[1] This atlas contains preliminary maps of six satellites of Saturn and includes the pictures and other data gathered primarily by the Voyager 1 and 2 spacecraft that were used to compile the maps. These satellites are the only ones for which a sufficiently comprehensive set of pictures is currently available for this kind of work. Maps have not yet been made of the irregularly shaped satellites because conventional map projection techniques are useful only for spherical or at least spheroidal bodies. Maps of Saturn and its largest satellite, Titan, cannot be made with Voyager pictures because they show only the dense clouds that cover those bodies. Voyager passed so far from the remaining satellites that the pictures taken are not adequate to support the compilation of maps.
Two versions of each satellite picture and two versions of each map are contained in the atlas. In addition, a grid showing the latitude-longitude system for each satellite, in the perspective view of each picture, is included. The two versions of each picture include a contrast-enhanced unfiltered version, and a contrast-enhanced high-pass filtered version. These two versions provide significantly different presentations of the images to augment the study and interpretation of the data. Appendix C discusses the computer-processing methods used to produce these pictures.
The two map versions include Mercator and polar stereographic projections of each map as well as Lambert azimuthal equal-area projections. The Mercator and polar stereographic projections are termed "conformal" by cartographers and are useful because the correct shapes of small features are preserved. Equal-area projections, on the other hand, are preferred for measuring surface areas. For example, a coin placed anywhere on an equal-area map will always cover the same number of square kilometers, even though the shapes of features may appear to be foreshortened near the edges of the maps.
Part or all of 14 planets and satellites have now been mapped with data from American spacecraft (Batson, 1980). Figure 1 is a montage of these objects, with Earth included for scale. The size of the image of each object is correct in proportion to the others. The names of American spacecraft with sensors capable of providing map data are shown next to the planets and satellites they have visited. Galileo and Venus Mapper had not flown at the time of this writing. Figure 2 shows the total known mappable area in our solar system. Planets that may not have solid surfaces beneath their cloudy atmospheres are not mappable in the traditional sense and are, therefore, excluded. Figure 3 shows the size of the Earth-Moon system with respect to the Saturnian system, along with the geometry of the Voyager trajectories with respect to the Saturnian system.
The process of mapping another planet involves special computer processing of spacecraft television pictures, making mosaics of those pictures, selecting names for newly discovered features, and compiling a.....
1.6 x 106 km2). Planets that may
not have solid (mappable) surfaces (Jupiter, Saturn, Uranus, and
Neptune) are not included. Pluto and its satellite, Charon, would add
only a minuscule amount to this diagram.
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Mimas |
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Enceladus |
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Tethys |
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Dione |
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Rhea |
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Iapetus |
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.....coherent portrayal of the surface on an appropriate map projection using airbrush drawing techniques. These techniques are discussed in appendixes C and D, and further information is contained in publications listed in the references section. The discussions are intended to provide a basic reference and orientation to readers who may be unfamiliar with some aspects of the mapping project. Appendix C, covering digital image processing, in particular was purposely written at an elementary level for the benefit of readers unfamiliar with many of the concepts of digital image processing. Full technical details of the processing are beyond the scope of the atlas.
The striking variations in the amount of detail shown on the maps are a direct function of the resolution of Voyager 1 and 2 pictures taken at different distances from the satellite. Some areas are portrayed with intricate detail, whereas others show only diffuse mottling, or nothing at all. Maps showing the resolution of available Voyager pictures are included at the beginning of the section for each satellite. Table 1 is a tabulation of the same information. The term "resolution" is used here to define the size of the area covered by a single picture element (pixel) on a planet or satellite. A surface feature must usually be large enough to be covered by at least two pixels if it is to be detected in the image. True resolution is more complicated: It varies as a function of scene contrast, the optical quality of the camera lens, various electronic recording and transmission factors in the camera and spacecraft, and the viewing geometry at the time a picture is taken.
The two most important visual aspects of an image of a cloudless planet or satellite are the surface forms, or topography, and the surface coloration, or albedo. Albedo is a measure of light reflected from surface materials (charcoal has a low albedo; snow has a high albedo) and is easiest to see and measure when the Sun is directly behind the observer or camera, so that shadows are not mixed with albedo markings. Albedo patterns sometimes vary in color, resulting in apparent differences between black and white pictures taken through different color filters. Albedo patterns cover vast areas on planets and satellites, and can usually be mapped from very low resolution data. Topography, on the other hand, is most visible when the Sun strikes the surface at an oblique angle, thus producing shadows, and is most sharply defined along the terminator, a line on the surface that divides day and night. Landforms tend to be much smaller than albedo patterns and, therefore, require much higher resolution to detect. They are rarely discernible on images in which each picture element covers an area that is more than 20 km in diameter.
Just as our Moon always presents the same face to Earth, the six moons in this atlas always present the same face to Saturn. The phenomenon of libration probably causes the sub-Saturn point to oscillate by some amount, undetectable through Voyager data. The International Astronomical Union (IAU) defines the prime meridian on each satellite by reference to a great circle line that passes through a specified, observable surface feature and the intercepts of the spin axis with the surface of the satellite (i.e., the North and South poles) (Davies et al., 1980). Astronomical convention defined by the IAU also specifies that longitude values increase in the direction of the movement of the Sun as viewed from the surface. Thus, if the Sun appears to rise in the east and move to the west, longitude values increase to the west (IAU, 1971). The terms "Saturn facing," "anti-Saturn facing," "leading," and "trailing" hemisphere are used in this atlas to specify areas on the satellites. The leading hemisphere is to the west of the central meridian, because the satellites revolve in a counterclockwise direction as viewed from above Saturn's north pole.
Some of Saturn's moons are not so orderly. Phoebe, for example, moves in the opposite direction to the other satellites in an orbit that is highly inclined to its orbital plane. There is evidence that the spin axis of Hyperion is highly inclined to the plane of its orbit, and that it does not always present the same face to Saturn.
Maps and photographs in this atlas are available to the public through Government distribution centers. Maps published by the U.S. Geological Survey, including many lunar and planetary maps in addition to those of the Saturnian satellites, are available at the following addresses:
Maps are ordered by the "I-number" shown on the captions.
Various enhancements of the pictures in the atlas are available from
Images are ordered by "picno," the picture number shown in each caption.
