For cartographic purposes, Mercury has been divided into 15 geographical regions (Figure 17). The Mariner 10 television cameras were able to take pictures of the planet s surface corresponding approximately to only 9 of these 15 regions because the same hemisphere was illuminated during all three encounters as a consequence of the synchronous nature of Mercury's rotation and the orbit of the spacecraft around the Sun. This Atlas is divided into 9 sections, each representing one of the cartographic regions. The name and H ( prefix for Mercury ) number of each region are shown in Figure 17. The regions presented are H-1, H-2, H-3, H-6, H-7, H-8. H-11, H-12, and H-15.
All sections of the Atlas are arranged in the following manner: A 1:5,000,000 shaded relief map and a computer-generated photomosaic are presented first on facing pages for general reference. Subsequent material includes enlargements of portions of the photomosaics, individual high-resolution pictures, mosaics of small areas, and stereo pairs located within the boundaries of the cartographic region. The photomosaics are designated by the letter A for the 1:5,000,000 format and the letters B, C, ... for the enlarged versions (e.g., 1-A, 1-B, ...). Individual pictures, small mosaics, and stereo pairs are designated by a numerical identification (e.g., 1-1, 1-2, ...), where the first digit denotes the cartographic region and the second digit is its identification within the region.
Footprint locations of individual pictures and stereo pairs have been plotted on the shaded relief maps. In cartographic regions H-1 and H-2, where high-resolution, third-encounter photographs were obtained on the planet's limb, footprints are provided both on the limb mosaic and on the shaded relief map. Footprint maps are identified by a 3 unit symbol ( e.g., 1-F1, 1-F2, ... ), u-here the first digit denotes the cartographic region and the last two symbols are its identification within the region.
The shaded relief maps are adapted from the 1:5,000,000 series rendered by airbrush artists in the Cartographic Section, Branch of Astrogeological Studies. U.S. Geological Survey ( USGS ), Flagstaff, Arizona. These maps show the topography without reference to albedo or sun direction, factors which combine to produce a surface appearance often quite different from that in the pictures. Names of surface features and the latitudes and longitudes on the maps make the first page the major reference for each section.
All photo products were produced using computer techniques and software developed in the Image Processing Laboratory (IPL) of the Jet Propulsion Laboratory (JPL), California Institute of Technology. The pictures have been high-pass filtered and contrast enhanced to accentuate surface detail. Pictures used in photomosaics or in stereo pairs have been geometrically transformed to an appropriate projection. A picture element size of 0.4 km was used in the mosaics and a video film converter was used to record the entire photomosaic on 8 by 10 in. film to preserve the resolution inherent in the photographs.
During the first flyby encounter, full coverage of both hemispheres was obtained with a resolution of about 2 km. Individual pictures were processed by IPL and mosaicked by USGS to produce Figure 18, a view of Mercury as seen from the approaching spacecraft, and Figure 19, a view of Mercury as seen from the departing spacecraft.
The second encounter, 6 months later, was on the illuminated side of the planet. Twenty sequences, composed of 18 pictures each, yielded coverage of an area extending from the equator to the south pole and encompassing the south portions of both hemispheres photographed earlier. The pictures were obtained at rapidly changing ranges and viewing angles, thus preventing the combination of the raw photographs into a global mosaic. Therefore, each frame was processed by IPL as an orthographic projection with an origin at 55° latitude and 100° longitude. Figure 20 is a mosaic of these pictures and provides a global view of Mercury as seen by the spacecraft as it passed above the point of projection. 'The only illuminated area not photographed during the three encounters was north of the equator between the limb coverage of the first encounter. This "gore" is especially evident in the H-2 region.
Pictures taken during the third encounter, 1 year after the first visit to the planet, are identified by their striplike configuration (see, for example, picture 1-13). Very high-resolution pictures were obtained in specific areas of interest identified during the first pass.
The Caloris basin, bisected by the morning terminator, is the most prominent feature discovered on Mercury. Because of its importance. a special mosaic, Figure 21, was constructed at JPL from the highest resolution pictures of the area obtained during all three encounters. Each photograph was scaled to a similar proportion and additional enhancement matched tone contrast from one picture to another. To eliminate foreshortening and suppress picture edge effects inherent in Figure 21, a stereographic projection was made of the Caloris Basin (photomosaic 3-F).