could have been a largely cold, dry world.
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“We’re not saying all clays on Mars formed by this process,” says coauthor Bethany Ehlmann, a planetary geologist at Caltech. However, “if most clays formed by a magmatic process, it says maybe water wasn’t so available on early Mars.”
Yet with limited data from the Red Planet, it’s too soon to know how pervasive this process might have been. “There is a lot more work to be done before this should be accepted as the prevailing paradigm for clays on Mars,” says Laszlo Kestay, director of the U.S. Geological Survey’s Astrogeology Science Center in Flagstaff, Ariz.
The researchers investigated the cooling-lava scenario because some Martian clays don’t appear to fit with previous explanations, Ehlmann says. Unable to test the idea directly, the team looked for clues on Earth. Some Martian meteorites contain clay minerals with hydrogen isotope compositions characteristic of water coming from Mars’ mantle and carried in lava — not from the atmosphere or surface — suggesting water-rich lava has produced some Martian clay.
The researchers also looked at clay deposits from French Polynesia’s Mururoa Atoll in the Pacific Ocean that formed from cooling lava. This clay reflects the same wavelengths of infrared light as Martian deposits, suggesting that both have similar mineralogical properties and thus probably formed in the same way.
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The team says cooling lava can account for the most geographically abundant Noachian clay minerals. But that doesn’t mean water didn’t flow on the surface during brief episodes, as evidenced by the planet’s ancient river valleys, says coauthor Alain Meunier of the University of Poitiers in France.
Ehlmann says scientists need to find a spot on Mars where Noachian-aged clay is found so that all three proposed clay-forming mechanisms can be tested. Unfortunately, the spot where NASA’s Curiosity landed is not a good test location because the clays there are slightly younger and are clearly part of a sedimentary rather than volcanic deposit.