Cartographic Mapping of the Icy Satellites using Cassini ISS images

Kurzfassung

The Cassini spacecraft started its tour through the Saturnian system in July 2004. The Imaging Science Subsystem onboard the orbiter consists of a high-resolution Narrow Angle Camera (NAC) with a focal length of 2000 mm and a Wide Angle Camera (WAC) with a focal length of 200 mm. The stated objective of the ISS is to obtain global coverage for all medium-sized icy satellites (Mimas, Enceladus, Tethys, Dione, Rhea, Ipaetus, and Phoebe) with a resolution better than 1 km/pixel and high-resolution coverage of se-lected areas. This goal was achieved with image se-quences obtained during close flybys supplemented by images from greater distances to complete the cover-age. Remaining gaps in the Northern parts of the satel-lite coverage will be filled until the end of the mission in September 2017. Though the Cassini-ISS camera takes images using many different filters, we used only images taken with the filters CL1, CL2 or GRN, as these images show similar contrast. The processing of the Cassini images follows the typical processing chain for framing cam-eras: radiometric correction, geometric correction and map projection, and mosaicking. For the Cassini mis-sion, spacecraft position and camera pointing data are available in the form of SPICE kernels [http://naif.jpl.nasa.gov]. While the orbit information is sufficiently accurate to be used directly for mapping purposes, the pointing information must be corrected using limb fits. High-resolution images that do not contain the limb were registered to limb images to improve the pointing. Digital global mosaics that are also called base-maps were prepared in simple cylindrical projection. Fig. 1 shows the basemap of Enceladus as an example. The most prominent features were marked with their names. Some features were already named based on the Voyager images. Many new feature names were suggested by the Cassini imaging team and approved by the International Astronomical Union (IAU). High-resolution atlases were produced to conform to the design and standards of the USGS airbrush maps and photomosaics which are widely used in planetary cartography. The selection of the atlas format depends on the resolution of the mosaics and the size of the satellites. Three different formats were used for the generation of the atlases: - The synoptic format for making planetwide maps on a single sheet was used for Phoebe. - The subdivision of the synoptic format for making planetwide maps with four quadran-gles on three sheets were used for Mimas and Iapetus. - The 15 quadrangles format for medium sized bodies and high-resolution imaging was used for Enceladus, Tethys, Dione, and Rhea. Examples of these atlases will be shown in this presentation.