By Jonathan Amos
Science correspondent, BBC News
Ice particles and water vapour spew from cracks in the moon's icy surface
There seems little doubt that Saturn's moon Enceladus hides a large body of liquid water beneath its icy skin.
The Cassini probe, which periodically sweeps past the little moon, has returned yet more data to back up the idea of a sub-surface sea.
This time, it is the detection of negatively charged water molecules in the atmosphere of Enceladus.
On Earth, such ions are often seen where liquid water is in motion, such as waterfalls or crashing ocean waves.
There are no "rollers" on the moon but it does have a very active region near its south pole where water vapour and ice particles shoot through cracks in the surface and rise high into the Enceladian sky.
"We see water molecules that have additional electrons added," explained Andrew Coates from University College London's Mullard Space Science Laboratory.
"There are two ways they could be added - from the ambient plasma environment, or it could be to do with friction as these water clusters come out of the jets, like rubbing a balloon and sticking it on the ceiling," he told BBC News.
Cassini has already detected sodium in the plumes - a signature of the dissolved salts you would expect to find in any mass of liquid water that had been in contact with rock deep within the world for a long period.
The latest observations were made using the Cassini plasma spectrometer (Caps). The instrument was originally flown to acquire data on Saturn's magnetic environment, by measuring the density, flow velocity and temperature of ions and electrons that enter the instrument.
It was never envisaged that Caps would also end up sampling jets at Enceladus and adding to what has become a very compelling story.
"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," said Dr Coates.
"The surprise for us was to look at the mass of these ions. There were several peaks in the spectrum, and when we analysed them we saw the effect of water molecules clustering together one after the other."
Dr Coates and colleagues report the Caps data in the journal Icarus. The measurments were made as the probe plunged through the plumes of Enceladus in a close fly-by in 2008.
ENCELADUS - AN ACTIVE MOON OF SATURN
Enceladus experiences tidal contortions as it orbits its parent planet
This energy is producing a "hotspot" at the satellite's southern pole
Big cracks (L) are 100 degrees warmer than the surrounding ice surface
These so called tiger stripes are the source of immense plumes (R)
Caps found not just the negatively charged water ions but hints of negatively charged hydrocarbons, too. Positively charged hydrocarbons at Enceladus have already been identified by Cassini's Ion and Neutral Mass Spectrometer (INMS).
Where Caps has definitely seen negatively charged hydrocarbons is at Saturn's largest moon, Titan. There, it found colossal ions, some measuring more than 13,000 amu (an amu is roughly the mass of a single hydrogen atom).
"If you have a methane and nitrogen atmosphere and you bombard it with particles from the Saturn's magnetosphere and ultraviolet light from the Sun, you can cook up really large molecules," explained Dr Coates.
"They get bigger as the altitude decreases. They are the source of Titan's haze and also maybe the source of the dunes on the surface as they rain down."
Cassini is a joint venture between the US space agency (Nasa), the European Space Agency (Esa) and the Italian Space Agency (ASI).