Cosmic dust can ferry in organic molecules.

During the solar system's first billion years, cosmic dust particles -- speck-sized shards of rock -- swarmed in the void, drifting steadily into Earth's primordial atmosphere and sprinkling the planet, some researchers have suggested, with the raw materials of life. Even today, slowly but steadily, the dust continues to rain onto Earth.

In March, space scientists reported collecting interplanetary dust particles from Earth's stratosphere and noted the presence of organic molecules on them (SN: 3/27/93, p.204). Now comes a detailed chemical analysis of those dust particles.

Simon J. Clemett, a chemist of Stanford University, and his colleagues report in the Oct. 29 SCIENCE the detection of polycyclic aromatic hydrocarbons (PAHs) on interplanetary dust. the PAHs bear unusual features the distinguish them from similar molecules found on terrestrial particles and meteorites. "This is the first time complex organic molecules have been clearly identified on interplanetary dust," says Claude R. Maechling, a chemist at Stanford and coauthor of the report. "There may be other species of organic molecules as well, but we've only just started to look for them."

"We're not claiming that interplanetary dust caused life to form -- not by any means," he adds. "But we do know two things. One is that there are organic molecules on interplanetary dust. The other is that interplanetary dust particles have, since the Earth's formation, shed a fair amount of material onto the Earth. So it's reasonable to say that the interplanetary medium delivered large amounts of organic materials onto Earth. But that's as far as we want to go."

The particles were gathered by a high-flying NASA ER-2 aircraft, whose sticky, silicone-coated, wing-mounted "flags" went dust collecting 20 kilometers above sea level. Of the many particles captured, the scientists focused on 17, ranging in size from 2 to 50 micrometers in diameter. They classified eight as interplanetary dust particles, seven as terrestrial contaminants, and two as of uncertain origin.

"Interplanetary dust particles have three special features that differentiate them from terrestrial particles," says physicist Robert M. Walker of Washington University in St. Louis, another report coauthor. "First, their elemental makeup is similar to that of chondritic mateorites, which contain mostly silicon, magnesium, iron, and aluminum. Second, they show an enrichment of odd isotopes, such ad deuterium and nitrogen-15. And third, they show the presence of nuclear particle tracks of heavy nuclei from solar flares. Those three factors give us proof positive for interplanetary dust particles."

The scientists crushed the rock specks between quartz plates and analyzed the resulting particles with a microprobe two-step laser mass spectrometer. They found "a rich mixture of nonvolatile, high-mass polycyclic aromatic hydrocarbons in two interplanetary dust particles," which they named Aurelian and Florianus.

"Both are certainly extraterrestrial, each has a chondritic elemental spectrum, and both have large deuterium enrichments," the team reports. "Furthermore, the N-15 enrichment recorded for Florianus is among the largest recorded for an extraterrestrial object." Interestingly, the organic molecules on the interplanetary dust were more complex than the quite different in makeup from those found in the terrestrial particles or those found previously in other meteorites, the researchers observe.

As for the implications of these findings, the researchers are cautious.

"These are abiotic compounds, not proteins or DNA," Maechling says. "We make no claims that these compounds have any biological activity. There are some theories that these polycyclic aromatic hydrocarbons could go on to form amino acids. But does that relate to life? We can't comment on that. That's a very hypothetical conclusion."