Ice in Comet Bowell

It has been widely accepted that frozen volatiles are the major constituent of a comet nucleus¹. However, the direct detection of these ices has proved to be difficult: bright comets which are easily observable are generally so close to the Sun that icy grains are too short lived to make an appreciable contribution to the coma brightness, while comets which are far enough from the Sun for ices to survive (heliocentric distance>2AU) ace usually too faint to be observed adequately. Observations of the reflected light from comets in the 1-5 m region of the spectrum are diagnostic of the presence of ices. Several attempts have been made to detect absorption bands in this region^2-5 A weak, unidentified band near 2.2 m may have been detected in Comets Bowell and Panther, both of which are at large heliocentric distance⁵; although for Comet Bowell the result could not be confirmed⁴. We now report the detection of a deep absorption at 3.25 m in Comet Bowell which provides the first direct evidence for the presence of H₂O ice in a comet.