The 9300-Å Absorption Band in the Spectrum of Jupiter

Abstract

Low-resolution spectrophotometry of Jupiter by Karkoschka (1994, Icarus111, 174–192) revealed a weak apparent absorption feature in the spectrum of Jupiter at 920–940 nm. This is the wavelength of the strong 2ν1+ν3 band of H2O, and Karkoschka tentatively identified the feature as jovian H2O absorption. If this spectral feature is indeed due to H2O absorption, it would offer the exciting possibility of monitoring the jovian H2O with visible band spectroscopy. This feature has been seen in previous low-resolution spectroscopy of Jupiter such as that of Woodman et al. (1979, Icarus37, 73–83), but was not identified as jovian in origin because of uncertainties in extinction corrections for the very strong telluric H2O absorption. Cochran et al. (1995, Bull. Am. Astron. Soc.27, 66) tentatively identified the feature as due to NH3, an identification that was supported by further low-resolution spectrophotometry by Karkoschka (1998, Icarus133, 134–146). Here we present high-resolution spectra (R=155,000) of Jupiter using the 2dcoudé spectrometer on the 2.7-m Harlan Smith Telescope at McDonald Observatory, in order to make a definitive identification of this absorption feature. Observations were obtained when Jupiter was near quadrature in order to Doppler shift potential jovian H2O features away from the corresponding telluric lines. We demonstrate conclusively that the absorption band identified by Karkoschka is not, in fact, due to H2O. We present further evidence that it is most likely to be an absorption band of NH3. We searched our spectra carefully to attempt to identify absorption lines which are actually due to H2O, without success.