Galileo NIMS Near-Infrared Observations of Jupiter's Ring System

We present the first observations of the jovian ring system viewed at near-infrared wavelengths in forward-scattered light. The eclipse of the Sun by Jupiter during the "C3" orbit of the Galileo spacecraft afforded favorable conditions for the Near Infrared Mapping Spectrometer (NIMS) to observe the ring system between 0.7 and 5.2 μm at a spectral resolution of ≈0.04 μm. NIMS images show the nearly edge-on main ring from just inside the ansa at 1.80 R_J down to 1.05 R_J (where 1 R_J=71398 km), at a spatial resolution of 450 km along the ring and 1130 km out of the ring plane. At these spatial resolutions, no fine structure is apparent in the ring. The large wavelength coverage of the NIMS observations permits an analysis of the scattering properties of the ring system even though the ring subtends only a small range of scattering angles. Our retrieved particle size distribution shows a decrease in the number of particles at larger radii although there is a reversal of this trend between 0.6 and 18 μm. This distribution can be modeled as the combination of a power law with index p=3.9±0.2 and a log-normal distribution of mean radius 4.5 μm. Analysis shows that Voyager data lacked the required wavelength coverage and viewing geometry to determine the particle distribution uniquely. We conclude that the NIMS data set is a better determinant of the particle size distribution, especially for particles larger than a micrometer. We suggest that the power law distribution is the result of collisional processes that form grains less than 1 micrometer, in agreement with previous theory. We suggest that the log-normal distribution is characteristic of another, yet undetermined, process that dominates grain formation and evolution for particles several micrometers in size.