ISO spectroscopic observations of short-period comets

J. Crovisier ¹, T. Encrenaz ¹, E. Lellouch ¹, D. Bockelée-Morvan ¹, B. Altieri ², K. Leech ², A. Salama ², M. Griffin ³, T. de Graauw ⁴, E. van Dishoeck ⁵, R. Knacke ⁶, & T.Y. Brooke ⁷

¹ Observatoire de Paris, Meudon, France

² ISO Science Operations Centre, Astrophysics Division of ESA, Villafranca, Spain

³ Queen Mary and Westfield College, London, UK

⁴ SRON, Groningen, The Netherlands

⁵ Leiden Observatory, The Netherlands

⁶ Penn. State, Erie, USA

⁷ Jet Propulsion Laboratory, Pasadena, USA

Two Infrared Space Observatory programmes (guaranteed time and open time) were devoted to high-resolution spectroscopic observations of comets. They were aimed at short-period comets. 22P/Kopff was observed on October-December 1996 with SWS and LWS. Due to the weakness of the object, which was at 1.9 AU from the Sun at that moment, only the ro-vibrational lines of water were detected, with SWS. They allowed a determination of the production rate of water in this comet and an analysis of its physical conditions. Comet 103P/Hartley 2 was observed close to its perihelion (at 1.04 AU from Sun and 0.82 AU from Earth) on January 1998 with SWS, LWS and CAM. The bands of H₂O and CO₂ at 2.7 and 4.3 m are detected, with [CO₂]/[H₂O] = 20 %. The 2.7 m band of H₂O is observed with a high signal-to-noise ratio, which permits to evaluate the rotational temperature of water averaged within the field of view to 20 K, and its ortho-to-para ratio to , corresponding to a spin temperature of 35 K. The 5-17 m spectrum of comet Hartley 2 observed with CAM-CVF shows the 9-12 m signature of silicates. Emission at 11.3 m (at a level of about 15 % of the continuum) indicates the presence of crystalline silicates. This is the first time crystalline silicates are found in a short-period comet. The ISO observations of the short-period, Jupiter-family comet P/Hartley 2, presumably originating from the Edgeworth-Kuiper belt, are to be compared to those of comet Hale-Bopp which came from the Oort cloud. They both show crystalline silicates, a high content of CO₂, and a low water ortho-to-para ratio. However, the dust-to-gas ratio of comet Hale-Bopp was much higher than that of Hartley 2.