An unusual "ice volcano" on Enceladus, one of the many moons of Saturn, appears to be spewing out plumes of water, Nasa scientists report.
The Cassini spacecraft, which has been orbiting Saturn since 2004, has taken images of bizarre volcanic features at the moon's southern pole which could indicate bodies of liquid water on the otherwise frozen satellite.
If water is being heated by volcanic activity to its liquid state then there is a slim possibility that life might also exist on Enceladus, said Carolyn Porco, the Cassini imaging team leader at the Space Science Institute in Boulder, Colorado.
"We realise that this is a radical conclusion - that we may have evidence for liquid water with a body so small and so cold," Dr Porco said. "However, if we are right, we have significantly broadened the diversity of solar system environments where we might possibly have conditions suitable for living organisms."
High-resolution images taken by the Cassini space probe show icy jets and towering plumes ejecting huge quantities of particles at high speed.
Nasa scientists have ruled out the idea that the particles are produced or blown off the moon's surface by vapour created when warm water ice converts directly to a gas. Instead, they believe that the jets may be erupting from pockets of liquid water above freezing point at the surface of the moon, like colder versions of the hot volcanic geysers found on Earth.
"Other moons in the Solar System have liquid-water oceans covered by kilometres of icy crust," said Andrew Ingersoll of the California Institute of Technology in Pasadena, California.
"What's different here is that pockets of liquid water may be no more than tens of metres below the surface," Dr Ingersoll said.
The Cassini , which has also flown close to the mysterious rings of Saturn, was inspecting the distinctive "tiger-stripe" pattern that dominates the southern pole of Enceladus' icy surface. The polar region appears to be cracked and littered with house-sized blocks of ice but in one region there is also a "hot spot" generated by volcanic activity. Any liquid water in the plume quickly freezes as it rises, until its ionized material reaches Saturn's atmosphere and replenishes the planet's E-ring. Some of the plume material may fall back to the moon's surface as fresh snow, brightening the plains between the moon's tiger-striped troughs, according to the journal Science where the latest research is published.
"Models of the plume suggest that it may be fed by several smaller jets of material and driven by warmer temperatures below the moon's icy surface," the journal says.
John Spenser, a Cassini scientist at Southwest Research Institute in Boulder, Colorado, said: "We previously knew of at most three places where active volcanism exists - Jupiter's moon Io, Earth and possibly Neptune's moon Triton.
"Cassini changes all that, making Enceladus the latest member of this every exclusive club, and one of the most exciting places in the solar system," Dr Spenser said. There has also been oxygen detected in the moon's atmosphere.
Cassini will get another chance to view Enceladus in the spring of 2008 when it flies within 220 miles of the moon.