Clay Minerals Suggest Martian Water Underground



Investigation of exposed clay minerals at thousands of Martian sites by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on NASA’s Mars Reconnaissance Orbiter suggests a long period of wet, warm conditions, mostly underground. Infrared light indicates terrains of different composition in false-color infrared images (left) of a crater site on Mars. Credit: NASA/JPL-Caltech/JHUAPL

A new NASA study suggests if life ever existed on Mars, the longest
lasting habitats were most likely below the Red Planet´s surface.



A new interpretation of years of mineral-mapping data, from more than
350 sites on Mars examined by European and NASA orbiters, suggests
Martian environments with abundant liquid water on the surface existed
only during short episodes. These episodes occurred toward the end of
hundreds of millions of years during which warm water interacted with
subsurface rocks. This has implications about whether life existed on
Mars and how its atmosphere has changed.



"The types of clay minerals that formed in the shallow subsurface are
all over Mars," said John Mustard, professor at Brown University in
Providence, R.I. Mustard is a co-author of the study in the journal
Nature. "The types that formed on the surface are found at very
limited locations and are quite rare."



Discovery of clay minerals on Mars in 2005 indicated the planet once
hosted warm, wet conditions. If those conditions existed on the
surface for a long era, the planet would have needed a much thicker
atmosphere than it has now to keep the water from evaporating or
freezing. Researchers have sought evidence of processes that could
cause a thick atmosphere to be lost over time.


Infrared light indicates terrains of different composition in false-color infrared images (left) of an escarpment site on Mars. Credit: NASA/JPL-Caltech/JHUAPL

This new study supports an alternative hypothesis that persistent warm
water was confined to the subsurface and many erosional features were
carved during brief periods when liquid water was stable at the
surface.



"If surface habitats were short-term, that doesn´t mean we should be
glum about prospects for life on Mars, but it says something about
what type of environment we might want to look in," said the report´s
lead author, Bethany Ehlmann, assistant professor at the California
Institute of Technology and scientist at NASA´s Jet Propulsion
Laboratory in Pasadena. "The most stable Mars habitats over long
durations appear to have been in the subsurface. On Earth, underground
geothermal environments have active ecosystems."



The discovery of clay minerals by the OMEGA spectrometer on the
European Space Agency´s Mars Express orbiter added to earlier evidence
of liquid Martian water. Clays form from the interaction of water with
rock. Different types of clay minerals result from different types of
wet conditions.



During the past five years, researchers used OMEGA and NASA´s Compact
Reconnaissance Imaging Spectrometer, or CRISM, instrument on the Mars
Reconnaissance Orbiter to identify clay minerals at thousands of
locations on Mars. Clay minerals that form where the ratio of water
interacting with rock is small generally retain the same chemical
elements as the original volcanic rocks later altered by the water.



The study interprets this to be the case for most terrains on Mars
with iron and magnesium clays. In contrast, surface environments with
higher ratios of water to rock can alter rocks further. Soluble
elements are carried off by water, and different aluminum-rich clays
form.



The study was performed using data from European and NASA orbiters, including the Mars Reconnaissance Orbiter (above). Image Credit: NASA

Another clue is detection of a mineral called prehnite. It forms at
temperatures above about 400 degrees Fahrenheit (about 200 degrees
Celsius). These temperatures are typical of underground hydrothermal
environments rather than surface waters.



"Our interpretation is a shift from thinking that the warm, wet
environment was mostly at the surface to thinking it was mostly in the
subsurface, with limited exceptions," said Scott Murchie of Johns
Hopkins University Applied Physics Laboratory in Laurel, Md., a
co-author of the report and principal investigator for CRISM.



One of the exceptions may be Gale Crater, the site targeted by NASA´s
Mars Science Laboratory mission. Launching this year, the Curiosity
rover will land and investigate layers that contain clay and sulfate
minerals.



NASA´s Mars Atmosphere and Volatile Evolution Mission, or MAVEN, in
development for a 2013 launch, may provide evidence for or against
this new interpretation of the Red Planet´s environmental history. The
report predicts MAVEN findings consistent with the atmosphere not
having been thick enough to provide warm, wet surface conditions for a
prolonged period.