NASA Cassini views water on Saturn's Enceladus

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,"

• 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).