COLUMBUS, Ohio -- Astronomers from Ohio State University and Lowell Observatory have confirmed the existence of cyanogen gas in Comet Hale-Bopp while the comet was still beyond the orbit of Jupiter, giving new evidence supporting one of two proposed theories explaining just how the gas is produced.
The conclusions, published this month in the journal Science, offer important new information about what mechanisms control the evolution of comets and whether the same processes are at work regardless of how close to or far from the sun the comet may be.
Comet Hale-Bopp was discovered by two astronomers July 23, 1995, and is now prominent in early evening and morning skies.
R. Mark Wagner, a research scientist with Ohio States astronomy department but based in Arizona, and David G. Schleicher, an astronomer at Lowell Observatory in Flagstaff, Az., made a set of observations of Comet Hale-Bopp in October, 1995, using the 4.5-meter Multiple Mirror Telescope. They took
detailed spectrographs of the comet through its head, or coma.
Wagner and Schleicher were looking for the telltale signs of cyanogen gas in the comets spectra. Cyanogen is a molecule composed of one atom each of carbon and nitrogen and a prominent compound found in most comets. As a comet gets closer to the sun, it releases more and more cyanogen.
What researchers did not know was the mechanism causing the release of cyanogen in the comet. Some experts believe that the gas is primarily liberated from dust particles in the icy body of the comet. Others suggest that the breaking apart of larger molecules in the comet -- hydrogen cyanide, for example -- is the primary source of the cyanogen.
"Our results suggest that even when the comet is far from the sun, the mechanism for the creation of cyanogen is the same at much closer distances," Wagner said. "Cyanogen cannot be released from the grains at slow speeds."
Scientists had once thought that cyanogen gas was released only after a comet had entered the H20 sublimation zone, a point on the comets orbit where it was about 3.0 astronomical units (AUs) -- about 280 million miles -- from the sun. (An AU is the distance between the sun and the earth.) "Thats where we saw it taking place when we observed Halleys Comet several years ago," Wagner said. But Wagner and Schleicher made their observations when Hale-Bopp was much farther away, 6.5 astronomical units (AUs) -- about 604 million miles -- from the sun. They were able to measure the spatial distribution of cyanogen gas throughout the comet, as well as its spectral profile.
"Its clear that this process takes place even further away from the sun. Sublimation or evaporation at much greater distances may be driven by things other than water -- carbon dioxide or carbon monoxide, for example.
"The distribution of cyanogen within Hale-Bopp at 6.5 AUs was similar to that of other comets at much closer distances to the sun," Wagner said. "That suggests the gas is formed through the same mechanism regardless of its distance from the sun."