PHOTOMETRIC DIVERSITY OF TERRAINS ON TRITON

Voyager disk-resolved images of Triton in the violet (0.41 mum) and gr een (0.56 mum) wavelengths have been analyzed to derive the photometri c characteristics of terrains on Triton. Similar conclusions are found using two distinct but related definitions of photometric units, one based on color ratio and albedo properties (A. S. McEwen, 1990, Geophy s. Res. Lett. 17, 1765-1768), the other on albedo and brightness ratio s at different phase angles (cf. P. Lee et al., 1992, Icarus 99, 82-97 ). A significant diversity of photometric behavior, much broader than that discovered so far on any other icy satellite, occurs among Triton 's terrains. Remarkably, differences in photometric behavior do not co rrelate well with geologic terrain boundaries defined on the basis of surface morphology. This suggests that in most cases photometric prope rties on Triton are controlled by thin deposits superposed on underlyi ng geologic units. Single scattering albedos are 0.98 or higher and as ymmetry factors range from -0.35 to -0.45 for most units. The most dis tinct scattering behavior is exhibited by the reddish northern units a lready identified as the Anomalously Scattering Region (ASR) by Lee et al., which scatters light almost isotropically with g = -0.04. In par t due to the effects of Triton's clouds and haze, it is difficult to c onstrain the value of thetaBAR, Hapke's macroscopic roughness paramete r, precisely for Triton or to map differences in thetaBAR among the di fferent photometric terrains. However, our study shows that Triton mus t be relatively smooth, with thetaBAR less than 15-20-degrees, and sug gests that a value of 14-degrees is appropriate. The differences in ph otometric characteristics lead to significantly different phase angle behavior for the various terrains. For example, a terrain (e.g., the A SR) that appears dark relative to another at low phase angles will rev erse its contrast (become relatively brighter) at larger phase angles. The photometric parameters have been used to calculate hemispherical albedos for the units and to infer likely surface temperatures. Based on these results, we determine that all but the most southerly regions (congruent-to south of the equator) of the reddish northern terrains are likely to have been covered with deposits of nitrogen frost at the time of the Voyager flyby, in agreement with the suggestion from the photometry that these units are overlain by a thin veneer of material. (C) 1994 Academic Press, Inc.