Thermal spectroscopy of Neptune: The stratospheric temperature, hydrocarbon abundances, and isotopic ratios

Abstract Portions of Neptune's disk-averaged spectrum were observed from the NASA/Infrared Telescope Facility in 1989 and 1990 with a cryogenic echelle array spectrometer, with resolution of ν / Δν ∼ 10 4 . A lower-resolution spectrum (Orton et al . 1990, Icarus 85, 257–265) was used in conjunction with the analysis of these spectra to provide a more reliable absolute intensity calibration. Together, these data imply a stratospheric temperature of 168 K near 1–10 μbar pressure. Considering the infrared data, Voyager UVS experiment, and Earth-based stellar occultation results, we adopt an uncertainty of ±10 K. The temperature profile adopted in this study is consistent with the size and shape of the H 2 J = 3−1 quadrupole feature which was detected in emission. The maximum mixing ratios of CH 4 , C 2 H 2 , and C 2 H 6 which are consistent with this temperature and its uncertainty are 7.5 −5.6 +18.6 × 10 −4 , 5.1 −4.3 +2.0 × 10 −8 , and 1.0 −0.8 +0.2 × 10 −6 , respectively. These C 2 H 2 and C 2 H 6 abundances are completely consistent with previous measurements, after correcting for different temperature profile assumptions. The combined measurements imply a CH 3 D/CH 4 ratio of 3.6 ± 0.5 × 10 −4 , from which we deduce D/H = 1.13 ± 0.16 × 10 −4 ; this is enhanced with respect to solar values and consistent with near-infrared high-resolution spectroscopy (deBergh et al. 1990, Astrophys. J. 355, 661–666). High-resolution measurements of 13 C 12 CH 6 and 12 C 2 H 6 imply that 12 C/ 13 C = 78 ± 26, consistent with solar and telluric values.