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Article Dans Une Revue Planetary and Space Science Année : 2008

Infrared observations of Saturn's rings by Cassini CIRS : Phase angle and local time dependence

Cedric Leyrat
  • Fonction : Auteur
Linda Spilker
  • Fonction : Auteur
Nicolas Altobelli
  • Fonction : Auteur
Stuart Pilorz
  • Fonction : Auteur

Résumé

Since the Saturn orbit insertion (SOI) of the Cassini spacecraft, in July 2004, the Cassini Composite Infrared Spectrometer (CIRS) has obtained a large number of thermal infrared spectra of Saturn's rings. Over the two and a half years of observations to date, ring temperatures were retrieved for a large range of unique geometries, inaccessible from Earth. Understanding their dependencies with phase angle and local time is a clue to understanding the thermal properties and dynamics of Saturn's ring particles. Azimuthal scans of rings, which have been obtained by CIRS at constant radial distance from the planet, have been planned to measure ring temperature variations with local hour angle. Over 47 azimuthal scans for Saturn's main rings (A, B, C and Cassini Division) have been retrieved to date, on both lit and unlit sides, at different phase angles and spacecraft elevations. The first measurements of the transient thermal episode of eclipse cooling in the planetary shadow have also been obtained for all three rings. In this paper, we present an overview of all azimuthal scans obtained by the Cassini/CIRS instrument so far and the dependencies of the temperature and the filling factor with the phase angle and the local hour angle. The ring temperature varies with longitude as the input heating flux coming from Saturn and the Sun changes. The decrease in temperature with the increasing phase angle on both the lit and the unlit sides and for most of the local time also suggests the presence of slowly rotating particles. The crossing of the planet's shadow generates drastic azimuthal variations in temperature, up to 20 K in the C ring. The strong anisotropy of emission observed outside the shadow between low and high phase angles decreases when ring particles cross the shadow, suggesting that particles are almost isothermal in the shadow. This suggests a thermal inertia associated with a rotating rate of particles low enough to have a thermal contrast on their surface. The temperature in the B ring is less sensitive to the phase angle effect on the lit side, suggesting that particles are close enough to form a flat layer at a scale larger than the particle's radius. On the unlit side, particles in the B ring are less sensitive to the lack of solar input than in the C ring or in the A ring. Azimuthal variations of the filling factor in the A ring are also detected with changing ring local time. This effect might be created by the presence of gravitational instabilities (wakes).
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Dates et versions

hal-03821744 , version 1 (19-10-2022)

Identifiants

Citer

Cedric Leyrat, Linda Spilker, Nicolas Altobelli, Stuart Pilorz, Cecile Ferrari. Infrared observations of Saturn's rings by Cassini CIRS : Phase angle and local time dependence. Planetary and Space Science, 2008, 56 (1), pp.117-133. ⟨10.1016/j.pss.2007.04.020⟩. ⟨hal-03821744⟩
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