A Comparison of Topography and Shaded Relief of the Beethoven Quadrangle of Mercury

Introduction: Significant differences exist between a digital elevation model (DEM) of the Beethoven quadrangle of Mercury, derived from Mariner 10 stereo images [1,2], the shaded relief map [3], and the image mosaic of the region. The images of the Beethoven quadrangle, located in the equatorial region (25oN to 25oS, 72oW to 144oW) of Mercury, were acquired at high sun angles by Mariner 10 [4]. The DEM reveals numerous impact craters not identified in the shaded relief map, particularly in areas mapped as smooth plains. Method: A DEM of the Beethoven quadrangle was derived from 180 Mariner 10 stereo pairs. The DEM does not cover the entire Beethoven quadrangle; image quality was too poor to produce good stereo pairs in much of the northern section of the quadrangle. We compared the topography to the image mosaic and the shaded relief map. We found impact craters in the DEM that are not easily recognized in the image mosaic and shaded relief map. To better quantify this effect, we performed crater counts. Because the area of the DEM is relatively small, we included all recognizable craters, regardless of their degradation state. If a crater was cut by the boundary or a gap (lack of data) within the DEM, we adopted the policy of counting it if 50% of the crater was present in the DEM. Craters with diameters of less than 30 km are not easily seen on the DEM due to its resolution, and thus craters of these sizes were not included in the crater counts. Crater counts were also performed on the areas of the shaded relief map that overlap the coverage of the DEM. Results: Figures 1 and 2 show examples of features identified in the DEM that are not easily recognized in the shaded relief or image mosaic maps. Each figure shows three panels. The top panel is a section of the DEM in a Mercator map projection of 1 km/pixel scale. The middle panel shows the same area in the shaded relief map; the bottom panel shows the same area in the image mosaic. The shaded relief map and image mosaic have the same map projection and scale as the DEM. Figure 1 shows a complex impact feature identified in the DEM, and its location (indicated by pink arrows) within the image mosaic and the shaded relief map. The two internal craters can be seen on the shaded relief map, but the larger crater that they superimpose is not easily discernable. The larger crater (87.4 km in diameter) is located at 4.8oS, 106.6oW. The geologic map of the Beethoven quadrangle [4] classifies this region as plains and terra undivided, a unit described of as flat, highly cratered, with scarps and ridges. This geologic unit is a continuation of a unit within the Kuiper quadrangle, usually assigned to areas where the sun angle was high and prevented good image quality [4].