A small moon that circles Saturn harbours a warm water ocean beneath its thick icy crust, according to research that makes the distant body a prime target in the search for life elsewhere.

The discovery means Enceladus, a 300 mile-wide moon of the ringed planet, is the only place beyond Earth that is known to be hydrothermally active, providing a natural heating system for the subsurface ocean.



Nasa’s Cassini probe has already mapped out a six-mile-deep saltwater ocean under the frozen south pole of Enceladus, but the latest measurements from the spacecraft go further in revealing the water to be warm, at least in places.



Hsiang-Wen Hsu at the University of Colorado in Boulder said that Enceladus appears to have a hot, porous core that heats the deepest water in the ocean to a temperature of at least 90C. On Earth there are similar hydrothermal vents on ocean floors that might have played a role in the emergence of life billions of years ago.



“A hydrothermal system like this could fulfill the basic criteria to have life: that is energy, nutrients and liquid water,” Hsu told the Guardian. “But we don’t know if the water was warm enough for long enough. If the temperature wasn’t stable, life may not have happened there.”



A Nasa image of Saturn and its rings. Photograph: NASA/JPL/Space Science Institute/EPA

Life would struggle to gain a foothold on Enceladus if it had no internal heating system because the Saturnian moon is too distant to be warmed by the sun.

Hsu and his colleagues reached the conclusion that Enceladus has a warm ocean under 25 miles of icy crust after months of work to trace the origins of tiny particles of silicon dioxide that had been collected by Nasa’s Cassini probe.

Using the spacecraft’s Cosmic Dust Analyser, the scientists found that dust particles in one of Saturn’s rings came from plumes that erupt from Enceladus. Among them were silica nanoparticles that most probably formed in chemical reactions between the water and rocky core on the ocean floor.

In lab experiments, the scientists confirmed that they could make similar particles by mixing silicon-bearing rocks with water and holding them at high temperature and pressure. The best conditions were salty and alkaline, according to a report in the journal, Nature.

Ralf Srama, a senior author on the study at the University of Stuttgart, said that the hydrothermal activity on Enceladus is happening today, because the particles in the dust are so young, and daily plumes of water vapour and dust are seen spraying out of Enceladus.

Researchers are not completely sure what makes the core of Enceladus hot, but part of the warmth could come from what scientists call tidal heating. “There are changing gravitational forces on the moon that squeeze the body as it orbits, and any squeezing leads to internal friction and heating,” Srama said.

“What we explain today is the geophysical processes inside the moon that can form these very tiny particles when you dissolve the rocky core in the water,” he added. “We think this water has been warm for millions of years, that’s the really exciting thing.”

Chris McKay, an astrobiologist at Nasa’s Ames Research Center in California, said the findings were important for astrobiology and made the case for more detailed studies of the plumes erupting from Enceladus.

“Overall the results of the plume analyses - this work and previous work by this same team - indicate that the ocean is habitable: it is composed of water of reasonable salinity and pH, contains organic material, and contains biologically useful energy sources,” he said.

“Finally, it shows that the plume contains interesting material that reveals what is occurring deep below the surface. Follow the plume!” he added.

Writing in an article to accompany the Nature report, Gabriel Tobie, at CNRS in Nantes, said: “Cassini will fly through the moon’s plume again later this year, but only future missions that can undertake improved in situ investigations, and possibly even return samples to Earth, will be able to confirm Enceladus’s astrobiological potential and fully reveal the secrets of its hot springs.”