New evidence that liquid water lies beneath the surface on the Saturn moon of Enceladus has been discovered by Nasa scientists, suggesting that life may exist.
Nasa's Cassini spacecraft flew through icy plumes created by ice volcanoes and detected negatively charged water molecules, in a clear sign an underground sea exists.
On Earth this short-lived type of ion is produced where water is moving, such as in waterfalls or crashing ocean waves.
British scientists, reporting in the journal Icarus, say it is known that the jets contained water but it was not clear before whether this might be liquid.
If there is liquid water on Enceladus, Nasa scientists believe Saturn's sixth-largest moon could have the conditions necessary to sustain life.
High-resolution images already taken by the Cassini spacecraft show that the icy surface of Enceladus has a spreading Earthlike crust that has changed over time.
On Earth the spreading of the sea floor is driven by molten rock and Nasa scientists speculated that the liquid beneath the south pole of Enceladus may be water
Cassini scientist Andrew Coates said the evidence gathered by Cassini pointed to other constituents for life, such as carbon, plus a source of heat to keep the water liquid.
"While it's no surprise that there is water there, these short-lived ions are extra evidence for subsurface water," said Dr Coates, from University College London's Mullard Space Science Laboratory.
"And where there's water, carbon and energy, some of the major ingredients for life are present.
“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.”
Similar negatively charged ions have been found on another satellite of Saturn, Titan, which is the only moon in the solar system with a thick atmosphere.
The data from Enceladus's icy spray was collected by an instrument on Cassini called a plasma spectrometer.
It measured the density, temperature and speed of ions and electrons it collected as it flew through the jets.
Cassini is a project of Nasa, the European Space Agency and the Italian Space Agency.
It has been a major success for U.S. and European scientists since the spacecraft began orbiting Saturn and studying its rings and moons since 2004.
Nasa has just extended the mission's life by seven years.
But the British scientists have been told to abandon their research thanks to swingeing cuts in science spending by the government.