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CONTACT: Stanford University News Service (415) 723-2558

COMMENT: Prof. Richard N. Zare, Chemistry (415) 723-3062
Simon J. Clemett, Chemistry (415) 723-4318

Interstellar molecules detected in the laboratory for the first time

STANFORD -- The forecast for the Milky Way is partly smoggy.

That is one possible implication of the first laboratory measurements of molecules from interstellar space.

A team of researchers, combining methods developed at Washington University to identify particles of interstellar dust with techniques developed at Stanford to pinpoint specific molecules in these dust grains, found particles in meteorites that may have originated in interstellar space beyond the solar system, or even may have formed around a distant star. A closer look showed that the stuff of stardust looks remarkably like the stuff of smog on Earth.

The far-ranging molecules are carbon compounds called polycyclic aromatic hydrocarbons, or PAHs. On Earth, they are best known as cancer-causing air pollutants created by partial combustion, such as a sooting flame, diesel exhaust or steak burned on the barbecue. They also are created naturally in forest fires and volcanic eruptions.

This discovery was reported jointly by graduate students Simon Clemett from Stanford University and Scott Messenger from Washington University in St. Louis on March 18 at the Lunar and Planetary Symposium at the NASA Johnson Space Center near Houston, Texas.

The finding provides new support for what is known as the "PAH hypothesis." When astronomers train their telescopes on interstellar nebulae and other features in the interstellar medium, almost everywhere they look they see signatures of chemical spectra that closely resemble those of PAH compounds. This is the first laboratory confirmation of those observations.

"At last, we can really get our hands on what we are seeing," said Richard N. Zare, the Marguerite Blake Wilbur Professor of Chemistry at Stanford who led the team that identified the molecular material.

The interstellar molecules were identified in microscopic grains of graphite extracted from four different meteorites. The researchers studied 124 grains from several meteorites. Of these, 89 (70 percent) showed appreciable concentrations of PAHs, while 58 (47 percent) appeared to have originated outside of the solar system, the scientists reported.

The researchers at Washington University, headed by Robert M. Walker, the McDonnell Professor of Physics, recovered the dust grains from the meteorites and performed the isotopic analyses that provide the evidence for their exotic origin.

In some of the grains, the graphite exhibited a highly unusual ratio of carbon isotopes that the scientists believe strongly suggests an interstellar provenance. Isotopes are atoms of the same element that have the same chemical behavior but slightly different weights. The PAHs in many of these grains also exhibited unusual carbon isotope ratios, strongly suggesting that these organic molecules were interstellar travelers as well.

Although astronomical measurements suggested that generic PAH compounds existed in the interstellar medium, the Stanford group was able to identify specific molecules of this type, including naphthalene (the stuff of mothballs) and a number of closely related compounds.

"These are very hardy molecules. So it may well be that PAHs are ubiquitous throughout the universe," Zare said.