Mid-Infrared Ethane Emission on Neptune and Uranus^*

We report 8-13 μm spectral observations of Neptune and Uranus from the NASA Infrared Telescope Facility spanning more than a decade. The spectroscopic data indicate a steady increase in Neptune's mean atmospheric 12 μm ethane emission from 1985 to 2003, followed by a slight decrease in 2004. The simplest explanation for the intensity variation is an increase in stratospheric effective temperature from 155 ± 3 K in 1985 to 176 ± 3 K in 2003 (an average rate of 1.2 K yr^-1) and a subsequent decrease to 165 ± 3 K in 2004. We also detected variation of the overall spectral structure of the ethane band, specifically an apparent absorption structure in the central portion of the band; this structure arises from coarse spectral sampling coupled with a nonuniform response function within the detector elements. We also report a probable direct detection of ethane emission on Uranus. The deduced peak mole fraction is approximately 1 order of magnitude higher than previous upper limits for Uranus. The model fit suggests an effective temperature of 114 ± 3 K for the globally averaged stratosphere of Uranus, which is consistent with recent measurements indicative of seasonal variation.