The discovery of a plume of water vapour and ice particles emerging from warm fractures (‘tiger stripes’) in Saturn's small, icy moon Enceladus1,2,3,4,5,6 raised the question of whether the plume emerges from a subsurface liquid source6,7,8 or from the decomposition of ice9,10,11,12. Previous compositional analyses of particles injected by the plume into Saturn's diffuse E ring have already indicated the presence of liquid water8, but the mechanisms driving the plume emission are still debated13. Here we report an analysis of the composition of freshly ejected particles close to the sources. Salt-rich ice particles are found to dominate the total mass flux of ejected solids (more than 99 per cent) but they are depleted in the population escaping into Saturn's E ring. Ice grains containing organic compounds are found to be more abundant in dense parts of the plume. Whereas previous Cassini observations were compatible with a variety of plume formation mechanisms, these data eliminate or severely constrain non-liquid models and strongly imply that a salt-water reservoir with a large evaporating surface7,8 provides nearly all of the matter in the plume.
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We thank G. Moragas-Klostermeyer and S. Helfert for their efforts in preparing the CDA plume observations. We acknowledge the work of the scientists and engineers of the Cassini Team at JPL. This work was supported by Deutsches Zentrum für Luft und Raumfahrt (DLR) and Deutsche Forschungs Gemeinschaft (DFG programme ‘The first 10 million years of the solar system’). J.H. was supported by the UK Science and Technology Facilities Council.
The authors declare no competing financial interests.
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Postberg, F., Schmidt, J., Hillier, J. et al. A salt-water reservoir as the source of a compositionally stratified plume on Enceladus. Nature 474, 620–622 (2011). https://doi.org/10.1038/nature10175
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Advances in Space Research (2020)