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past feature
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Molecular
Hitchhiking on a Comet
A team of scientists have used mass
spectrometry to show that organic
molecules hitchhiking aboard a comet could
survive an impact with Earth and thus have
seeded life on this planet. The results
give credence to the theory that the raw
materials for life came from outer space
and were assembled on Earth into the
ancestors of today's proteins and DNA.
Interestingly, more than 70 amino acids
have been found in meteorites, and eight
of them are in common with those found in
humans and all other life on Earth.
Jennifer Blank of the Department of Earth
and Planetary Science at the University of
California, Berkeley and her colleagues
Randy Winans and Mike Ahrens of Argonne
National Laboratory, and
engineer-mathematician Gregory Miller of
the Lawrence Berkeley National Laboratory,
reported their preliminary findings at the
national meeting of the American Chemical
Society in San Diego, California.
Simulating a high-velocity comet collision
with Earth, the team shot a soda-can sized
bullet into a target containing a teardrop
of water mixed with amino acids, the
building blocks of proteins. The ballistic
test was designed to simulate the type of
impact that would have been frequent on
Earth some four billion years ago. The
severity of the laboratory impact was akin
to an oblique collision at an angle of
less than 25 degrees to the Earth's
surface.
To test whether water and organic
compounds could survive the high pressures
and high temperatures of a collision,
Blank and her colleagues worked for three
years to design a steel capsule that would
not rupture when hit with a
mile-per-second (1.6 kilometer-per-second)
bullet fired from an 80-mm bore cannon at
the University of Chicago and later at Los
Alamos National Laboratory. The target - a
two-centimeter diameter stainless steel
disk about a half-centimeter thick - was
able to withstand about 200,000 times
atmospheric pressure without bursting. A
small cavity was filled with water
saturated with five amino acids: three
from the list of 20 that comprise all
proteins in humans (phenylalanine, proline
and lysine) and two varieties detected in
the Murchison meteorite (aminobutyric acid
and nor-valine) that plummeted to the
ground in 1969 in Australia.
The liquid contents were analyzed
afterwards at Argonne using liquid
chromatography and mass spectroscopy to
determine the species and concentrations
of molecules present. The survival of a
large fraction of the amino acids and
their polymerization during the collision
validate the idea of an extraterrestrial
origin of organic compounds.
The next hitchhikers that the scientists
plan to subject to a shock test are
bacterial spores, which some have proposed
arrived on Earth via comets, jump-starting
evolutionary processes.
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