EARTHLY
BACTERIA could have reached distant planets
and moons after being flung into space by massive
meteorite impacts, scientists suggest.
The proposal neatly reverses
the panspermia theory, which suggests that life
on Earth was seeded by microbes on comets or
meteorites from elsewhere. Both theories envision
life spreading through the Solar System in much
the same way that germs race around a crowded
classroom, says Jeff Moore, a planetary scientist
at NASA's Ames Research Centre in Moffett Field,
California.
Small
meteorites: "Once one planet
comes down with life, they all get it."
Impacts on Mars and the Moon are known to throw
rocks into space that end up on Earth as small
meteorites.
But spraying Earth rocks towards
the edge of the Solar System is more difficult,
because the material has to move away form the
Sun's strong gravity.
Computer
model: To find out just how
many rocks could reach the outer Solar System,
a team of scientists used a computer model to
track millions of fragments ejected by simulated
massive impacts, such as the one that created
the Chicxulub crater some 65 million years ago.
Similar sized events are thought
to have happened a few times in Earth's history.
The researchers looked in part at how many Earthly
fragments would reach environments thought to
be relatively well suited to life, such as Saturn's
moon Titan and Jupiter's moon Europa.
"I assumed the answer
would be very, very few," says Brett Gladman,
a planetary scientist at the University of British
Columbia, Vancouver, Canada, who led the team.
But Gladman was surprised to find that within
5 million years, about 100 objects would hit
Europa, while Titan gets roughly 30 hits.
He presented the results at
Lunar and Planetary Science Conference in League
City, Texas, recently. But could bacteria survive
the sudden heat and acceleration of being thrown
into space? Other researchers at the conference
suggest that they can. Wayne Nicholson, a microbiologist
from the University of Florida in Gainesville,
has tested the idea with a gun the size of a
house at NASA's Ames Research Centre.
He and his colleagues fired
a marble-sized pellet at about 5 kilometers
per second into a plate that contained bacterial
spores in water, in order to simulate a meteorites
impact. The debris that scattered upwards was
caught in sheets of foam, and the team found
that about one in 10, 000 bacteria survived.
Titan
is different story: Many astrobiologists
believe that bacteria, once in space, could
survive cosmic-radiation exposure during their
trip. Unfortunately, a crash-landing on Europa
would almost certainly sterilize the few rocks
that made it that far.
"But Titan is a different
story," says Gladman. The moon thick atmosphere
would first shatter the meteorite before slowing
the fragments down; the same process happens
with meteorite impacts on Earth. The heat of
landing could even melt the ice and open up
a short-lived pool of liquid for the visitors,
he adds.
At the conference, Gladman
was asked whether, assuming a few bugs did make
it safely on to Titan's surface, they could
ever really thrive in the moon's chilly climes
of about -170 degrees C. "That's for you
guys to work out," he told the audience.
