Spectroscopic Evidence of Organic Molecules Released by the Dust of Halley’s Inner Coma

Abstract

New spectroscopic arguments supporting the probable presence of organic molecules in the material released by comet Halley are deduced from the data obtained by the Vega three-channel spectrometer. Two excesses of emission on the UV side of the OH and CN bands at 305 and 383 nm are interpreted as being due to “prompt” radiation emitted by electronically excited OH and CN radicals directly produced by the photolysis of water vapor and an organic X-CN molecule. A broad-band emission progressively appears between 342 and 375 nm when the solar scattered continuum has been substracted. This emission increases approximately as the inverse of the projected distance to the nucleus. It is interpreted as a fluorescence emission of organic molecules, possibly condensed polycyclic hydrocarbons. Present observations support the hypothesis of grains coated with organic material and give arguments in favor of a probable interstellar origin for cometary dust.