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

Abstract

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 #m, 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 -I.The 2.5-5 ~m IKS spectra definitely show strong narrow signals at 2.7 and 4.25 ~m, attributed to the ~; vibrational bands of H20 and CO2, respectively, and a broader signal in the region 3.2-3.5/tm, 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 #m could correspond to the vl and l,s bands of H2CO 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 H20 and CO2, 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 a° sec -1 for H20, ~2.7 x 1028 sec -1 for CO2, ~5 × 1028 sec -l for CO, and 4 xl02s sec -~ for H2CO, if attributions to CO and H2CO are correct. The production rate of carbon atoms in CH-bearing molecules is -9 x 10 29 sec -1 assuming fluorescence of molecules in the gas phase, but could be much less if the 3.2-3.5 pm 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 #m, tentatively attributed to OCS and molecules with the CN group, respectively. Broad absorption at 2.8-3.0 #m, as well as a narrow emission at 3.15/tm, which follow well the D -1 intensity variation, might be due to water ice. Emission at 2.8 /~m is also possibly present, and 404 0019-1035/88 $3.00 Copyright ,L~ 1988 by Academic Press, Inc. All rights o...

Molecular Abundances in Comets

“…Carbon dioxide, which cannot be observed from the ground due to the absence of allowed rotational lines and to strong telluric absorption of its IR bands, was strongly suspected from the existence of C02 + . CO2 was observed only in P/Halley through its strong band at 4.3 μπι by VEGA-1 (Combes et al 1988) and through mass spectroscopy by NM S (Krankowsky et al 1986). In the UV spectrum of comet P/Hartley 2 1991 XV observed with the HST, Weaver et al (1994) identified bands of the CO Cameron system around 200 nm that they attributed to CO prompt emission following photodissociation of CO2.…”

“…The problems in deriving water production rates from these indirect observations were discussed in a special session of a preceding Asteroids Comets Meteors conference (Lagerkvist et al 1990). Up to now, water was directly observed in only two comets, through its IR vibrational bands : in P/Halley from the Κ AO ) and from the VEGA IKS and TKS IR spectrometers (Combes et al 1988;Krasnopolsky et al 1986), in comet Wilson 1987 VII from the KAO (Larson et al 1988). It is expected that future space observations in the submillimetre and far-infrared ranges (e.g.…”

“…Its fundamental vibrational band at 4.7 μπι was marginally detected by the VEGA-1 infrared spectrometer in P/Halley (Combes et al 1988) and one of its rovibrational lines was tentatively detected in Austin 1990V (DiSanti et al 1992b. CO was also studied by the NMS in P/Halley (Eberhardt et al 1987).…”

“…The detection of formaldehyde was first claimed in P/Halley through its IR bands at 3.56 μπι (Combes et al 1988; subsequently reanalyzed by Mumma and Reuter, 1989) and at 6 cm radio wavelength (Snyder et al 1989). The derived abundance was 4 %.…”

Carbonaceous Compounds in Comets: Infrared Observations

“…A spectrum of Comet Halley taken by the IKS-VEGA spectrometer shown in Figure 1 (Combes et al 1988) illustrates the kind of spectral data that were derived from the Halley studies. The 2-to 5-fim spectral region is especially useful for investigation of molecules, because resonant fluorescent excitation is efficient there (Section 3.1).…”