Spectroscopic study of comet Halley by the Vega 2 three-channel spectrometer

Abstract

The three-channel spectrometer (TKS) aboard the Vega 2 spacecraft recorded infrared and visible spectra near the nucleus of comet Halley. Spectra in the range 0.95–1.9 µm at a distance of 300 km from the nucleus reveal the H₂O 1.38-µm band and OH bands, the latter excited by dissociation of a parent molecule. Their production rates are 4 × 10²⁹ and 1.7 × 10³⁰ molecules s⁻¹, respectively. The dust particle density is 0.35 g cm⁻³, typical of fluffy particles of interplanetary dust. Emissions of 10 species are observed in the range 275–715 nm at ∼4,000 km from the nucleus, and 6 of them are analysed in terms of production rates: Q_OH = 2×10³⁰, Q_CH = 10²⁹, $Q_{{\text{C}}_{\text{2}}}$ = 6×10²⁷, $Q_{{\text{C}}_{\text{3}}}$ = 3×10²⁷, Q_CN = 10²⁷ and Q_NH = 2 × 10²⁶, all in molecules s⁻¹. The difference between the OH and H₂O production rates is puzzling and needs further analysis. Ionic emissions, not discussed here, are prominent in the tailward part of the comet.