Further evidence about water ocean on Enceladus

Cassini has found further evidence to suggest that the Saturn's sixth largest moon Enceladus has a reservoir of water - perhaps an ocean - beneath its surface. A subsurface ocean could provide a suitable environment for the formation of life. The finding, published in the most recent (25 June) issue of Nature, was made by a European instrument team that detected sodium salts in ice grains of Saturn's outermost ring, which is primarily replenished by material from the jets of Enceladus.

The water-ice jets on Enceladus were first discovered in 2005, with key contributions from a UK led team, using the Dual Technique Magnetometer instrument. These jets expel tiny ice grains and vapour, some of which escape the moon's gravity, forming Saturn's outermost ring, the E ring. Cassini's Cosmic Dust Analyser has examined the composition of those grains and found salt within them.

'We believe that the salty minerals deep inside Enceladus washed out from rock at the bottom of a liquid layer,' said Frank Postberg, Cassini scientist on the Cosmic Dust Analyser at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany and lead author of the study.

Scientists on Cassini's cosmic dust detector team conclude that liquid water must be present because it is the only way to dissolve significant amounts of minerals to account for the levels of salt detected. The process of sublimation - the mechanism in which vapour is released directly from solid ice in the crust - cannot account for the presence of salt.

The makeup of the E ring grains, determined when thousands of high-speed particle hits are registered by Cassini, provides indirect information about the composition of the plume material and what's inside Enceladus. The E ring particles are almost pure water ice, but nearly every time the dust analyser has checked for composition, it has found at least some sodium within the particles.

'Our measurements imply that besides table salt, the grains also contain carbonates like soda, both components in concentrations that match the predicted composition of an Enceladus ocean,' said Postberg. 'The carbonates also provide a slightly alkaline pH value. If the liquid source is an ocean, that, coupled with the heat measured near the moon's South Pole and the organic compounds found within the plumes, could provide a suitable environment on Enceladus for the formation of life precursors.'

In another study published in Nature, researchers doing ground-based observations of the vapour cloud rather than the ice grains did not see sodium, an important salt component, in the vapour. That team notes that the amount of sodium being expelled from Enceladus is actually less than observed around many other planetary bodies. They argue that if the plume vapour does come from ocean water, the evaporation must happen slowly deep underground, rather than as a violent geyser erupting into space, because a violent salt-water geyser would eject sodium into the vapour cloud.

'Finding salt in the plume gives evidence for liquid water below the surface. The lack of detection of sodium vapour in the plume gives hints about what the water reservoir might look like,' said Sascha Kempf, Cassini scientist on the Cosmic Dust Analyser from the Max Planck Institute for Nuclear Physics.