A COLOR-RATIO MAP OF MERCURY, B. Hapke, C. Christman and B. Rava, Dept. of Geology and Planetary Sci., U. of Pittsburgh, Pittsburgh, PA 15260, and J. Mosher, Jet Propulsion Lab., Pasadena, CA 91103. A color-ratio map of the portion of Mercury viewed by Mariner 10 was con- structed from images taken through the orange and W filters (effective wave- lengths 578 and 355 nm, respectively). Images taken on the first flyby were photometrically decalibrated, geometrically corrected and registered. Pixel- by-pixel OR/W ratios were formed and used to construct images in which the brightnesses are proportional to the ratios. Thus, in these images the brightness is proportional to the ratio of the spectral reflectances of the planet at the two wavelengths, with relatively redder areas brighter than relatively bluer areas. The images thus formed were badly contaminated with spots of spurious color caused by blemishes on the vidicon faces. In order to avoid this problem a map was constructed from the images using at least two independent color-ratio images for each region. Approximately seven ratio levels could be resolved; regions corresponding to these levels were hand-colored. Red and orange corresponds to the highest ratios and indigo and black to the lowest ratio. The spectral ratio for the red-colored units is about 25% higher than the ratio for the black-colored units. The map will be published in the Proceedings of the Conference. Just as albedo contrasts on Mercury are smaller than on the moon (I), color differences are less pronounced also, implying more chemical homogeneity, at least as far as those elements which affect color, particularly iron, are concerned. The color units were compared with the geologic terrain units of Trask and Guest (2). In general, there was very little correlation between the two types of maps. This result implies that most of the geologic terrain units are variable in composition. Probably the terrain is controlled by external processes, while the composition is controlled by internal processes. Other conclusions may be summarized as follows. 1. The intercrater plains, which may be among the oldest exposed units, are mainly blue and green in color, with some yellow. 2. The heavily-cratered terrain and old craters and basins tend to be blue and green. 3. The Caloris, Odin, Tir and Borealis Planitia have a similar yellow color and may be similar in composition. 4. Smooth plains are mostly green in the approaching hemisphere and yellow in the departing hemisphere, although some are blue and green. No color features similar to lava flows on the lunar maria were found. 5. Young craters, rays and basins (e.g., Kuiper, Bronte and Degas) tend to be very blue. The geochemical implications of this have been previously dis- cussed by Hapke et a1 (3). 6. No instance was found where a color boundary coincided with a scarp. This result strengthens the interpretation of the scarps as thrust faults (Strom et a1 (4). 7. The E-W part of the looping "ray" N. of Harunobu on the departing hemi- sphere coincides with a color boundary. This part of the "ray" may be a sun- ward-facing scarp. 8. Many instances occur of a crater or basin having a floor of one color and rim or ejecta blanket of a different color (e.g., Kuiper, Bartok, Balzac, crater interior to Renoir, crater interior to Checkhov, craters N. of Durer). 9. The bright floor of Lermontov is yellow, but the yellow unit extends past the NE rim of the crater and its boundary does not coincide with the boundary of the bright albedo unit. 10. The reddest region on the planet is a small crater between Theophanes and Lysippus. The center of this red unit is a mound in the center of this crater.
0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System A COLOR-RATIO MAP OF MERCURY
Hapke, B. et a1
REFERENCES 1. Murray, B. et a1 (1974) Science, 184, 459. 2. Trask, N. and Guest, J. (1975) J. Geophys. Res., 80, 2461. 3. Hapke, B. et a1 (1975) J. Geophys. Res., 80, 2431. 4. Strom, R. et a1 (1975) J. Geophys. Res., 80, 2478.
0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System