Grains of rock spewed into deep space suggest a small moon of Saturn has hydrothermal vents, boosting theories it may harbour microbial life, scientists said Wednesday.

Reporting in the journal Nature, astrophysicists in the United States offered a solution to a decade-old mystery over dust observed streaming from Saturn's rings.

The grains are disgorged from a mineral-rich, balmy sea beneath the planet's ice-crusted moon Enceladus, they suggested.

It is the first indication of ongoing hydrothermal activity beyond Earth, a possible clue to the existence of warm, water-rich conditions conducive to life beyond our planet.

"It is very exciting that we can use these tiny grains of rock, spewed into space by geysers, to tell us about conditions on, and beneath, the ocean floor of an icy moon," said Sean Hsu of the University of Colorado at Boulder, who led the four-year probe.

A curiosity of the Solar System, Enceladus looks like a ball of ice, 500 kilometres (310 miles) in diameter, its mostly smooth exterior marked by a few shallow, wavy ridges and small pockmarks from space impacts.

At its south pole, Enceladus disgorges fountains of ice crystals from what is thought to be a sub-surface ocean about 10 km (six miles) deep -- about the size of Lake Superior, the second largest freshwater body on Earth.

One theory is that the water can exist in liquid form, despite the deep chill of the outer Solar System, thanks to a phenomenon called tidal heating.

The guts of Enceladus are squeezed and released by the gravitational pull of mighty Saturn, causing friction and thus warmth.

- Space oddity -

But the wonders of Enceladus do not appear to end there.

The unmanned US probe Cassini found nano-sized (billionths of a metre) grains streaming out of Saturn's so-called "E" ring, which is believed to comprise Enceladus crystals.

For years, scientists have been trying to figure what these specks are, and how they got there.

Based on measurements from Cassini's onboard instruments of the grains' chemical signature, Hsu's team concluded they were likely grains of silica -- the mineral found on Earth in quartz and sand -- from the Enceladus sea.

They then used computer simulations and lab tests to calculate how this could happen.

Very hot water -- at least 90 degrees Celsius (194 degrees Fahrenheit) -- that is slightly alkaline and super-saturated with silica, must be shooting upwards from the moon's spongy rocks, before coming into contact with cooler water at the ocean floor.

The big drop in temperature causes the grains to form, which are eventually spouted from the south pole with water, which instantly freezes on contact with space.

The grains' tiny size suggests the process is fast -- a 50-km (30-mile) trip from their hydrothermal origins to outer space that would take only a few years, otherwise the specks would be much bigger.

"Ten years ago, it was a big mystery why the nano-grains were made of silica rather than water ice," said co-author Sascha Kempf.

"Now we know the observations were correct. We know where the silica particles are coming from, and why we are seeing them. We learned something very unexpected."

In a commentary also carried by Nature, Gabriel Tobie at the University of Nantes in western France saw an intriguing parallel with a hydrothermal field back on Earth, dubbed Lost City.

It is a spread of limestone chimneys some 60 metres (200 feet) tall, discovered on the floor of the mid-Atlantic in the early 2000s.

The funnels spit fluids rich in hydrogen and methane at temperatures of 90 C, creating a niche habitat for tough, specialised microbes.

Lost City has been put forward as a model of how microscopic life could be nurtured on other planets.

"The current findings confirm" the idea is valid, said Tobie.

"But only future missions... will be able to confirm Enceladus' astrobiological potential and fully reveal the secrets of its hot springs."