Flash-frozen seawater sprays from Enceladus

New measurements confirm a liquid ocean under the icy crust of Saturn's moon

June 22, 2011

Is there an ocean on Enceladus or not? This question had been occupying researchers since the Cassini space probe discovered fractures in the icy crust around the southern pole of Saturn’s moon which eject plumes of water vapour and ice grains. Earlier measurements of the composition of these particles, which had been undertaken with the dust detector of the Max Planck Institute for Nuclear Physics, provided indications for a liquid ocean. Other scientists rejected this explanation, however. New data from a close fly-by of Enceladus have now dispelled these doubts.

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Enceladus has a diameter of 500 kilometres or so, making it one of Saturn’s smaller moons. It has a rocky core below an icy crust some 80 kilometres thick. Around its southern pole the surface has a series of fractures and fissures. These “tiger stripes” eject plumes of water vapour and tiny ice particles. The water vapour is ejected at supersonic speed from individual vents and thus feeds the diffuse outer E-ring around the gigantic planet.

In 2009 the scientists published the analysis of the chemical composition of ice particles in Saturn’s E-ring. The researchers detected three types of ice particle in the data, which was obtained with the Cosmic Dust Analyzer (CDA) of the Max Planck Institute for Nuclear Physics aboard the Cassini space probe. One of the types of ice particle contains salts in a quantity and composition which supports the assumption of there being an ocean between icy crust and rocky core. A discussion then followed about whether the salt-rich ice particles could not also have been formed without liquid water.

When Cassini flew through the plumes at an altitude of only 21 kilometres above the south pole of Enceladus, it gave the researchers the opportunity to directly scrutinize the freshly ejected ice grains. The dust detector found the same three ice particle types as in the E-ring, although their proportions changed markedly as a function of the distance from the source: near the source the salt-rich particles dominate; further away, the pure ice particles are in the majority (just like in the E-ring).

The fraction of particles containing silicates or organic material is slightly higher in the plumes. Moreover, the salt-rich ice grains are larger and slower than those without salt. When Cassini flew through the supersonic jet of a vent, the CDA detected an increased proportion of the small, salt-free particles compared to the material ejected by the other “tiger stripes”.

For Frank Postberg, researcher at the Max Planck Institute for Nuclear Physics and at the University of Heidelberg, there is only one plausible explanation for these findings and for the sources of the plumes: large saltwater reservoirs, fed by an ocean between the icy crust and the rocky core of Enceladus. The salt-rich ice particles are flash-frozen seawater spray and comprise the lion’s share of the ejected grains, while the pure ice particles are produced from water vapour – mainly in the vents. Since they are lighter, they are more strongly entrained in the jet of water vapour, and make it to the E-ring. Most of the heavier, salt-rich particles, on the other hand, fall back to the surface of Enceladus.

GH / HOR

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