Letter to the Editor

The Herschel-SPIRE submillimetre spectrum of Mars^*

¹ STFC Rutherford Appleton Laboratory, Harwell Innovation Campus, Didcot, OX11 0QX, UK e-mail: bruce.swinyard@stfc.ac.uk
² Max Planck Institute for Solar System Research, Katlenburg-Lindau 37191, Germany
³ Bluesky Spectroscopy, Lethbridge, Canada
⁴ LESIA, Observatoire de Paris, France
⁵ School of Physics and Astronomy, Cardiff University, Cardiff, UK
⁶ Physikalisches Institut, University of Bern, Switzerland
⁷ Space Research Centre, Polish Academy of Sciences, Warsaw, Poland
⁸ LERMA, Observatoire de Paris, France
⁹ European Space Astronomy Centre, European Space Agency, Madrid, Spain
¹⁰ Instituto de Astrofisica de Andalucia, Granada, Spain
¹¹ California Institute of Technology, Pasadena, USA
¹² Instituut for Sterrenkunde, Katholieke Universiteit Leuven, Belgium
¹³ Argelander Institute for Astronomy, University of Bonn, Germany
¹⁴ Astronomy Department, University of Michigan, USA
¹⁵ Université de Bordeaux, Laboratoire d'Astrophysique de Bordeaux, France
¹⁶ Laboratory of Molecular Astrophysics, CAB-CSIC. INTA, Madrid, Spain
¹⁷ Joint Astronomy Center, Hilo, USA
¹⁸ Joint ALMA Observatory, Chile
¹⁹ Institute d'Astrophysique et de Geophysique, Université de Liège, Belgium
²⁰ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, USA
²¹ Department of Physics and Astronomy, University of Lethbridge, Canada
²² 1st Physics Institute, University of Cologne, Germany
²³ SRON, Netherlands Institute for Space Research, Netherlands

Received: 1 April 2010
Accepted: 21 May 2010

Abstract

We have obtained the first continuous disk averaged spectrum of Mars from 450 to 1550 Ghz using the Herschel-SPIRE Fourier-transform spectrometer. The spectrum was obtained at a constant resolution of 1.4 GHz across the whole band. The flux from the planet is such that the instrument was operated in “bright source” mode to prevent saturation of the detectors. This was the first successful use of this mode and in this work we describe the method used for observing Mars together with a detailed discussion of the data reduction techniques required to calibrate the spectrum. We discuss the calibration accuracy obtained and describe the first comparison with surface and atmospheric models. In addition to a direct photometric measurement of the planet the spectrum contains the characteristic transitions of ¹²CO from J 5–4 to J 13–12 as well as numerous H₂O transitions. Together these allow the comparison to global atmospheric models allowing the mean mixing ratios of water and ¹²CO to be investigated. We find that it is possible to match the observed depth of the absorption features in the spectrum with a fixed water mixing ratio of 1×10^-4 and a ¹²CO mixing ratio of 9×10^-4.