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.
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.”
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.
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.
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.”
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
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.
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.
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.
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.
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.
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
manages the Mars Reconnaissance Orbiter for NASA’s Science Mission
Directorate in Washington. APL provided and operates CRISM.