The far-infrared spectra of Jupiter and Saturn

We present far-infrared, disk-averaged spectra of Jupiter and Saturn that were measured with the Long Wavelength Spectrometer aboard the Infrared Space Observatory. The Long Wavelength Spectrometer covered the wavelength region between 43 and 197 mum and achieved in grating mode a resolving power of 150-300. The spectra observed in the far infrared are compared to synthetic spectra calculated from atmospheric radiative transfer models using expected values for the constituent vertical concentration profiles. Rotational transitions of ammonia and phosphine are responsible for the broad absorption features observed: Strong ammonia absorption manifolds are obvious against the background continuum slope, appearing at 63, 72, 84, 101 and 125 put in both Jupiter and Saturn. Also phosphine absorption features are clearly present; we found them at the expected wavelengths of 113 and 141 mum in both planets and at 81, 87, 94, 103, 126 and 160 mum on Saturn only. Our findings are in good agreement with the results obtained with other instruments, in particular the Short Wavelength Spectrometer of the Infrared Space Observatory. On Jupiter we obtained the best fit to the observations with models having an NH3 mixing ratio of 2 x 10(-4). We fitted Saturn's measured spectrum with models where the ammonia mixing ratio amounted to 10-4. For phosphine we found an abundance of 7(-3.5)(+7) x 10(-6) in the troposphere of this planet. (C) 2003 Elsevier Ltd. All rights reserved.