Volatiles on triton: The infrared spectral evidence, 2.0-2.5 μm

We report new infraredspectra of Triton in which the methane band at 2.3 μm is shown in greater detail than in previous data. The spectral band at 2.15 μm reported by D. P. Cruikshank, R. H. Brown, and R. N. Clark (1984, Icarus 58, 293-305) is also present. Comparisons are made with methane ice spectra and with laboratory spectra of methane dissolved in liquid nitrogen. Based on these comparisons, we conclude that the 2.3-μm band in Triton's spectrum cannot be attributed entirely to methane gas; the methane also exists either in the solid state or possibly as a weak solute nitrogen. If the band is caused by methane frost, the grain size must be exceedingly small. The strength of the 2.3-μm band does not show an obvious correlation with the satellite's orbital position in the new data set, but it is notably weaker than it was in data obtained at lower spectral resolution in 1980. The new spectral data support the earlier evidence for both methane and molecular nitrogen on Triton, but the physical state of these constituents remains uncertain.