SUBSTANTIAL OUTGASSING OF CO FROM COMET HALE-BOPP AT LARGE HELIOCENTRIC DISTANCE

When comet C/1995 O1 (Hale-Bopp) was discovered(1), at a distance of s even astronomical units from the Sun, it was more than one hundred tim es brighter than comet Halley at the same distance. A comet's brightne ss is derived from the reflection of sunlight from dust grains driven away from the nucleus by the sublimation of volatile ices. Near the Su n, sublimation of water ice (a main constituent of comet nuclei) is th e source of cometary activity; but at its current heliocentric distanc e, Hale-Bopp is too cold for this process to operate. Other comets hav e shown activity at large distances(2), and in the case of comet Schwa ssmann-Wachmann 1, carbon monoxide has been detected in quantities suf ficient to generate its observed coma(3,4). Here we report the detecti on of CO emission from Hale-Bopp, at levels indicating a very large ra te of outgassing. Several other volatile species were searched for, bu t not detected. Sublimation of CO therefore appears to be responsible for the present activity of this comet, and we anticipate that future observations will reveal the onset of sublimation of other volatile sp ecies as the comet continues its present journey towards the Sun.