Asteroid
and comet impacts on Earth can cause catastrophic extinction events. They can
also bring life back, new research shows.
Many
scientists believe that a massive rock from space came
crashing down 65 million years ago at the end of the Cretaceous Period. The
resulting blast set forests ablaze. The skies of Earth were filled with ash
that blocked out the sun, and the planet went cold. Vegetation died in the
absence of sunlight. Shortly thereafter, the dinosaurs and many other life forms
on Earth went extinct. Millions of years of evolution were wiped clean in an
instant.
It's
frightening that one instantaneous event could completely change the face of
life on Earth. However, a new study supports longstanding suggestions that asteroid
impacts could also help spread life throughout the universe.
Rocks
that are ejected from the Earth — or any other life-bearing planet — by an
asteroid impact might actually protect microbes living inside them while they
float through space. These rocks could then fall to the surface of other
planets, or even back to their planet of origin.
In
this way, the microbes could return to their home planet and
"re-colonize" the surface after the disastrous effects of the
asteroid impact have worn off.
Blast off
In
order for organisms to survive a trip into orbit, they must endure a series of
life-threatening events.
First
there's the asteroid impact itself. Then there's the force of being launched
into space. Next, they must travel in the harsh environment of space
until a planet's gravity reels them in. This means facing an environment of
extreme cold, intense radiation
and vacuum exposure. Finally, they need to fall down through the atmosphere,
experiencing extreme pressure, heating and the shock of landing.
Previous
studies have shown that some rock-inhabiting organisms, known as "endoliths,"
might be able to survive a trip through space and a plunge through a planet's
atmosphere to the surface. However, nobody knew whether these organisms could
survive the initial trip into space.
Recently,
an international team of researchers, led by Gerda Horneck of the Institute of Aerospace Medicine in Köln, Germany, selected a number of
hardy microbes from Earth and tested their ability to hitchhike aboard rocks
similar to martian meteorites.
The
organisms used in the study included bacterial endospores, endolithic
cyanobacteria and lichens. This selection provided a wider range of organisms
than in other studies performed to date, including not just simple bacteria but
also more complex eukaryotic organisms.
Smashing life
The
researchers looked at previous studies of martian
meteorites that provided information about the kinds of forces needed to
eject rocks from a large planet. Using this data, the researchers developed a
series of tests designed to simulate these pressures on the selected organisms.
By
smashing the life-containing rocks between metal plates, the researchers were
able to determine which organisms are capable of surviving different pressures
caused by asteroid impacts and ejection into space. Ultimately, they discovered
that a wide range of organisms would be capable of surviving impacts on Mars or
Earth.
"Our results enlarge the number of potential organisms
that might be able to reseed a planetary surface after early very large impact
events, and suggest that such a re-seeding scenario on a planetary surface is
possible with diverse organisms," the researchers report.
The research is detailed in the Spring 2008 issue of the journal
Astrobiology.
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