MARTIAN CLAYS TELL STORY OF A WET PAST

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.

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