Sub-millimetre spectroscopy of Saturn’s trace gases from Herschel/SPIRE

¹ Atmospheric, Oceanic & Planetary Physics, Department of PhysicsUniversity of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK
e-mail: fletcher@atm.ox.ac.uk
² Dept. of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
³ Space Science & Technology Department, Science & Technology Facilities Council, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK
⁴ Institute for Space Imaging Science, Department of Physics and Astronomy, University of Lethbridge, Lethbridge, Alberta, T1K 3M4, Canada
⁵ Blue Sky Spectroscopy, 9 / 740 4 Ave S, Lethbridge, Alberta, Canada, T1J 0N9
⁶ LESIA–Observatoire de Paris, CNRS, Université Paris 06, Université Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France
⁷ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
⁸ Univ. Bordeaux, LAB, UMR 5804, 33270 Floirac, France
⁹ CNRS, LAB, UMR 5804, 33270 Floirac, France
¹⁰ Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
¹¹ Joint Astronomy Centre, 660 N. AÕohoku Place, Hilo, HI 96720, USA

Received: 7 November 2011
Accepted: 31 December 2011

Abstract

Aims. We provide an extensive new sub-millimetre survey of the trace gas composition of Saturn’s atmosphere using the broad spectral range (15–51 cm^-1) and high spectral resolution (0.048 cm^-1) offered by Fourier transform spectroscopy by the Herschel/SPIRE instrument (Spectral and Photometric Imaging REceiver). Observations were acquired in June 2010, shortly after equinox, with negligible contribution from Saturn’s ring emission.

Methods. Tropospheric temperatures and the vertical distributions of phosphine and ammonia are derived using an optimal estimation retrieval algorithm to reproduce the sub-millimetre data. The abundance of methane, water and upper limits on a range of different species are estimated using a line-by-line forward model.

Results. Saturn’s disc-averaged temperature profile is found to be quasi-isothermal between 60 and 300 mbar, with uncertainties of 7 K due to the absolute calibration of SPIRE. Modelling of PH₃ rotational lines confirms the vertical profile derived in previous studies and shows that negligible PH₃ is present above the 10- to 20-mbar level. The upper tropospheric abundance of NH₃ appears to follow a vapour pressure distribution throughout the region of sensitivity in the SPIRE data, but the degree of saturation is highly uncertain. The tropospheric CH₄ abundance and Saturn’s bulk C/H ratio are consistent with Cassini studies. We improve the upper limits on several species (H₂S, HCN, HCP and HI); provide the first observational constraints on others (SO₂, CS, methanol, formaldehyde, CH₃Cl); and confirm previous upper limits on HF, HCl and HBr. Stratospheric emission from H₂O is suggested at 36.6 and 38.8 cm^-1 with a 1σ significance level, and these lines are used to derive mole fractions and column abundances consistent with ISO and SWAS estimations a decade earlier.