RADIAL-DISTRIBUTION OF THE OH RADICAL IN HALLEY INNER COMA

A radial distribution of the OH radical in Halley's inner coma is pres ented. This distribution is obtained from the Vega 2 spacecraft with t he three-channel spectrometer TKS which allows a measurement of the in tensity of the OH(0,0)(A2SIGMA+ - X2PI(i)) band at 309 nm. The models computed to explain this distribution show that it is necessary to con sider the coma as an optically thick medium in order to explain the co nstant intensity observed near the nucleus, at cometocentric distances shorter than 2000 km. The two main processes involved in our calculat ions are the absorption of solar photons by water molecules, which lim its the photodestruction rate of H2O near the nucleus, and the saturat ion of the OH lines.