Anomalous-scattering region on Triton

Abstract

A broad region with unusual light-scattering properties is identified on Neptune's largest satellite Triton. Photometric analysis of Voyager 2 high-resolution images, acquired over a range of phase angles, shows that this “Anomalous-Scattering Region” (ASR) extends from at least 350°E to 60°E longitude, and 10°N to 40°N latitude. The ASR material is unusual compared to “Average Triton” regolith in that it is only weakly backward scattering at all Voyager 2 camera wavelengths (0.41 μm ≤ λ ≤ 0.59 μm). Fits of Hapke's model to disk-resolved photometric data yield a Henyey-Greenstein asymmetry factor g of −0.10 ± 0.02 for the ASR in the clear filter, compared to about −0.25 for Average Triton. The ASR also displays a combination of phase-dependent green/violet color ratios and clear-filter albedo that appears to be distinctive. With spatial continuity as an assumed additional property, we use these characteristics to map the global distribution of the ASR in areas for which photometric coverage is incomplete. We find that the ASR might actually form an almost continuous band of material running roughly parallel to Triton's equator and extending northward from about 10°N, at least up to the terminator (near 30°N). A likely explanation for the ASR's unusual photometric behavior and for the strong latitudinal control of its distribution is the presence of as transparent, optically thin, and seasonally controlled veneer of well-annealed solid N 2. The relatively dark, reddish color may result from products of ex situ irradiation processing of CH 4 or other hydrocarbons present in intimate mixture within the transparent anomalous-scattering layer and/or in the substrate underlying it. Because mapping of the ASR was indirect, these various possible configurations could imply radically different areal extents for the ASR on Triton.