Les anneaux de Saturne revisités par les images de la sonde spatiale Cassini : Evolution dynamique de l'anneau F et étude photométrique des anneaux principaux

Abstract : In the Solar system, the planetary rings represent a fantastic opportunity of studying a majority of phenomena taking place in the thin discs. One can find discs at all redshifts and on all scales of the Universe. Planetary discs are very different~: among the jovian rings, one finds a halo of fine and diffuse dust; the rings of Uranus are very compact, like radially confined strings and the system of rings of Neptune consists of azimuthally stable arcs. However our interest goes on Saturn which has the most complex and widest system of rings known to date~: 484.000 km and a vertical extension which increases with the distance to Saturn (typically less than 1km to 10.000 km). The interest of such a matter organization around Saturn plus its many moons (more than one forty including 8 of a size of several hundreds kilometers) gave birth to the exploration mission CASSINI, supposed to allow the development and the refinement of models set up at the flybies of the two interplanetary probes VOYAGER. The CASSINI Mission began its nominal tour on january, 15th 2005 after the orbital insertion the 1st july 2004 and the dropping of HUYGENS probe on january, 14th 2005 on Titan's surface. The purpose of this thesis consists to revisite two subjects unsolved of long date in the photometric and dynamic behaviours of the Saturn's rings.

In a first part, we try to solve the problem of accretion of matter within the Roche limit by studying the F ring. This ring, since its discovery in 1979 by Pioneer 11, is involved in a most various dynamic theories to explain its complex multi-radial structure and its variable azimuthal structure.
We showed that the multi-radial structure of this ring can be understood by the existence of a spiral which is rolled up around a central area, bright, eccentric and inclined~: the core. The lifespan of this spiral is not the same one as the core, suggesting that the processes which create the spiral are periodic.
Moreover, we showed that the structure of the core is roughly stable on a scale of one year, but is very unstable on a shorter spatial and temporal scales, which is explained by multiple interactions with the satellite Prometheus and the ephemeris satellites. May be that these satellites came from the core and re-interact with him. Additional observations are necessary to refine this theory.

In a second part, we were interested in photometry of the Saturn's rings i.e. to the way that the ring particles reflect sunlight.
We were interested on the one hand in the opposition effect, an intense brightness observed in the main rings for the first time in 1878 by Müller at zero phase angle (the angle between the Sun, the rings and the observer), and which since was vainly explained by geometric optics theories (in particular by the means of the theory of shadow hiding). The use of models combining geometric optics and quantum optics (thanks to the theory of the coherent backscattering which allow to Philip Anderson to receive the Physics Nobel Prize in 1977) made it possible to understand a part of the opposition effect observed in the Saturn's rings. We show that five generally assertions on the shadow hiding and the coherent backscattering used by the community are false. Our study of the opposition surge caused by the coherent backscattering also showed than the taking into account of vectorial nature of light is necessary to understand this quantum phenomenon.
In addition, we studied the overall photometric behaviour of the Saturn's rings by obtaining for the first time the full phase curves, going from 0° to 180° of phase angle, at several optical wavelengths. We showed that the rings have the widest range of albedo and anisotropy of all planetary objects joined together. This could be explained by size distributions, compositions and very different filling factors, which we determined thanks to some photometric models and which are in agreement with dynamic simulations and hydrodynamic analytical computations.
Document type :
Astrophysique [astro-ph]. Université Paris-Diderot - Paris VII, 2007. Français

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  • HAL Id : tel-00255723, version 1




Estelle Déau. Les anneaux de Saturne revisités par les images de la sonde spatiale Cassini : Evolution dynamique de l'anneau F et étude photométrique des anneaux principaux. Astrophysique [astro-ph]. Université Paris-Diderot - Paris VII, 2007. Français. <tel-00255723>




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