Properties of Saturn's Ring Particles from Occultations and Thermal Observations

Grant #: NNX08AP76G
Senior Scientist: Stuart Pilorz

This is a  two year study relevant to the archiving and analysis of Cassini data pertaining to Saturn’s main rings. This research consolidates modeling of ring optical thickness with data from the Ultra-Violet Imaging Spectrograph (UVIS) and Visual and Infrared Mapping Spectrometer (VIMS), and uses the products of this modeling to isolate and archive previously inseparable physical effects in observations made by the Cassini InfraRed Spectrometer (CIRS). Analysis of the resulting data set leads to better constraints on particle properties and ring structure. In a broad context, the research is aimed at advancing our understanding of the interplay between ring particle properties and the dynamical evolution of ring systems.

 

Saturn’s rings display a wide range of dynamical phenomena, from irregular structure spanning thousands of kilometers, through density and bending waves that can span hundreds of kilometers, to clumping of particles in self-gravity wakes only tens of meters across. The resulting structure determines the optical thickness seen by observers from various orientations and with various fields of view, and also modulates the thermal forcing that particles within the rings experience due to incident flux from the Sun and Saturn. Infrared thermal emission from the rings contains information on the surface layers of individual particles via their emissivity, and on their spin states via the relation between the time-dependent flux seen by a particle within the structure of the rings and its thermal relaxation time, which is determined by the thermal inertia of the regolith.

The CIRS instrument observes footprints on the rings spanning from hundreds to thousands of kilometers, and information about individual particles is convolved with information about the ring structure. The observed spectra are expressible in terms of a physical temperature and a filling factor, b, which is a heretofore un-separable product of the geometrical cross section of emitters within a field of view (FOV), their thermal emissivity, and another factor dependent upon the temperature distribution in the FOV. A significant obstacle in previous analyses is that classical models of the rings’ geometrical cross section do not accurately represent the rings. This study uses a model for viewing-dependent geometrical cross section generated from UVIS and VIMS observations of stellar occultations, and applies it towards correcting the filling factor parameter returned from analysis of CIRS spectra. This work uses a database and analysis tools we developed for the IR CIRS data up to April 1, 2006, to allow one to efficiently stratify derived temperatures and filling factors, and parse them by the viewing geometry. After deriving cross-section models using the UVIS/VIMS data, the database is updated progressively to incorporate the filling factor for each CIRS spectrum, convolved appropriately to its footprint; to derive thermal emissivity maps using a subset of observations that permit isolating its effect; and to archive the emissivity and another temperature dependent parameter for each footprint. These three parameters are applied to searching for first order physical effects and relationships in observed temperature phase curves and directional emission. The studies rely on the database for correlative investigation, and upon existing radiative transfer, ray tracing and thermal models in conjunction with dynamical simulations to place modelling constraints on particle properties and ring structure. In summary, with  this work we enhance a UVIS/VIMS-based model of the geometrical dependence of the line-of-sight optical thickness of the main rings, archive cross sections, emissivities, and temperature dependent factors for each CIRS spectrum to April 1, 2006, and analyse the effects of these parameters using a combination of numerical models previously developed by the P.I. and collaborators.