Liquid water on Enceladus from observations of ammonia and 40Ar in the plume

Abstract

Since Cassini spacecraft images revealed plumes of water vapour and ice particles erupting from Saturn's moon Enceladus in 2006, the search for the water source has been on. Possibilities include liquid water under the ice shell and ice that is subject to heating. The chemical composition of the jets can give clues as to their source. The 9 October 2008 Cassini fly-by provides a data set ideal for the purpose — mass spectrometry with the best signal-to-noise ratio so far obtained, sufficient to allow the identification of chemicals present in trace amounts. Waite et al. use these data to reveal the presence of ammonia in the plume, strong evidence for the existence of at least some liquid water. The overall composition of the plume suggests that it arises from both a liquid reservoir (or ice derived from one) and from the degassing of ice containing volatile materials. Jets of water ice from surface fractures near the south pole of Saturn's icy moon Enceladus produce a plume of gas and particles. The source of the jets may be a liquid water region under the ice shell. Here, ammonia is reported to be present in the plume, providing strong evidence for the existence of at least some liquid water. Jets of water ice from surface fractures near the south pole1 of Saturn’s icy moon Enceladus produce a plume of gas and particles2,3,4,5. The source of the jets may be a liquid water region under the ice shell—as suggested most recently by the discovery of salts in E-ring particles derived from the plume6—or warm ice that is heated, causing dissociation of clathrate hydrates7. Here we report that ammonia is present in the plume, along with various organic compounds, deuterium and, very probably, 40Ar. The presence of ammonia provides strong evidence for the existence of at least some liquid water, given that temperatures in excess of 180 K have been measured near the fractures from which the jets emanate8. We conclude, from the overall composition of the material, that the plume derives from both a liquid reservoir (or from ice that in recent geological time has been in contact with such a reservoir) as well as from degassing, volatile-charged ice.