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03/17/93

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Scientists make first measurements of organic molecules on interplanetary dust

STANFORD -- Scientists at Stanford University and Washington University in St. Louis have made the first measurements of specific organic molecules - the kind that might have been involved in "seeding" the earth with life - on particles of interplanetary dust.

The teams, led by Professor Robert M. Walker at Washington and Professor Richard N. Zare of Stanford, reported on their findings Wednesday, March 17, at the Lunar and Planetary Science Conference XXIV in Houston.

The report does not mean they have found the precursors of life on the dust, but rather that they have made the first measurements of the kinds of molecules, polycyclic aromatic hydrocarbons (PAHs), scientists would expect to find if the precursors of terrestrial life did arrive on earth from space. Scientists have speculated on that scenario for years but until now no one has been able to measure these molecules on these dust particles.

Interplanetary dust particles (IDPs), ubiquitous in the interplanetary medium, are some of the most primitive materials in the solar system.

Although they are typically less than 10 microns in diameter (.00039 inches or one-tenth the diameter of a human hair), extensive measurements have shown unusual enrichments in the heavy isotopes of both the elements hydrogen and nitrogen. The size, however, precluded measurements of organic molecules in IDPs.

The researchers solved this problem with a double blast of laser light.

The IDPs were collected in the stratosphere by aircraft operated by NASA, and sent first to the McDonnell Center for the Space Sciences at Washington University in St. Louis. Particles were removed from the dust collectors and crushed between two quartz plates.

The Washington team, which included Patrick D. Swan, a McDonnell space engineer, then separated the plates, and material from one plate was transferred to gold foil for analysis to determine if a particle was of extraterrestrial origin. Similar material from the second plate was transferred to a potassium bromide mount and measured by infrared spectroscopy.

On the gold foil specimen, they measured the ratio of the abundances of different isotopes of the same element, which allowed them to conclude definitively that they had some particles that came from outer space.

The companion mount was then shipped to Stanford where it was placed in a vacuum chamber and irradiated with a pulsed infrared laser to evaporate the organic molecules. A second pulsed laser, slightly delayed, ionized organic molecules that contain certain aromatic rings. The ions were sorted by mass.

The researchers found PAHs in the particles, especially large non-volatile molecules such as coronene (C24H12).

The hydrocarbons are flat or planar molecules having one or more six-member carbon rings fused with one or more common sides between them. Coronene has six aromatic rings that surround a central six-sided ring as an extended hexagon.

Preliminary data indicated that some IDPs are at least as rich in these molecules as are carbonaceous chondrites, a class of stony meteorites that contain 1-2 percent carbon. The researchers found several kinds of PAHs, including many highly alkylated species. Their combined concentrations are estimated to be at the level of parts per million.

The Stanford team included graduate students Simon Clemett and Rick Maechling.

"Although we have the instrumentation to see minute quantities of molecules on dust," Zare said, "we rely solely on the previous studies by Walker and his research team to know that we are investigating extraterrestrial particles. It is this team that first determined isotope ratios which unambiguously identified some particles as originating not from this planet."

The researchers said the excitement in the work was being able to have the sensitivity necessary to observe organic molecules on these tiny particles.

"In one sense, the result is not surprising," Walker said. "We have known that carbon is a major constituent of IDPs, and there have been numerous indications that organic molecules were probably present.

"However, it took the superb experimental skills of the Stanford group to make the first definitive molecular measurements.

"The results open up a number of new avenues for research," Walker said.

They said they were sure the organic molecules found were not contaminants and were on the particles when they were collected in the stratosphere.

"People ask me to comment on the significance of this discovery," Zare said. "It is premature to make vast cosmological generalizations, but it certainly appears that organic molecules on microscopic particles can survive the harsh environment associated with space outside our Earth."

-jns-

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