Scientists flying at 41,000 feet -- above much of Earth's obscuring atmosphere -- have directly confirmed the existence of water in Halley's comet for the first time. In doing so, they have also demonstrated the effectiveness of a new technique for directly studying a wide range of chemical molecules in a comet nucleus. This should enable astrochemists to analyze the primary makeup of comet nuclei. And that, in turn, should give them new insights into the nature of the primordial nebula out of which the sun and planets once condensed. Comet nuclei are believed to be relatively unblemished samples of that original nebula.
Michael Mumma, of the National Aeronautics and Space Administration's Goddard Space Flight Center in Greenbelt, Md., says he is elated with the instant success of this new technique. He says, ``This is the first time water has been detected directly in any comet at all.''
Up to now, scientists have only been able to infer the presence of cometary chemicals indirectly. A vast cloud of dust and gas called the coma boils up to obscure a comet nucleus by the time it comes close enough for Earth-based scientists to observe it. Sunlight and bombarding solar particles break up the gas molecules into fragments, which have been detected by ultraviolet (UV) light. Scientists then try to work backward to deduce what the original, or parent, molecules might have been.
Earlier observations found atomic oxygen and hydrogen and a combination called the OH radical in Halley's coma. These suggested the presence of water. But, Dr. Mumma notes, ``molecules other than water could have been the parents.'' So scientists have needed a way of detecting water and other chemical molecules directly. That's what Mumma and his colleagues, Harold A. Weaver at The Johns Hopkins University and Harold P. Larson at the University of Arizona, now have provided.
They have made practical a concept of how water behaves in a comet's coma that was first proposed by Mumma. Water molecules fluoresce under the influence of solar infrared (IR) radiation. Mumma and his colleagues developed a theory predicting that, in the cold environment of a coma, this fluorescence should shine brightly at a relatively few wavelengths. In technical terms, there should be about 10 bright spectral lines. Thus, given proper equipment and using it above most of the water vapor in Earth's atmosphere, observers should easily pick up the telltale fluorescence.
Last Dec. 21, 22, and 23, the Arizona-Goddard-Hopkins team took the detector -- called a Fourier transform infrared spectrometer -- to 41,000 feet in the US National Aeronautics and Space Administration's Kuiper research aircraft. Mumma says analysis of their data has shown that they picked out water in Halley's coma clearly on the first try, Dec. 21.
They confirmed that observation Dec. 23, having used the equipment for other purposes in the meantime. They found three times as much water the second time. Halley had undergone a sudden outburst, which was seen by ground observers and by a Japanese spacecraft studying Halley in UV light. Mumma says this seems to be typical of the variability of a comet as it approaches the sun.
There now is no doubt that water is a major constituent. This helps confirm the theory that comets are largely dirty snowballs -- water ice with dust and other chemicals mixed in. Now that they have pinned down water, scientists will be looking for those other constituents.
The IR analysis technique should be able to detect many other parent molecules of cometary gases. This, Mumma says, means scientists now have a way to study the composition of comet nuclei from aircraft or space-shuttle instruments.