In recent years, NASA's overriding focus in Martian exploration has been to "follow the water" — especially the water of modern-day Mars.

Today it's all frozen. This past week the Phoenix lander has been clawing away at the rock-hard slab of ice just a few inches below its footpads. The smart money says that a thick layer of this "white gold" lies barely buried across much of the planet's polar regions.

But for decades spacecraft pictures have been telling us that ancient Mars was a far different place, and that liquid water freely coursed across its surface. How widespread were the flows, scientists wonder, and for how long? The key to knowing whether the Red Planet was ever hospitable to life is buried in those hard-to-know details.

One leap in our understanding came in 2005, when the European Space Agency's orbiter called Mars Express used its infrared spectrometer to discover extensive deposits of phyllosilicates (clay minerals) on the surface. The implication was clear: liquid water, and a lot of it, had saturated the ancient rocks and altered their chemistry.

Clay deposits on Mars

Ancient rivers ferried clay-like minerals (green in this colorized image) into a lake that flooded the floor of Jezero crater on Mars. Click on the image for a larger view.

NASA / JPL / JHU-APL / MSSS / Brown Univ.

Now a far-more detailed view of water-driven chemistry has been revealed by NASA's Mars Reconnaissance Orbiter and its powerful CRISM infrared spectrometer. In the July 17th issue of Nature, John Mustard (Brown University), Scott Murchie (Applied Physics Laboratory), and 34 collaborators describe just how ubiquitously water affected early Mars.

CRISM has identified thousands of clay deposits in the ancient southern highlands of Mars, thanks largely to the 20-fold improvement over Mars Express's OMEGA spectrometer in resolving details on the ground. And it's spotted new types of clay minerals rich in aluminum and chlorite and even hydrated silica (what we call opal here on Earth).

It's far from certain that all these rock-and-water minglings took place on the surface. Sometimes they did. For example, in June 2nd's Nature Geoscience, a research team led by Brown graduate student Bethany Ehlmann describes how clay minerals permeate two deltas laid down on the floor of a 30-mile-wide Martian crater called Jezero.

But conceivably, says Murchie, much (and maybe most) of the chemical alteration occurred deeper down. Liquid water could have percolating through subterranean cracks for hundreds of millions of years — even if the temperature topside remained near or below freezing.

There's more of this story to come, Murchie teases, as the CRISM team starts delving into the chronological arrangement of the deposits and identifying more minerals. As Ehlmann notes, "These clay minerals offer just a taste of the geologic setting" that will allow the team to reconstruct the planet's ancient environment.


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