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.
"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.
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
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
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