Nitrogen on Triton

Abstract

The near-infrared spectrum of Triton is characterized by strong absorption bands of methane, probably in the solid state. An additional absorption band at 2.16 μm is tentatively identified as the density-induced (2-0) band of molecular nitrogen in the liquid state. The fundamental overtones of this band system cannot presently be observed because of limitations of the terrestrial atmosphere or spectral signal precision. Using the absorption coefficient for this band derived from laboratory observations and from the literature, it is calculated that Triton must have a layer of nitrogen at least tens of centimeters deep over much of its surface; this quantity is plausible in terms of the cosmic abundance of nitrogen and by comparison with Titan where a massive atmosphere of nitrogen exists. The Triton spectrum has been modeled with liquid nitrogen and solid methane, and it is found that the shape of the continuum in two spectral regions can be properly accounted for by adding a spectral component corresponding to fine-grained water frost. It is speculated that yet another component, a dark, solid, photochemical derivative of methane, may occur as a trace contaminant of the surface materials. If much of the surface of Triton is liquid, the radiometric observations of the satellite must be reinterpreted to derive the radius and surface albedo. If there is liquid nitrogen exposed on the surface, the atmosphere of Triton is probably dominated by nitrogen rather than methane because of the much higher vapor pressure of the former. At the calculated subsolar temperature of Triton, the vapor pressure of nitrogen implies a surface atmospheric pressure in the range 0.13 to 0.30 atm.