The astronomers observed Comet Hyakutake with a high spectral resolution spectrograph attached to the NASA Infrared Telescope Facility at Mauna Kea, Hawaii, on April 8. Working at a wavelength of 3 micrometers in the infrared spectral region (about 6 times the wavelength of visible light), the astronomers detected the emission lines from acetylene gas for the first time in a comet. The symmetry of the acetylene molecule makes it difficult to detect at radio wavelengths, where many previous cometary molecules have been discovered.
At the time of the observations, Comet Hyakutake was 70 million kilometers (44 million miles) from the Earth. At its closest approach to the Earth, Comet Hyakutake was only 15 million kilometers (9 million miles) from the Earth. Since comets rarely come this close to the Earth, Comet Hyakutake drew considerable attention from astronomers around the world.
The abundance of acetylene in Comet Hyakutake was estimated to be between 0.3 to 0.9% that of water vapor, or 3 to 9 acetylene molecules for every 1000 water molecules. Water vapor was the most abundant gas emitted by Comet Hyakutake, and by comets in general as they near the sun. The amount of water produced by Comet Hyakutake at the time of the observations was about 6000 kilograms per second (6 tons per second). The amount of acetylene produced was about 50 kilograms per second (110 pounds per second).
On Earth acetylene gas is highly flammable and is used as a fuel in welder's torches. Its presence in comets confirms that comets formed at very low temperatures, trapping hydrocarbons like acetylene as ices during the formation of the comet. As the comet approached the sun, acetylene vapor was released from the ice in the nucleus of the comet. (The nucleus is the solid part of the comet and is the source of all the gases and dust released by the comet in its travel around the sun.)
These observations are significant because the abundance of acetylene relative to water provides some insight into the origin of this comet. It is suspected that comets may have formed directly from, and preserve, ice-covered dust particles of the interstellar medium, the starting material for the entire solar system. In some models of the composition of ices in the interstellar medium, the predicted abundances of acetylene ice relative to water ice are close to that which was observed in comet Hyakutake. Thus it is possible that the acetylene in Hyakutake was incorporated directly into the comet from interstellar dust particles. This process occurred at the birth of the solar system.
The results are reported in the October 17, 1996 edition of the scientific journal: Nature.
For further information, please contact :
Dr. Alan Tokunaga
Institute for Astronomy
University of Hawaii
2680 Woodlawn Dr.
Honolulu, HI 96822
Dr. Tim Brooke
Jet Propulsion Laboratory
4800 Oak Grove Dr.
Pasadena, CA 91109