Mapping of the Icy Saturnian satellites

Kurzfassung

The Saturnian system contains 47 satellites of different sizes. This paper deals with the mapping of the so-called medium-sized icy satellites Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus, and Phoebe. Voyager-1 and Voyager-2 obtained a large number of images from the icy Saturnian satellites during their journeys through the Saturnian system in 1980 and 1981. These images constitute the basis for the planning of mapping during the Cassini mission. The Cassini Imaging Science Subsys-tem (ISS) consists of two framing cameras. The narrow angle camera is a reflecting telescope with a focal length of 2000 mm and a field of view of 0.35 degrees. The wide angle camera is a refractor with a focal length of 200 mm and a field of view of 3.5 degrees. Each camera is outfitted with a large number of spectral filters which, taken together, span the electromagnetic spectrum from 0.2 to 1.1 micrometers. At the heart of each camera is a charged coupled device (CCD) detector consisting of a 1024 square array of pixels, each 12 microns on a side. The stated objective of the ISS team is to ob-tain global coverage for all medium-sized icy satellites with a resolution better than 1 km/pixel and high-resolution images. This goal is being achieved with image sequences obtained during close fly-bys supplemented by images from greater distances to complete the coverage. Close flybys of all me-dium sized satellites except Mimas are planned during the nominal mission of the Cassini spacecraft until summer 2008. The first flybys during the mission were those of Phoebe in June 2004 and Iapetus in December 2004 followed by three flybys of Enceladus in February, March, and July 2005 and fly-bys of Tethys, Dione, and Rhea in fall 2005. Imaging of the medium-sized icy satellites is ongoing and will continue until the end of the Cassini mission, making it possible to improve the image mosaics during the tour. The starting points of global mosaics for any satellite are the Voyager mosaics in which areas can be replaced gradually by higher-resolution Cassini images as data become available. At some point in time new mosaics can be generated on the basis of Cassini image data, where Voyager data fill the gaps between the Cassini images. In these maps, the satellite coverage, as expected by the end of the nominal Cassini mission, can be visualized. The global mosaics are usually produced using images of a similar resolution. However, some areas of the satellites are imaged at very high resolution. The data set of Phoebe is the only one that is complete, as no more high-resolution images are expected during the mission. Also, we will not obtain new high-resolution Enceladus images until 2008. Therefore standard maps were generated for these two satellites. The nomenclature was proposed by the Cassini imaging team and has yet to be validated by the IAU.