The 2.5-12 μm spectrum of comet halley from the IKS-VEGA experiment

The infrared instrument IKS flown on board the VEGA space probes was designed for the detection of emission bands of parent molecules, and for a measurement of the size and temperature of the thermal emitting nuclear region. The instrument had three channels with cooled detectors: an "imaging channel" designed to modulate the signal of the nucleus and two spectroscopic channels operating at 2.5-5 and 6-12 μm, respectively, equipped with circular variable filters of resolving power ∼50. This paper presents and discusses the results from the spectral channels. On VEGA 1, usable spectra were obtained at distances D from the comet nucleus ranging from 250,000 to 40,000 km corresponding to fields of view 4000 and 700 km in diameter, respectively. The important internal background signal caused by the instrument itself, which could not be cooled, had to be eliminated. Since no sky chopping was performed, we obtain difference spectra between the current spectrum and a reference spectrum with little or no cometary signal taken at the beginning of the observing sequence ( D ∼ 200,000 km). Final discrimination between cometary signal and instrumental background is achieved using their different time evolution, since the instrumental background is proportional to the slow temperature drift of the instrument, and the cometary signal due to parent molecules or dust grains is expected to vary in first order as D^-1. The 2.5-5 μm IKS spectra definitely show strong narrow signals at 2.7 and 4.25 μm, attributed to the ν₃ vibrational bands of H ₂O and CO ₂, respectively, and a broader signal in the region 3.2-3.5 μm, which may be attributed to CH-bearing molecules. All these signals present the expected D^-1 intensity variation. Weaker emission features at 3.6 and 4.7 μm could correspond to the ν₁ and ν₅ bands of H ₂CO and the (1 - 0) band of CO, respectively. Molecular production rates are derived from the observed emissions, assuming that they are due to resonance fluorescence excited by the Sun's infrared radiation. For the strong bands of H ₂O and CO ₂, the rovibrational lines are optically thick, and radiative transfer is taken into account. We derive production rates, at the moment of the VEGA 1 flyby, of ∼10 ³⁰ sec ^-1 for H ₂O, ∼2.7 × 10 ²⁸ sec ^-1 for CO ₂, ∼5 × 10 ²⁸ sec ^-1 for CO, and 4 × 10 ²⁸ sec ^-1 for H ₂CO, if attributions to CO and H ₂CO are correct. The production rate of carbon atoms in CH-bearing molecules is ∼9 × 10 ²⁹ sec ^-1 assuming fluorescence of molecules in the gas phase, but could be much less if the 3.2-3.5 μm emission is attributed to CH stretch in polycyclic aromatic hydrocarbons or small organic grains. In addition, marginal features are present at 4.85 and 4.45 μm, tentatively attributed to OCS and molecules with the CN group, respectively. Broad absorption at 2.8-3.0 μm, as well as a narrow emission at 3.15 μm, which follow well the D^-1 intensity variation, might be due to water ice. Emission at 2.8 μm is also possibly present, and might be due to OH created in vibrationally excited states after water photodissociation. The 6-12 μm spectrum does not show any molecular emission, nor emission in the 7.5-μm region. The spectrum is dominated by silicate emission showing a double structure with maxima at 9.0 and 11.2 μm, which suggests the presence of olivine.