Did
Earth seed life elsewhere in the Solar System?
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 Center in
Moffett Field, California. "Once one planet comes down with life,
they all get it."
Spreading germs
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 edges of the Solar System is more difficult, because
the material has to move away from 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 a simulated massive impact, 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 the Lunar and Planetary Science
Conference in League City, Texas, on 16 March.
Tough journey
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 Center.
He and his colleagues fired a marble-sized pellet at about 5
kilometres per second into a plate that contained bacterial spores
in water, in order to simulate a meteorite impact. The debris that
scattered upwards was caught in sheets of foam, and the team found
that about one in 10,000 bacteria survived. "It's an experimental
validation of a fairly well established calculation," says
Moore.
Crash landing
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's thick
atmosphere would first shatter the meteorite before slowing the
fragments down; the same process happens with meteorite impacts on
Earth. "It's a nice safety net," Gladman says. 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°C. "That's for you
guys to work out," he told the audience. "I'm just the delivery
boy."
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Leaving on other planets or even moons may not be impossible in the
future. Hmm...