Voyager photometry of Triton: Haze and surface photometric properties

Abstract

We have analyzed Voyager whole‐disk observations of Triton at violet (0.41 μm), blue (0.48 μm),and green (0.56 μm) filter wavelengths, using a model which combines an improved version of Hapke's photometric equation with a thin atmospheric haze layer in the appropriate spherical geometry. The model accurately describes the phase curves over a wide range of phase angles (10° ≤ α ≤ 159°) and agrees well with disk‐resolved brightness scans along the photometric equator and mirror meridian. An upturn in the phase curves seen at the highest phase angles observed (Smith et al., 1989) can be explained by including scattering in a thin atmospheric haze layer with optical depths ranging from 0.066 in the violet to 0.036 for the green filter data. The haze is forward scattering with g ∼ +0.6, requiring particle sizes of ≳0.2 μm. The haze single‐particle scattering albedo is near unity at all three wavelengths, suggesting the haze particles are relatively neutral ice condensates. The geometric albedo, phase integral, and spherical albedo of Triton in each filter corresponding to our best fit Hapke and atmospheric parameters are derived. The bolometric Bond albedo of Triton calculated from our results is 0.85 ± 0.05. If the 16‐μbar N2 atmosphere detected by the Voyager radio occultation experiment (Tyler et al., 1989) is in vapor equilibrium with the surface (therefore implying a surface temperature of 37.7 K), then our Bond albedo implies a surface emissivity of 0.46 ± 0.16.