La composition chimique des noyaux cométaires : bilan de plus d'un siècle d'observations

Observatoire de Paris, Section de Meudon, 92195 Meudon, France

Résumé

Les noyaux cométaires sont les corps du Système Solaire les plus primitifs, c'est à dire ceux qui ont subi le moins de modifications chimiques ou métamorphiques depuis leur formation, il y a 4,6 milliards d'années. L'étude de leur composition est donc une clé pour la compréhension de l'origine et de la formation du Système Solaire. Le retour de la comète de Halley en 1986 a été observé par plus d'un millier de chercheurs grâce à des moyens sans précédents allant des réseaux coordonnés d'observations au sol dans tous les domaines de longueur d'onde au survol du noyau de la comète par cinq sondes spatiales. Notre connaissance de la composition moléculaire, élémentaire et isotopique du matériau émis par les noyaux cométaires s'en est trouvée grandement améliorée et nous en présentons ici le bilan. Les noyaux cométaires se révèlent être constitués de solides qui se sont condensés près de sites de nucléosynthèse, dans le milieu interstellaire et dans la nébuleuse solaire.

Abstract

The 1986 apparition of Halley's comet has been observed by more than one thousand astronomers. Five fly-by missions together with coordinated earth-based observations have led to a quantum leap forward in our knowledge of comets. The view that comets constitute the most pristine material in the Solar System was one of the strongest motivations for cometary observations and in situ measurements. One thousand of comets have been observed at that time and all of them belong to the Solar System. Cometary nuclei are among the smallest bodies of the Solar System and did not achieve the high pressures and temperatures which caused the extensive evolutionary alteration that occured in planets. Comets are privileged over meteoritic/asteroidal material because they formed farther from the Sun, as testified by the presence of abundant volatile ices, and because they spent the major part of their life in the distant Oort cloud, at 50000 UA from the Sun, where the temperature and the pressure are typically those of the interstellar medium, i.e. very low. The zone of formation of comets is not known and is situated between 20 UA (in the vicinity of geant planets) and 50000 UA, depending upon the models. Since presolar condensates are present in our Solar System, as evidenced by numerous isotopic anomalies in primitive meteorites, it is expected that the study of the composition of cometary nuclei will lead to far-reaching inferences regarding the chemical and physical state of the primordial nebula in wich Solar System bodies formed. The composition of comets has been investigated for more than one century through spectroscopic observations of their atmosphere which develops in the inner Solar System under the influence of solar radiation. But conclusive informations on the nature of cometary ices and refractory material have been mostly obtained recently through observations and in-situ measurements of the atmosphere of Halley's comet. The molecular composition of cometary ices appears quite different from that resulting of the condensation of a cooling gas with solar elemental composition under equilibrium conditions. This indicates that a large fraction of ices condensed at very low temperatures and that some of them could be of interstellar origin. The presence of a large fraction of organic refractory material in cometary grains, their extreme heterogeneity at submicron scales, the close association between volatile and non-volatile elements are consistent with interstellar condensation. The bulk elemental composition of comet Halley is close to that of condensable interstellar material, if we except the apparent deficiency of nitrogen in comets. Anomalous isotopic ratios 13C/12C are suspected in some cometary grains, suggesting the survival of solids condensed near nucleosynthetic sources. All this confirms the pristine nature of cometary nuclei and supports the formation of comets from and on interstellar condensates. Clearly we need a sample-return mission to gain a deeper understanding of cometary composition and origin and to put new constraints on formation models of the Solar System.