Pollutants in space

Do cancer-causing compounds exist beyond earth's atmosphere?

Published: Thursday 15 June 2000

imageOne would assume that carcinogenic compounds like polycyclic aromatic hydrocarbons ( PAHs) were confined just to this world alone. But for nearly 20 years astronomers have been debating whether molecules of PAHs are as common in space as they are on earth. PAHs are customarily found in vehicular exhaust and cigarette smoke and are made up of two or more fused benezene rings.

Scientists have been puzzled by the fact that certain interstellar gas clouds in regions of space too cold to produce thermal emissions nonetheless produce characteristic emission bands in their infrared spectra. These bands are astonishingly similar to the emission bands produced by PAHs, but they are not exactly the same, and the failure of chemists to reproduce precisely the observed spectra in the laboratory has led some astronomers to argue that the emission bands are caused by small solid particles, such as soot or hydrogenated amorphous carbons. But recently astronomers going over data obtained by Europe’s Infrared Space Observatory ( ISO ), believe they have evidence of the presence of PAHs in space.

Some astronomers claim that PAHs could form in the atmospheres of carbon-rich stars and be ejected into interstellar space when the stars die. In the frigid conditions of space, such molecules would sometimes be struck by ultraviolet photons, causing the carbon atoms in the benezene rings to vibrate. Changes in vibrational energy are emitted as infrared light - a process called relaxation - just as a sound is emitted from a gong when it is struck. Because of the many different shapes and sizes of PAHs, their spectra are dominated by bands of emissions, rather than discrete peaks.

Critics of this model argue that the emission bands just don’t match those of PAHs in the lab. “The spectroscopy obtained (in lab experiments) with solid materials, such as anthracite, give a better spectral agreement,” says Ceócile Reynaud of France’s Atomic Energy Commission in Saclay. PAH supporters counter that in cold interstellar space, there is simply not enough heat to produce thermal emissions from such solid particles. Very little ultraviolet radiaton will cause relaxation in PAHs, however, so this could still occur even in the cold environs of interstellar clouds.

Now, a team led by Christine Joblin of the cnrs Space Study Center for Radiation in Toulouse, France, believes it has convincing evidence. By studying the high-resolution spectra obtained with iso’ s short wave infrared spectrometer, they found that the emission bands are made up of a forest of sharp peaks. “This structure cannot be explained by solids - one expects wide and smooth bands - and we can explain this structure by assuming that we are dealing with a family of molecules,” says Joblin.

To finally settle the matter, chemists will have to duplicate this detailed emission structure in the laboratory.

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