PLANETARY RINGS

J.N. Cuzzi, I. Mosqueira, M. Showalter, F. Poulet (NRC)

In addition to the natural curiosity inspired by their exotic appearance, planetary rings present a unique dynamical laboratory for understanding the properties of collisional particle disks which might help us understand the accretion of the planets. Ames scientists are involved in a number of different aspects of planetary ring studies.

An ongoing Hubble Space Telescope (HST) program to observe the rings while they "open up" as seen from Earth over the last five years (see Figure) has produced over 100 images in a variety of filters. Analysis of these images using a newly developed surface scattering code has led us to the conclusion that the increasing redness of the rings which we found occurs as the angle between the Sun and Earth increases, is caused by unusually rough surfaces on the ring particles. This supports the concept that a ring "particle" is actually an ensemble or aggregation of smaller "particles" - a lumpy snowman-like of fractal structure. Further analysis will help us gain insight into how this structure varies across the rings, on scales that can never be observed directly (tens of meters or less). In addition, this modeling and analysis has established that the abrupt brightening of the rings as the Sun-ring-Earth angle gets very small, which has been previously ascribed to the disappearance of shadows, is more likely due to optical interference effects within the grainy surface of individual particles. This result helps us reconcile the brightening with dynamical expectations that the ring particles are collapsed into a fairly thin, dense layer due to inelastic collisions rather than being many particles thick as had been previously thought. We also hunted down a discrepancy between Voyager and HST color observations of the rings, tracing it to an incorrect Voyager calibration. We can now compare Voyager and HST color data directly, and find that the two data sets are in very good agreement from the standpoint of spatial color variations. We find that variations of color, which trace out particle compositional variations, vary with radius and ring opacity in a way that is quite unusual and will be addressed in future analysis.

The systems of large (and small) regular moons that orbit the gas giant outer planets have always been cited as "solar systems in miniature" but their own origin has remained a puzzle. One recent area of interest is the two outer Galilean moons of Jupiter (Ganymede and Callisto), which are of very similar mass and size, yet have very different internal structure. We have developed a two-stage accretion scenario, which postulates a long-lived, secondary accretion stage onlyfor Callisto involving debris which forms in a very extended disk of material extending far out beyond the boundaries of the current satellite system. A small amount of gas remaining in this disk causes solid material to drift slowly inwards onto the outermost moon, accreting without providing much heating. In addition, a study of the thermal internal evolution of a realistic Callisto was carried out, including ice phase change boundaries and plastic ice convection, showing that a sufficiently slow accretion rate would indeed preclude melting of the icy component and prevent complete differentiation of the icy and rocky material.

Ames maintains the Planetary Data System's Rings Node (http://ringmaster.arc.nasa.gov/), which archives and distributes ring data from NASA's spacecraft missions and from Earth-based observatories. We now have on line the entire archive of images from the Voyager missions to the giant planets, with catalogs to help users find the images they need. We have also produced interactive search and geometrical visualization utilities to assist Cassini scientists in planning observations of the rings during the upcoming tour (2004-2008). Ames also provides the Cassini project with the Interdisciplinary Scientist for Rings and Dust, who chaired the Rings Discipline Working Group this year as it worked through initial ring science sequence planning.

Figure caption: HST observations of Saturn since 1996, during which time the ring opening angle has increased from about 4 degrees to about 24 degrees. Data of this quality is available to us in over eight color filters. Voyager data is of higher spatial resolution, but only three filters are available - fortunately they are nearly identical to three of the HST filters.