[meteorite-list] Did Earth Seed Life Elsewhere in the Solar System?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri Mar 17 14:48:16 2006
Message-ID: <200603171903.k2HJ3OE20485_at_zagami.jpl.nasa.gov>

http://www.nature.com/news/2006/060313/full/060313-18.html

Did Earth seed life elsewhere in the Solar System?

Impacts on our planet could have sprayed life into space.

Mark Peplow
nature.com
March 17, 2006

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."
Received on Fri 17 Mar 2006 02:03:24 PM PST