Probing the internal structure of the nuclei of comets

International audienceThere is no direct evidence about the internal structure of cometary nuclei, which are mostly hidden by their gas and dust comae, and have not yet been orbited by any spacecraft. Their densities are low, typically of about 400 kg m−3 for 9P/Tempel 1 (that was impacted by the Deep Impact probe) and 67P/Churyumov–Gerasimenko (that is the target of the Rosetta mission). Such low densities are in favour of a high macro-porosity, or a high micro-porosity, or both. Observations of disruption or splitting of nuclei indeed suggest that some huge sub-nuclei or some meter-sized fragments could be the building blocks of comets. Analysis, from in-situ measurements and from remote light scattering observations, of the structure of the dust particles, which significantly consist of fluffy aggregates of submicron-sized grains, could be in favour of a fractal structure. However, the presence of huge icy grains in the innermost coma, and of flat layers on the surface of 9P/Tempel 1, are clues to the complexity of these objects, which have suffered drastic erosion phenomena on their elongated orbits. It is expected that the Rosetta mission will provide a fair understanding of the structure of the deep interior of the nucleus of 67P/Churyumov–Gerasimenko, thanks to the on-board CONSERT experiment

Modeling the Dynamics of Cometary Fragments: Application to Comet C/1996 B2 Hyakutake

International audienceA tridimensional model for the dynamics of cometary fragments is presented, using a direct integration of the vectorial sum of forces. This model is validated with a complete set of images of Comet C/1996 B2 Hyakutake, between March 22 and April 6, 1996, UT, by making the assumption that the bright condensations visible in the near-nucleus coma were minicomae surrounding nucleus fragments. It is concluded that the observations can be explained with the emission of a major fragment on March 21.4, 1996, UT, accompanied by six minor fragments. The emission of that major fragment followed an increase in the comet's activity by a few days. The main consequences for cometary space missions are discussed

Le Système Solaire revisité

sous la direction de Jean Lilenste

Early Inner Solar System Impactors: Physical Properties of Comet Nuclei and Dust Particles Revisited

During the epoch of early bombardment, terrestrial planets have been heavily impacted by cometary nuclei and cometary dust particles progressively injected in the interplanetary medium. Stardust and Deep Impact missions confirm that the nuclei are porous, loosely consolidated objects, with densities below 1,000 kg m−3, and that they often release small fragments of ices and dust. Recent numerical simulations of the light scattering properties of cometary dust particles indicate that they are highly porous, most likely fractal, and rich in absorbing organics compounds (with a mixture ratio of e.g. 33 to 60% in mass for comet Hale–Bopp). Taking into account the fact that porous structures survive more easily than compact ones during atmospheric entry, such results reinforce the scenario of the early terrestrial planets enrichment – in organics needed for life to originate – by comets