Voyager photometry of Iapetus |
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| Authors: | Steven W Squyres Bonnie Buratti Joseph Veverka Carl Sagan |
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| Institution: | Theoretical Studies Branch, NASA Ames Research Center, Moffett Field, California 94035, USA;NAS/NRC Research Associate, Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, California 91109, USA;Laboratory for Planetary Studies, Cornell University, Ithaca, New York 14853, USA |
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| Abstract: | Voyager images of Iapetus ranging in phase angle from 8 to 90° were used to define the satellite's photometric properties and construct an albedo map of its surface. The images confirm that the albedo distribution has a roughly hemispheric asymmetry, as had been inferred from earlier analyses of the disk-integrated lightcurve. On the darker leading hemisphere albedo contours are roughly elliptical in shape and centered at the apex of orbital motion, flattened at the poles and elongated along the equator. The reflectance within the darker material is lowest (0.02–0.03) at the apex, and increases with increasing distance from the apex. The albedo pattern on the brighter trailing hemisphere is more complex. Reflectance increases gradually with increasing distance from the interface with the darker material, and reaches a maximum near the poles. Reflectances of 0.3–0.4 in the brighter material are common, and the highest values probably reach 0.6. The transition in reflectance contours between the two materials is gradual rather than sharp, and albedo histograms of images centered on the visually perceived boundary are weakly bimodal. The dark material on Iapetus is reddish, the bright material somewhat less so. |
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