Letter to the Editor

Direct detection of the Enceladus water torus with Herschel^⋆,⋆⋆

¹ Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany
e-mail: hartogh@mps.mpg.de
² LESIA, Observatoire de Paris, 5 place Jules Janssen, 92195 Meudon, France
³ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91107, USA
⁴ Université de Bordeaux, Observatoire Aquitain des Sciences de l’Univers, CNRS, UMR 5804, Laboratoire d’Astrophysique de Bordeaux, France
⁵ SRON, Groningen, The Netherlands
⁶ Herschel Science Centre, European Space Astronomy Centre, Madrid, Spain

Received: 30 May 2011
Accepted: 22 June 2011

Abstract

Cryovolcanic activity near the south pole of Saturn’s moon Enceladus produces plumes of H₂O-dominated gases and ice particles, which escape and populate a torus-shaped cloud. Using submillimeter spectroscopy with Herschel, we report the direct detection of the Enceladus water vapor torus in four rotational lines of water at 557, 987, 1113, and 1670 GHz, and probe its physical conditions and structure. We determine line-of-sight H₂O column densities of  ~4 × 10¹³ cm^-2 near the equatorial plane, with a  ~50 000 km vertical scale height. The water torus appears to be rotationally cold (e.g. an excitation temperature of 16 K is measured for the 1113 GHz line) but dynamically excited, with non-Keplerian dispersion velocities of  ~2 km s^-1, and appears to be largely shaped by molecular collisions. From estimates of the influx rates of torus material into Saturn and Titan, we infer that Enceladus’ activity is likely to be the ultimate source of water in the upper atmosphere of Saturn, but not in Titan’s.