New structure, density and composition measurements of Enceladus’ water plume were obtained when the Cassini spacecraft’s Ultraviolet Imaging Spectrograph observed the star zeta Orionis pass behind the plume on 24 October 2007, as seen in this animation.
Changes in the starlight as it dimmed while passing through the plume allowed the spectrograph to identify the plume’s physical and chemical composition. The spectrograph detected four high-density gas streams composed of water vapour. The density of the water vapour is twice that of the broad plume of gas that surrounds each jet.
This measurement confirms the theoretical analysis performed prior to the flyby that showed it was safe for Cassini to fly very closely past Enceladus, even through part of the plume, during the 12 March 2008 flyby.