Letter to the Editor

First results of Herschel-PACS observations of Neptune^*

¹ LESIA, Observatoire de Paris, 5 place Jules Janssen, 92195 Meudon, France e-mail: emmanuel.lellouch@obspm.fr
² Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
⁴ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Belgium
⁵ SRON, Groningen, The Netherlands
⁶ JPL, California Institute of Technology, Pasadena, USA
⁷ Space Research Centre, Polish Academy of Science, Warszawa, Poland
⁸ Blue Sky Spectroscopy Inc., Lethbridge, Alberta, Canada
⁹ European Space Astronomy Center, Madrid, Spain
¹⁰ Instituto de Astrofísica de Andalucía (CSIC), Granada, Spain
¹¹ California Institute of Technology, Pasadena, USA
¹² Rutherford Appleton Laboratory, Oxfordshire, U K
¹³ Deutsches Zentrum für Luft- und Raumfahrt (DLR), Bonn, Germany
¹⁴ University of Michigan, Ann Arbor, USA
¹⁵ Université de Bordeaux, Observatoire Aquitain des Sciences de l'Univers, CNRS, UMR 5804, Laboratoire d'Astrophysique de Bordeaux, France
¹⁶ Laboratorio de Astrofísica Molecular, CAB. INTA-CSIC, Spain
¹⁷ Joint Astronomy Center, Hilo, USA
¹⁸ LERMA, Observatoire de Paris, and Univ. Pierre et Marie Curie, Paris, France
¹⁹ F.R.S.-FNRS, Institut d'Astrophysique et de Géophysique, Liège, Belgium
²⁰ Herschel Science Centre, ESA Centre, Madrid, Spain
²¹ University of Lethbridge, Canada
²² University of Bern, Switzerland
²³ University of Cologne, Germany
²⁴ Max-Planck-Institut für Radioastronomie, Bonn, Germany
²⁵ Experimental Physics Dept., National University of Ireland Maynooth, Co. Kildare. Ireland

Received: 31 March 2010
Accepted: 14 May 2010

Abstract

We report on the initial analysis of a Herschel-PACS full range spectrum of Neptune, covering the 51–220 μm range with a mean resolving power of ~3000, and complemented by a dedicated observation of CH₄ at 120 μm. Numerous spectral features due to HD (R(0) and R(1)), H₂O, CH₄, and CO are present, but so far no new species have been found. Our results indicate that (i) Neptune's mean thermal profile is warmer by ~3 K than inferred from the Voyager radio-occultation; (ii) the D/H mixing ratio is (4.5 ± 1) × 10^-5, confirming the enrichment of Neptune in deuterium over the protosolar value (~2.1 × 10^-5); (iii) the CH₄ mixing ratio in the mid stratosphere is (1.5 ± 0.2) × 10^-3, and CH₄ appears to decrease in the lower stratosphere at a rate consistent with local saturation, in agreement with the scenario of CH₄ stratospheric injection from Neptune's warm south polar region; (iv) the H₂O stratospheric column is (2.1 ± 0.5) × 10¹⁴ cm^-2 but its vertical distribution is still to be determined, so the H₂O external flux remains uncertain by over an order of magnitude; and (v) the CO stratospheric abundance is about twice the tropospheric value, confirming the dual origin of CO suspected from ground-based millimeter/submillimeter observations.