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J. Crovisier (Obs. Paris)
The Infrared Space Observatory (ISO) was well suited to investigate cometary volatiles through the fluorescence of their fundamental bands of vibration and to measure the composition and physical properties of cometary dust.
C/1995 O1 (Hale-Bopp) was observed as a target of opportunity several times pre- and post perihelion, when the comet was between rh = 2.8 and 4.9 AU from the Sun. For the first time, the full spectrum of a comet could be observed from 2.4 to 195 \mum. Low-resolution spectra (with the PHT-S instrument) allowed the detection of H2O, CO2 and CO, the determination of their production rates and their evolution with rh. From high-resolution spectra observed at rh = 2.8 AU with the short-wavelength spectrometer, a detailed study of the water vibrational bands determined the rotational and spin temperatures of water. In the far-infrared, several rotational lines of water are observed with the long-wavelength spectrometer. The spectrum from 7 to 45 \mum shows prominent features that may be ascribed to crystalline silicates and more specifically to Mg-rich olivine (like forsterite). Features observed in emission at 44 and 65 \mum and in absorption at 3.1 \mum suggest that water ice grains were still present at rh = 2.9 AU.
Similar observations were made on the short-period comet 103P/Hartley 2 close to its perihelion, at rh \approx 1 AU. Crystalline silicates were also revealed in this Jupiter-family comet. In both comet Hale-Bopp and P/Hartley 2, the ortho-to-para ratio of water is found significantly lower than 3 (the high-temperature limit), corresponding to spin temperatures of 25--35 K. These temperatures could be preserved since the formation of cometary water and have a primordial signification.
These two comets as well as several weak or distant other comets were observed with nine broad band filters of ISOPHOT in the range 3.6--175 \mum. This allowed the determination of the colour temperature of cometary dust and its evolution with rh.
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