Extremophile
bacteria can tough it out in the Siberian permafrost, but the environment of
Mars may still be too hostile for such hardy life.
That's
the finding of a recent study conducted by Andrew C. Schuerger, a microbiologist
at the University of Florida and the Space Life Sciences Lab at NASA's Kennedy
Space Center in Florida, and colleagues.
"Very
seldom have microbes that grow well under cold or high salt conditions been
subjected to Martian
conditions," said Schuerger.
Harsh
ultraviolet (UV) light proved particularly devastating for the survival of
cold-resistant microbes under simulated surface conditions on Mars.
Such
findings not only hone the search for traces of Martian life, but also could
help focus NASA's procedures to prevent
contamination of Mars by Earth microbes.
Cold
Case for a Small Suspect
Past
studies have focused on moderate-temperature microbes that pose a more common
risk of contaminating spacecraft parts and hitching a ride into space. NASA has
long worried that contamination by Earth microbes could upset ongoing efforts
to find Martian or other extraterrestrial life.
Schuerger
and his colleagues chose to test Psychrobacter cryohalolentis, an
extremophile that thrives under extremely dry conditions and at temperatures as
low as minus 10 degrees C. That choice allowed them to push scientific
understanding of what may or may not survive on the Martian surface.
"If
we can find any of extremophilic
terrestrial species that are capable of growth and replication under
Martian conditions, it puts the search for life on Mars a bit closer to
success," Schuerger explained.
Testing
under Martian surface conditions meant turning to the Mars Simulation Chamber
(MSC) at the Kennedy Space Center, where researchers simulated everything from
dust free skies to global dust storm conditions. The study grew out of the
undergraduate work of David Smith from Princeton University, who has since gone
on to conduct his Ph.D. research at the University of Washington.
The
team also tested how well a salt mineral matrix similar to parts of the Martian
surface could protect against UV, and how well the extremophiles dealt with
very dry conditions.
No
Escape from UV
Turns
out that P. cryohalolentis finds precious little shelter from UV
radiation, even under global dust storm conditions or sitting inside a salt
mineral matrix.
"We
found that the UV radiation was readily and easily able to penetrate the
salt-organic matrix that the bacterial cells were embedded in," Schuerger
said.
Such
findings suggest that this type of extremophile poses little risk of spreading
across the surface of Mars — at least not on its current-day hostile surface
environment. P. cryohalolentis even fared poorly under the dry
conditions on the lab bench, let alone inside the Mars simulator.
The
study points to a need for future investigations on how much UV protection a
microbe can find in salt mineral layers within the Martian regolith.
"The
research emphasizes the point that you're not going to automatically get
protection from UV when embedded within salt deposits," Schuerger noted.
"Some salt encrustations might protect with just a few millimeters of
salt, while others might need 5 to10 centimeters."
UV
is not the only challenge facing hitchhiking microbes. Schuerger's past
research found at least 13 separate factors on Mars that can kill Earth
microbes, not counting perchlorate salts uncovered by NASA's Phoenix Mars
Lander in the polar region of Mars.
Digging
Deeper Into the Future
Still,
current NASA missions to Mars take precautions with sterilization and clean
rooms to prevent contamination prior to launch.
"I'm
not overly worried that we're contaminating landing sights," Schuerger
said. "I'm not suggesting that we relax planetary protection protocols,
either, but current ones appear to be very good."
That's
not to say that NASA missions don't pose any contamination risk, given the dust
that may get kicked up and cover surviving microbes with a protective layer, he
added. Future missions that have landers or rovers digging deeper beyond the
top 10 or 20 centimeters of the Martian surface may require additional risk
studies.
"Whether
or not microbes will survive when we drill down to depth, that's a wide open
question at this point," Schuerger cautioned. Studies like Schuerger's,
and more complex robotic or human
missions to Mars over the next several decades, may provide some answers.
The
full study is detailed in the March issue of the journal Astrobiology.
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