Publications (5)31.2 Total impact
-
Article: Sensitivity Studies of Thermal Observations of Saturn's Rings Using a Coupled Thermal/Radiative Transfer Model
[show abstract] [hide abstract]
ABSTRACT: Observations of Saturn's rings taken with the Cassini Infrared Spectrometer (CIRS) reveal that thermal emission from the rings often has a pronounced directional variation, as well as variation with local hour angle and radius. Spectra taken with the CIRS Focal Plane 1 (FP1) detector in the thermal-infrared region (10 to 500 cm-1) resemble a scaled Planck function with a well resolved peak indicative of temperatures near those expected for the ring particles, between approximately 80 and 110 K. The size of the FP1 footprint on the rings varies from hundreds to thousands of kilometers, and the spectra are representative of an ensemble of particles, containing valuable information about the ring structure and particle properties. Simple inversion techniques are of limited use for extracting this information, so we take a forward modelling approach to the inverse problem, investigating the properties of the mapping from physical parameters onto predicted thermal emission. We model ring emission using a radiative transfer calculation that contains a thermal model for particle heating and emission, including spin effects. Given illumination conditions and a vector of parameters describing a vertically varying particle size distribution, spin distribution, thermal inertia, albedo and optical depth, it calculates a self consistent emission spectrum accounting for mutual shading, interparticle heating, and directionally varying emission. For a ringlet at a given radius, each parameter vector generates a set of predicted emission spectra varying with hour angle and direction, providing a mapping from the parameter space onto a high dimensional space of observation vectors. We present analyses of the dimensionality of some sets of Cassini observations, evaluate the agreement of some simple mono- and multi-layer toy models with these, and present sensitivity studies highlighting the interplay of parameters. Limitations of the model are discussed. This research was carried out at JPL/Caltech, under contract with NASA.07/2005; 37:761. -
Article: Azimuthal temperature variations in Saturn's rings as seen by the CIRS spectrometer onboard Cassini
[show abstract] [hide abstract]
ABSTRACT: The thermal emission of ring particles depends both on their surface properties and on their local dynamics. In particular, the thermal gradient between day and night hemispheres is characteristic of their thermal inertia and of their rotational characteristics. The heating by Saturn and the cooling in the planetary shadow should also generate large azimuthal temperature variations which share the same dependence on these properties (Ferrari and Leyrat 2005). Finally the thermal contrast between the lit and unlit faces of the ring should vary with the vertical distribution of particles and their vertical excursion about the ring plane during their revolution around the planet. The CIRS spectrometer on board the Cassini spacecraft is able to observe most of Saturn's rings thermal emission in the infrared between 7 and 1000 μ m. An observational campaign, which is dedicated to the detection and study of these azimuthal asymmetries, began in March 2005. All three main rings A, B, and C have been observed at both low ( ˜ 40o) and high ( ˜ 130o) phase angles. First results of these observations will be presented and discussed. They will also be compared to ground-based observations of Saturn's rings made by the VISIR/VLT spectro-imager during the same period at very small phase angle (6o, Leyrat et al, this DPS). This work is supported by the Programme National de Planetologie07/2005; 37:764. -
Article: Cassini CIRS Observations and Thermal Modelling to Constrain Vertical Structure and Particle Spin Rates in Saturn's Rings
[show abstract] [hide abstract]
ABSTRACT: We present the status of thermal observations and modelling as regards constraining the vertical structure characteristics of Saturn's A, B, and C Rings. In chief, we address the standard models of mono-layer vs multi-layer rings, and whether there is evidence that large particles are phase locked. We address Voyager IRIS and Cassini CIRS observations of thermal infra-red spectra taken at various viewing geometries in the context of the radiation of an ensemble of radiators, some phase locked, some not, and with various configurations. The direct inverse problem is extremely ill-determined, and we present a coupled radiative transfer and thermal model for the rings. We discuss application of this to the Cassini CIRS observations to date, and the prospects of constraining the dynamical parameters. This research was conducted at JPL/Caltech under contract with NASA. We would like to acknowledge support from Mike Flasar and the CIRS Instrument Team.04/2005; 37:528. -
Article: Temperatures, winds, and composition in the saturnian system.
[show abstract] [hide abstract]
ABSTRACT: Stratospheric temperatures on Saturn imply a strong decay of the equatorial winds with altitude. If the decrease in winds reported from recent Hubble Space Telescope images is not a temporal change, then the features tracked must have been at least 130 kilometers higher than in earlier studies. Saturn's south polar stratosphere is warmer than predicted from simple radiative models. The C/H ratio on Saturn is seven times solar, twice Jupiter's. Saturn's ring temperatures have radial variations down to the smallest scale resolved (100 kilometers). Diurnal surface temperature variations on Phoebe suggest a more porous regolith than on the jovian satellites.Science 03/2005; 307(5713):1247-51. · 31.20 Impact Factor -
Article: C ring fine structures revealed in the thermal infrared
[show abstract] [hide abstract]
ABSTRACT: We analyze data sets obtained with the Composite Infrared Spectrometer (CIRS) onboard the Cassini spacecraft after the Saturn Orbit Insertion (SOI). Using the mid-IR interferometer's FP3 channel (600–1100 cm−1), we derive radial temperature profiles for the C ring with a spatial resolution never achieved before. For the first time, the C ring's plateaus and ringlets can be clearly separated from the optically thinner background and their thermal behavior is studied separately for different viewing geometries. In particular, thermal phase curves derived for the plateaus reveal an interesting surge near 0° phase, not observed in the background. We show that mutual shadowing in the plateaus can explain the existence of the surge but is not sufficient to model the phase curves in detail. By analogy with thermal emission of asteroid surfaces we discuss the possible influence of small scale and large scale roughness of the ring structure itself. Because infrared emissivity cannot be derived without being deconvolved from the ‘structural’ filling factor, we examine temperature and filling factors measurements at opposition where the filling factor is most constrained. The occurrence of higher temperatures in the plateaus than in the background near opposition likely arises from enhanced mutual heating between particles, multiple scattering and surface roughness combined with a higher single-scattering albedo.Icarus.
