Stratospheric ethane on Neptune: Comparison of groundbased and Voyager IRIS retrievals

Abstract

Ethane abundances and altitude distributions in Neptune's stratosphere, retrieved from groundbased single line measurements and from the band spectrum obtained by Voyager IRIS, were compared and used to test current photochemical models. Infrared heterodyne spectroscopic measurements of the C 2H 6 ν 9 RR ( K = 4, J = 5) emission line doublet at 840.9763 cm −1 were taken at the NASA Infrared Telescope Facility in May–June 1989, just prior to the Voyager 2 encounter with Neptune in August of the same year. Analysis of Voyager IRIS spectra has also revealed ethane emission from the broad band centered at 822 cm −1 which includes our single line measurement (Bézard et al., 1991, J. Geophys. Res. 96, 18,961–18,975). The infrared heterodyne data were reanalyzed using Voyager updated parameters and thermal profiles and model-determined ethane altitude distributions identical to those used by Bézard et al. Analysis of the line spectrum and band emission yielded consistent mole fraction distributions. Heterodyne-retrieved ethane mole fractions were ∼30% greater than those from IRIS, but within the experimental uncertainties (30 and 35%, respectively, for a given thermal profile). For a constant with a height ethane distribution profile an ethane mole fraction of 1.9 × 10 −6 was obtained for the heterodyne data. Even though the contribution functions for the heterodyne data peak higher in the stratosphere (∼1 scale height), the retrieval uncertainties and widths of the contribution functions do not permit direct retrieval of the altitude distribution of ethane. The new nominal thermal profile and low eddy mixing in the lower stratosphere are consistent with predictions by Kostiuk et al. (1990, Icarus 88, 87–96) using pre-Voyager atmospheric models. The sensitivity of retrievals to changes in the thermal structure was tested. Both heterodyne and IRIS retrieved ethane mole fractions can vary up to a factor of 4 over the range of reasonable stratospheric temperature profiles (± 10 K), with the heterodyne results being more sensitive to changes at low pressures (<0.1 mbar).