Image tube spectra of Pluto and Triton from 6800 to 9000 A

A three-stage Varo image tube was used to obtain spectra of Pluto and Triton as well as comparison stars for the spectral region from 6800 to 9000 A, and the question of whether an adsorption feature near 8900 A indicates the presence of an atmosphere is considered. The feature, more definitive for Pluto than for Triton, occurs in a wavelength region which produces a steeply diminishing response in the image tube. If certain conditions are assumed, the pressure indicated by the feature is the pressure that can be expected from the equilibrium vapor pressure of a methane frost. If the absorption is spurious, the analysis indicates upper limits for methane on Pluto and Triton.