Observations of Saturn with HIFI, performed initially in June 2009 and in more details in June 2010 within the framework of the KP-GT ..Water and related chemistry in the Solar System", have revealed unexpected absorptions in the core of several emission lines of water from Saturn.s atmosphere (557 GHz, 987 GHz, 1113 GHz and 1670 GHz). These absorptions cannot occur in Saturn itself; rather we show that they are due to absorption from water in the ..Enceladus torus", i.e. a cloud of material originating from Enceladus. active plumes, spreading around Saturn and forming a broad toroidal structure centered around Enceladus. orbit at 4 Saturn radii. Based on a comet-like fluorescent excitation model, our preliminary analysis of these data indicate line-of-sight water column densities of (1-3)x10^13 cm-2 and a radial extent of about 2.5 Saturn radii. This discovery provides an entirely new method to probe physical conditions (density, structure, and composition) in the Enceladus torus. Here we propose a detailed follow-up on these observations. The goals are (i) to monitor the variation of these absorptions with viewing geometry, taking advantage thatthe change of aspect in the Saturn system (with the satellite and ring system becoming progressively more ..open") over the upcoming years (ii) to search for H2O emission directly originating from the torus (iii) to search for several additional compounds such as NH3 (known to be produced by Enceladus. plumes), OH (seen in the UV from HST) and several ionized species (H3O+, H2O+, OH+, expected from torus ionization). The ensemble of data will hopefully provide us with (i) an improved understanding of the excitation conditions in the torus (e.g. on the role of electrons) (ii) an improved understanding of its composition and chemistry (iii) a detailed 3-dimensional view (radial, vertical and longitudinal) of the torus and the ability to directly test physically-based model torus models.
Herschel was launched on 14 May 2009! It is the fourth 'cornerstone' mission in the ESA science programme. With a 3.5 m Cassegrain telescope it is the largest space telescope ever launched. It is performing photometry and spectroscopy in approximately the 55-671 µm range, bridging the gap between earlier infrared space missions and groundbased facilities.