Evidence of
a key mineral on Mars has been found at several locations on the planet's surface,
suggesting that any microbial life that might have been there back when the
planet was wetter could have lived comfortably.
The findings
offer up intriguing new sites for future missions to probe, researchers said.
Observations
made by NASA's Mars Reconnaissance Orbiter (MRO), which just completed
its primary mission and started a second two-year shift, found evidence of
carbonates, which don't survive in conditions hostile to life, indicating that
not all of the planet's ancient watery environments were as harsh as previously
thought.
The
findings are detailed in a study in the Dec. 19 issue of the journal Science
and will be presented today at the annual meeting of the American Geophysical
Union in San Francisco.
Escaping
the acid bath
Over the past
several years, evidence for water on Mars has been stacking up: Scientists have
found
gullies formed by running water, possible ancient
lakes, and minerals formed by interaction with water.
But just
how much water there was and how suitable it was for Martian microbes or other
primitive life forms have been harder puzzles to solve.
Most
evidence has pointed to a period when water on the planet's surface formed
clay-rich minerals, followed by a time of drier conditions, when salt-rich,
acidic water affected much of the planet. These later conditions would have proven
difficult for any Martian life — if it ever existed — to endure or to leave any
traces for scientists to find.
Because
carbonates dissolve quickly in acid, finding them shows that at least some areas
of the planet escaped the acid bath and were less hostile to life.
If
primitive life sprang up in these pockets, it could have persisted and left
clues of its existence.
"Primitive
life would have liked it," said study author Bethany Ehlmann, a graduate
student at Brown University in Providence, R.I. "It's not too hot or too
cold. It's not too acidic. It's a 'just right' place."
The sites
are dry now, however, so researchers are not expecting to find biology on the
surface in those locations.
Pinning
down sites
NASA's
now-defunct Phoenix
Mars Lander also found carbonate signatures in surface samples it analyzed
earlier this year. Researchers have also found carbonates in Martian meteorites
that fell to Earth, as well as in windblown Mars dust observed from orbit. But
this dirt and dust could have been mixtures from many areas, so the origins of
carbonates have been unclear.
Scientists
had expected to find "extensive layers of carbonate" because they
would be a natural consequence of the chemistry between Mars' carbon
dioxide-rich atmosphere and the weathering products formed by water acting on
volcanic rocks, said MRO project scientist Richard Zurek of NASA's Jet
Propulsion Laboratory in Pasadena, Calif. But no carbonate deposits could ever
be found.
But now, MRO's
finer resolution has allowed scientists to finally locate carbonate.
"These
carbonate exposures are small, which I think explains why we haven't seen them
on the surface before," Ehlmann said during the AGU press conference.
The areas
with carbonates found by MRO's CRISM (Compact Reconnaissance Imaging
Spectrometer for Mars) instrument were: Nili Fossae, which is 414 miles( 667
kilometers) long and lied at the edge of the Isidis impact basin; some sides of
eroded mesas; sedimentary rocks within Jezero crater; and rocks exposed on the
sides of valleys in the crater's watershed. Traces were also found in Terra
Tyrrhena and Libya Montes.
These are
locations where future rovers and landers could visit to search for life. (Nili
Fossae was one of the final seven candidate sites for the Mars Science
Laboratory, whose launch has now been delayed until 2011, but it didn't make
the MSL final-four list.)
"This
is opening up a range of environments on Mars," said study co-author Jack
Mustard, a professor at Brown.
CRISM has
scoured Mars' surface looking for signs of carbonates, but this is the first
time it has turned any up. This could indicate that this is "a local
environment on Mars that is somewhat unique," Ehlmann said.
Still,
there is the possibility that other regions will turn up carbonate signatures,
or that carbonate layers could be hidden beneath the surface, she added.
The
researchers have multiple theories for how the carbonates might have formed,
including slightly heated groundwater percolating through olivine-rich rocks exposed
at the surface and altered by running water or precipitation in small, shallow
lakes.
The study
was funded by NASA and the National Science Foundation.
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