Unexpected populations of charged molecules and dust tasted by the Cassini spacecraft as it plunged through the plumes of Enceledus provide further evidence that the moon harbours liquid water beneath its icy shell.

Cassini began exploring the Saturnian system nearly five years ago, and during that time has revealed Enceladus as an active world that spews water-rich plumes from cracks in its surface out into space. During the 12 March 2008 flyby of the moon, Cassini's plasma spectrometer discovered negatively charged ions within the plume material, many of which are water, but also include other hydrocarbon species.

On the Earth, negative water ions are present where liquid water is in motion, such as waterfalls or ocean waves; negatively charged ions are also known to exist on Titan and in comets.

“While it’s no surprise that there is water there, these short-lived ions are extra evidence for sub-surface water and where there’s water, carbon and energy, some of the major ingredients for life are present,” says lead author Andrew Coates from University College London’s Mullard Space Science Laboratory, whose results are published in the journal Icarus. “The surprise for us was to look at the mass of these ions. There were several peaks in the spectrum, and when we analyzed them we saw the effect of water molecules clustering together one after the other.”

At Titan, the plasma spectrometer detected extremely large negative hydrocarbon ions with masses up to 13,800 times that of hydrogen, with the largest seen at the lowest altitudes that Cassini flew – 950 kilometres above the moon's surface. These large ions are the likely source of Titan's smog-like haze.

The findings at Enceladus add to the mounting evidence that some of the major ingredients for life are present in this distant world.

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