Equatorial Frost on Triton
J. Hillier (JPL), J. Veverka (Cornell University)
Lee et al. (1992) and Hillier et al. (1994) found the equatorial regions of Triton to be unusually forward scattering with a nearly isotropic particle phase function in contrast to the moderately backscattering particles typically found on icy satellites and elsewhere on Triton. Within this region Helfenstein et al. (1992) discovered three features which, though visible at low phase ( ), are almost entirely obscured at high phase ( ). They hypothesize that the disappearance of the features at high phase angles is due to the obscuring effect of an overlying frost layer.
Using a 2-layer surface photometric model we find that obscuration by a uniform frost layer is incapable of explaining the loss of contrast at high phase angles. Further variations between the regions are required. For example, an increase from 0.0 to 0.1 in the asymmetry parameter for the dark regions or an optical depth at least 1.75 times larger over the dark regions can explain the observed loss of contrast. In either case, the loss of contrast is best explained if the frost layer is optically thin with an optical depth less than one though optical depths up to 3 - 4 cannot be ruled out.
Helfenstein et al. (1992) Science, 255, pg. 824; Hillier et al. (1994) Icarus, 109, pg. 296; Lee et al. (1992) Icarus, 99, pg. 82.
Portions of this work were performed while John Hillier held a National Research Council-JPL Research Associateship.