Cassini UVIS observations of Saturn's rings

The Cassini Ultra-violet Imaging Spectrograph (UVIS) is Dart of the remote sensing payload of the Cassini Orbiter spacecraft. Its science objectives i nclude investigation of the chemistry, clouds, and energy balance of the Ti tan and Saturn atmospheres; neutrals in the magnetosphere; D/H ratio for Ti tan and Saturn; and structure and evolution of Saturn's rings. The UVIS has two spectrographic channels which provide images and spectra covering the ranges from 56-118 nm and 110-190 nm. A third optical path with a solar bli nd CsI photocathode is used for high signal to noise ratio stellar occultat ions by rings and atmospheres. A separate hydrogen-deuterium absorption cel l (HDAC) measures the relative abundance D/H from their Lyman-alpha emissio n. The rings of Saturn are the best-studied of planetary rings and contain the majority of the ring material in the solar system. The four-year Cassini t our provides multiple observation opportunities and long time coverage. The UVIS observations include photometry, imaging, spectroscopy, and stellar o ccultations. Numerous diffraction-limited star occultations by the rings ar e a prime objective for the UVIS. The 2 ms integration period in this mode will give a ring radial resolution of better than 20 m. The counting rate i s 50x greater than the Voyager star occultations in a resolution element 5x smaller. Multiple opportunities on the same Saturn passage will define tem poral and azimuthal variation. We expect to observe waves, wakes and ring e dges-all characteristics of ring dynamics and history. The imaging resoluti on is 1 mrad, or 1000 km from a viewing range of 10(6) km. The UVIS is sens itive to the shortest wavelengths of all the remote sensing experiments, an d thus the scattered light from the smallest ring particles. In combination with images from ISS and VIMS, CIRS spectra, and radio occultations, we wi ll define the size distribution or ring particles larger than r = 0.1 mu m. Observing the differential opacity of the rings from 110-190 nm and the po ssible "aureole" created by ring particle forward scattering will further c onstrain the ring particle size distribution. Water ice absorption is stron g in this portion of the UV : some compositional implications for the ring particles are possible from the ring spectral reflectance. The UVIS is sens itive to H (1216 Angstrom) and O (1304, 1356 Angstrom) emissions near the r ings which will better define the ring atmosphere. The atmosphere observati ons constrain ring-magnetosphere interactions, ring evaporation and particl e lifetime, as well as interchange of material between rings, moons and mag netosphere in the Saturn system. The high spectral, spatial, azimuthal and time coverage from Cassini will provide data to refine and test our best cu rrent models of the short-term and long-term ring dynamics. (C) 1998 Elsevi er Science Ltd. All rights reserved.