Letter to the Editor

First results of Herschel-SPIRE observations of Titan^⋆

¹ LESIA–Observatoire de Paris, CNRS, Université Paris 6, Université Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France
e-mail: regis.courtin@obspm.fr
² University College London, Department of Physics and Astronomy, Gower Street, London WC1E 6BT, UK
³ University of Lethbridge, Institute for Space Imaging Science, Department of Physics and Astronomy, Lethbridge, Alberta T1K 3M4, Canada
⁴ Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany

Received: 20 October 2011
Accepted: 10 November 2011

Abstract

We investigate the composition of Titan’s stratosphere from high-resolution submillimetric observations performed with the SPIRE instrument on the Herschel satellite. From the flux density spectrum measured in the 20 − 52 cm^-1 interval at an apodized resolution of 0.08 cm^-1, we determine the stratospheric abundances of CH₄, CO, HCN, as well as the isotopic ratios ¹²C/¹³C = 87  ±  6 in CO and 96  ±  13 in HCN, ¹⁴N/¹⁵N = 76  ±  6, and ¹⁶O/¹⁸O = 380  ±  60. The last of these results is the first documented measurement of Titan’s ¹⁶O/¹⁸O ratio in CO, with a value 24% lower than the terrestrial ratio.