Structural and Compositional Evolution of Saturn's Rings

Grant #: NNX12AP54G
Senior Scientist: Paul Estrada

We propose to study the compositional and structural evolution of Saturn's main rings subsequent to meteoroid bombardment, with the ultimate goal of obtaining a better understanding of the rings' origin. In the last cycle, we completed the development of a suite of evolutionary models that can study ring structure and composition in tandem. This suite of models includes the modernization of structural ballistic transport codes of previous workers and their integration with the compositional "pollution transport'' model we developed previously. The result is a robust code capable of modeling both structural and compositional changes over time on both local and global scales. This new model will further benefit from an improved understanding of key ring physical quantities (such as particle spectral albedo) that are being determined by ongoing collaborative efforts described in this proposal. For instance, a Monte Carlo ring layer radiative transfer code has been completed (under a separate grant) which includes locally inhomogeneous ring structure such as self-gravity wakes. We have also shown how to obtain the autocorrelation length scale of "objects'' in the rings from UVIS stellar occulation data, and obtained new color ratios and scans from the ISS data. The size of the largest "objects'' connects the ring optical depth to its mean density. VIMS occultation and spectral data in different viewing geometries are also available to complement and extend our compositional analysis. 

Our main science goals include: (1) addressing ring structural evolution questions left unanswered by previous workers; (2) applying our compositional evolution models to a broader range of ring features and regions; (3) exploring evidence for radial variation of intrinsic ring material; (4) constraining hidden mass in the rings; and (5) exploring evidence for ring features which may be younger than a few x 10^8 years. We will achieve these goals by: (a) running simulations using a focused range of parameters to simulate prominent ring features, study fine-scale structure, and test hypotheses regarding ring mass and geological age; (b) performing an exhaustive compositional study using ring layer and particle regolith radiative transfer models which relate composition to observed color and brightness; and (c) continued code improvement including the addition of important ring physics. The wealth of available Cassini data that continues to be processed, analyzed, and/or interpreted in conjunction with this proposal includes VIMS spectral and ISS 8- and 15-filter color observations of the rings in many new combinations of viewing geometries, and dozens of stellar occultations cutting all parts of the main rings at a number of longitudes and elevations. This data set allows the most comprehensive multiple wavelength study of the ring's non-icy constituent abundances to date, and provides improved constraints on key ring properties which are needed for better dynamical and radiative transfer modeling.