Thermal properties of Saturn's rings : from CAMIRAS to the CASSINI mission.

This thesis presents a study of the rotational properties of particles in Saturn's A and C rings. The own rotation of particles is a dynamical key parameter which plays a role during mutual collisions. The distribution of the spin rate w depends indeed on relative speeds but also on the surface quality of regolith (porosity and roughness) and thus on the internal structure of particles. To constrain w, we need to interpret the thermal emission (function of thermal inertia Tau and w) of the disc according to the phase angle and the planetocentric longitude, as it is still impossible to observe particles separately. Azimuthal variations of temperature observed with various phases angles are modulated by the cooling of the particles, when they cross the shadow of the planet. The heating and cooling rates make it possible to measure thermal inertia, whereas the differences in temperature with the phase angle inform us on the anisotropy of emission associated with the spin rate. Azimuthal variations of temperature were observed in the mid-infrared, at weak phase angle with imagers CAMIRAS (CFHT) and VISIR (VLT), and at multiple phases angles with IRIS (Voyager) and CIRS (Cassini) spectrometers, offering respectively a total or partial azimuthal coverage. A thermal model of planetary ring made up of icy spherical particles in rotation and distributed according to a monolayer structure was developed to interpret the observed temperatures. It enables us to determine how the temperature of the disc, submitted to the multiple sources of heating (Sun, Saturn... etc), varies with the phase angle and the longitude according to the thermal properties of particles. The significant asymmetrycal emission to weak or strong phase shows that the largest particles, which contain a significant fraction of the C ring mass, have an average spin rate w/Omega =0.5±0.4.This result, obtained with the assumptions of a mono size distribution and a monolayer vertical structure, is compatible with results of dynamical simulations. The thermal inertia of the regolith in the C ring (Tau = 6.0 ± 4Jm-2K-1s-1/2) is 3 orders of magnitude weaker than that of the crystalline water ice, and confirms a very porous structure, probably generated by cracks on the surface of the particles. They are probably the consequence of the permanent patching due to the mutual collisions, or the forces of tensions related to the signi- ficant variations in temperature with each orbit. In addition to azimuthal variations in temperature of the A ring, related to the spin rate and the cooling of particles in the Saturn's shadow, a modulation of brightness temperature, correlated with variations of optical depth, is put. This variation could be highlighted by the observations of CIRS at multiple phases and is explained by the presence of gravitational instabilities known under the name of "wakes". The thermal emission of the A ring observed with VISIR, after the taking into account of CIRS observations, is similar to that coming from plane structures in which the particles form aggregates. The high angular resolution accessible with the VLT enabled us, for the first time, to measure the azimuthal variations of temperature in this ring and to deduce its thermal inertia from it. The found value (Tau = 4±3Jm-2K-1s-1/2), under the assumption of a monolayer plane structure, is virtually identical to that of the rings C and B, indicating a probably similar surface quality.