Layers of clay-rich rock have been found in Mars' Mawrth Vallis, a potential landing site for future rovers. This work, published in the August 8 issue of Science, suggests that abundant water was once present on Mars and that hydrothermal activity may have occurred.

The Mawrth Vallis outflow channel is a feature in Mars' northern highland region, a heavily cratered, ancient area of the Red Planet whose geology is a time capsule offering revelations to those who can read it. A team of researchers led by planetary scientist Janice Bishop of the SETI Institute has used the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard the Mars Reconnaissance Orbiter (MRO) to examine infrared light reflected from clays situated in the many-kilometer wide channel. Mawrth Vallis resembles a dried-up, broad river valley through which water may have flowed.

The infrared spectra from CRISM show an extensive swath of phyllosilicate-bearing material. This is a type of iron and magnesium-rich clay that forms in liquid water, and can be found on Earth in oceans and river beds. It is familiar to anyone who's nearly broken a shovel while trying to plant a tree. There is also evidence in the spectra for hydrated silica, which in its pure, clean form is known as opal.

The researchers have combined their data on the composition of soils in this region with topographic information collected by MOLA, the Mars Orbiter Laser Altimeter. They find that clay units in this region were emplaced in a layered fashion, with aluminum clays lying on top of hydrated silica and iron/magnesium clays. These clays were likely formed when water came in contact with basalt - which is the dominant component of the Martian highlands, and probably was produced from volcanic ash, which once blanketed the planet.

"We were surprised by the variety of clay minerals in this region," says Bishop. "But what's interesting is that we find the same ordering of the clay materials everywhere in Mawrth Vallis. It's like a layer-cake of clays, one on top of another. All these layers are topped with a 'frosting' of lava and dust. We can see the clay layers where an impact crater has carved a hole through the surface or where erosion has exposed them."

Since phyllosilicates have been found in a number of outcrops on Mars in CRISM images, these new data suggest that whatever mechanism formed clays at Mawrth Vallis has probably operated over much greater areas of the Red Planet. Alteration by liquid water may have been widespread on early Mars.

These observations improve on others obtained by French scientists with the Mars Express/OMEGA instrument. While those measurements were made with resolutions of hundreds of meters per pixel, the CRISM instrument can boast a sharper gaze: eighteen meters per pixel. Additional scrutiny by MRO's HIRISE camera, with a resolution of 26 cm per pixel, show distinct textures and layering in each of the clay units.

Bishop is careful to note that this work is part of the long-term effort to establish just how widespread, and for what period of time, liquid water may have existed on Mars.

"This is not evidence for life," she notes. "But it does suggest the long-term and common presence of liquid water - and concomitant active chemistry - on the Red Planet in the distant past."

Other members of this research team include Eldar Z. Noe Dobrea and Ralph Milliken of the Jet Propulsion Laboratory, Nancy McKeown of the University of California, Santa Cruz, Mario Parente, of Stanford University, Bethany Ehlmann and John F. Mustard of Brown University, Joseph R. Michalski, Francois Poulet and Jean-Pierre Bibring of the Institut d'Astrophysique Spatiale (France), Gregg A. Swayze of the U.S. Geological Survey, and Scott L. Murchie of the Johns Hopkins University Applied Physics Laboratory. Murchie is principal investigator for CRISM, which was built and is operated by the Applied Physics Laboratory.

The Jet Propulsion Laboratory of the California Institute of Technology, Pasadena, manages the Mars Reconnaissance Orbiter mission for NASA's Science Mission Directorate. Lockheed Martin Space Systems, Denver, is the prime contractor for the project and built the spacecraft.