Microbial stowaways on Mars rovers could raise false alarms for astrobiologists hoping
to find evidence of life -- or worse, could wipe out native
Martians waiting in the soil. A new study suggests that current
techniques for cleaning Mars rovers could let some of the hardiest
life forms, single-celled salt-lovers and tiny animals called
tardigrades, slip through.
"We might actually select for these organisms," said Adam Johnson, a graduate student at Indiana
University and lead author of a paper to be published in the
journal Icarus. "They would be the most likely
thing to be able to survive."
Johnson and colleagues subjected some of Earth's toughest life
forms from the most extreme environments they could find to 40 days
in a mockup Mars environment.
The subjects included bacteria from Siberian permafrost; single-celled microorganisms called haloarchaea from briny saltwater in Mexico;
yeastlike organisms from cold saline springs in the Canadian
arctic; and tardigrades (also known as water bears), the world's
toughest multicellular animal, which have been known to survive
trips into space.
"We threw a lot of organisms at the experiment," Johnson said.
"A lot of studies just focus on one, but we really just threw the
kitchen sink at it."
The researchers cooked up a batch of simulated Martian soil,
called regolith, from volcanic basalt rocks taken from two
different outcrops in Oregon. They baked the mixture for 12 hours
at 750 degrees Fahrenheit to make sure it was free of organic
Then they mixed carefully measured samples of the organisms into
the soil, and let the concoction sit in a glass chamber meant to
simulate the Martian atmosphere for a week. After that first week,
Johnson set the chamber to mimic the daily temperature variations,
solar cycle and ultraviolet radiation at the Martian surface for 40
Earlier studies had found that the single biggest threat to
Earthly microbes was ultraviolet radiation, but being buried in
just a millimeter of soil could protect organisms enough to keep
Johnson and colleagues expected organisms that like extreme
cold, called psychrophiles, to fare best in the negative-40-degree
Surprisingly, cold-loving critters dropped off quickly in their
first week in the chamber, when temperatures were held at a balmy
50 degrees Fahrenheit.
"Really temperature and atmosphere doesn't matter," Johnson
said. "It seemed to be the actual conditions of the regolith and
[the organisms] themselves that determined the chance of
Most of the creatures died from drying out, or from the harsh
chemical conditions in the fake Martian soil.
"They become almost mummified," Johnson said. "It looks like organisms go almost
into a freeze-dried type state."
The only organisms that made it were the salt-loving haloarchaea
from Mexico, and the
hardy tardigrades. Tardigrades can slow down their metabolisms
by a factor of 10,000 under harsh conditions, allowing them to dry
up without dying. Based on their success in the simulated Mars
chamber, Johnson thinks they could last more than 300 days on
"This is the first study where we've actually shown that an
organism could potentially last for several hundred days on the
surface of Mars," he said.
Current techniques for sterilising spacecraft use dry-heat
treatments and chemicals similar to those that could be produced in
the Martian soil. Whatever organisms survive those treatments are
also the most likely things to survive and thrive once they reach
Mars, Johnson said.
"Everybody knows that this is not the greatest way to go about
it, but that's the way they do it," said astrobiologist Rocco
Mancinelli of the SETI Institute, a coauthor of the paper. "I
personally think it has to be revamped."
"This paper points out the need for ongoing re-examination and
updating of sterilisation and detection methods used for planetary
protection purposes during cleaning and preparation of spacecraft,"
said Margaret Race of the SETI Institute, who
studies how best to avoid contaminating Mars with earthly life, and
vice versa, but was not involved in the new study. "We are
continuing to find microorganisms that surprise us in their