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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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