Detection of Enceladus Torus from Submillimeter Observations with Herschel/HIFI
Résumé
The presence of water vapor in the stratospheres of the Giant Planets and Titan has been established from their emission in rotational lines longwards of 30 microns. Emission in the fundamental H2O line at 557 GHz has been spectrally resolved from heterodyne observations with SWAS at Jupiter and Saturn and ODIN at Jupiter, showing emission line-widths of about 20 km/s, due to planetary rotation. Initial observations of Saturn with the HIFI instrument on Herschel, performed in June 2009, revealed an additional 20 % deep, 5 km/s broad absorption superimposed to the emission, that was not seen in the SWAS observation performed in 1999 (Bergin et al. 2000). This absorption is certainly not due to water in Saturn itself, as the brightness temperature in the line core would indicate implausibly cold temperatures ( 95 K) in Saturn's upper atmosphere, and as the line-width is too narrow. Instead, we interpret it as water vapor absorption along the line-of-sight coming from the Enceladus H2O torus, the difference with the SWAS results being presumably related to the different viewing geometries. We developed an excitation model for water in Enceladus torus including excitation by solar and Saturn's radiation. Extended observations performed in June 2010, and covering the H2O 557, 987, 1113 and 1670 GHz (including a 5-point map at this frequency) confirm this interpretation, as does the fact that the 1097 GHz H2O line does not show absorption. An initial modelling of the data indicates line-of-sight water column densities of (1-3)x1013 cm-2. These observations provide a new method to study physical conditions in the Enceladus torus.