THERMAL SPECTROSCOPY OF NEPTUNE - THE STRATOSPHERIC TEMPERATURE, HYDROCARBON ABUNDANCES, AND ISOTOPIC-RATIOS

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 ∼ 104. 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 H2 J = 3-1 quadrupole feature which was detected in emission. The maximum mixing ratios of CH4, C2H2, and C2H6 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 C2H2 and C2H6 abundances are completely consistent with previous measurements, after correcting for different temperature profile assumptions. The combined measurements imply a CH3D/CH4 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 13C12CH6 and 12C2H6 imply that 12C/13C = 78 ± 26, consistent with solar and telluric values. © 1992.