The forecast for the Milky Way is partly smoggy.
That, at least, is one of the possible implications of the first laboratory measurements of molecules that were formed in interstellar space or, perhaps, around another star.
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 burning a steak on the barbecue. They also are created naturally in forest fires and volcanic eruptions.
This discovery was jointly reported recently by graduate students Simon Clemett from Stanford University and Scott Messenger from Washington University at the Lunar and Planetary Symposium at the NASA Johnson Space Center near Houston.
The finding provides new support for what is known as the "PAH hypothesis." Astronomers who have trained their telescopes on interstellar nebulae and other features in the interstellar medium have seen spectral signatures that closely resemble those of PAH compounds almost everywhere they have looked. This is the first laboratory confirmation of these observations.
"At last, we can really get our hands on what we are seeing," said chemistry Professor Richard N. Zare, who led the Stanford team that identified the molecular material.
The interstellar molecules were identified in microscopic grains of graphite extracted from four different meteorites. The Washington University researchers, headed by Robert M. Walker, Ph.D., the McDonnell Professor of physics and director of the McDonnell Center for the Space Sciences in Arts and Sciences, recovered the dust grains from the meteorites and performed the isotopic analyses that provide the evidence for their exotic origin. About half of the graphite grains exhibited highly unusual ratios of carbon isotopes that the scientists believe demonstrate that they are stardust. 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 isotopic ratios, strongly suggesting that these organic molecules were fellow interstellar travelers as well.
Isotopic measurements coupled with transmission electron microscope studies of their internal structures have been used by Washington University scientists to understand the nature and origin of the stardust. Walker noted, "The discovery of the indigenous molecules gives us still another important handle for determining the chemical and physical conditions in the stellar environment where the stardust condensed."
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 two closely related compounds, acenaphthene and phenanthrene.
"These are very hardy molecules. So it may well be that PAHs are ubiquitous throughout the universe," Zare said.
Although scientists have been studying the chemical composition of meteoritic material for some time, it was only in 1987 that scientists discovered that some meteorites contain particles of dust that are circumstellar in origin. The discovery meant that the analysis of such grains could provide important clues to the nature of the dust cloud from which the solar system formed and of the interstellar medium from which other stars formed.
It is only recently, however, that laboratory techniques were developed at Stanford that are sensitive enough to identify specific organic molecules present in these dust grains. Two-and-a-half years ago, Zare and Walker collaborated on the first measurements of organic molecules in interplanetary dust particles collected in the stratosphere.
The newly identified interstellar molecules are of interest to scientists not only because of their origin but also because of their age. They are most likely the oldest molecules ever studied in the laboratory, with an age pre-dating the formation of the solar system. They also appear to have traveled the furthest of any molecules that have undergone laboratory analysis.
The fact that these molecules are no different from those made on Earth adds to scientists' basic conviction that things "out there" -- in the rest of the universe -- and "back then" -- billions of years ago -- were "not all that different, chemically speaking, than they are here and now," Zare said.
Other members of the Walker team include physics research associate Xia Gao, Ph.D.
Please send comments and suggestions to: