A spatially resolved high spectral resolution study of Neptune’s stratosphere

Abstract

Using TEXES, the Texas Echelon cross Echelle Spectrograph, mounted on the Gemini North 8-m telescope we have mapped the spatial variation of H 2, CH 4, C 2H 2 and C 2H 6 thermal-infrared emission of Neptune. These high-spectral-resolution, spatially resolved, thermal-infrared observations of Neptune offer a unique glimpse into the state of Neptune’s stratosphere in October 2007, L S = 275.4° just past Neptune’s southern summer solstice ( L S = 270°). We use observations of the S(1) pure rotational line of molecular hydrogen and a portion of the ν 4 band of methane to retrieve detailed information on Neptune’s stratospheric vertical and meridional thermal structure. We find global-average temperatures of 163.8 ± 0.8, 155.0 ± 0.9, and 123.8 ± 0.8 K at the 7.0 × 10 −3-, 0.12-, and 2.1-mbar levels with no meridional variations within the errors. We then use the inferred temperatures to model the emission of C 2H 2 and C 2H 6 in order to derive stratospheric volume mixing ratios (hence forth, VMR) as a function of pressure and latitude. There is a subtle meridional variation of the C 2H 2 VMR at the 0.5-mbar level with the peak abundance found at −28° latitude, falling off to the north and south. However, the observations are consistent within error to a meridionally constant C 2H 2 VMR of 3.3 - 0.9 + 1.2 × 10 - 8 at 0.5 mbar. We find that the VMR of C 2H 6 at 1-mbar peaks at the equator and falls by a factor of 1.6 at −70° latitude. However, a meridionally constant VMR of 9.3 - 2.6 + 3.5 × 10 - 7 at the 1-mbar level for C 2H 6 is also statistically consistent with the retrievals. Temperature predictions from a radiative-seasonal climate model of Neptune that assumes the hydrocarbon abundances inferred in this paper are lower than the measured temperatures by 40 K at 7 × 10 −3 mbar, 30 K at 0.12 mbar and 25 K at 2.1 mbar. The radiative-seasonal model also predicts meridional temperature variations on the order of 10 K from equator to pole, which are not observed. Assuming higher stratospheric CH 4 abundance at the equator relative to the south pole would bring the meridional trends of the inferred temperatures and radiative-seasonal model into closer agreement. We have also retrieved observations of C 2H 4 emission from Neptune’s stratosphere using TEXES on the NASA Infrared Telescope Facility (IRTF) in June 2003, L S = 266°. Using the observations from the middle of the planet and an average of the middle three latitude temperature profiles from the 2007 observations (9.5° of L S later, the seasonal equivalent of 9.5 Earth days within Earth’s seasonal cycle), we infer a C 2H 4 VMR of 5.9 - 0.8 + 1.0 × 10 - 7 at 1.5 × 10 −3 mbar, a value that is 3.25 times that predicted by global-average photochemical models.