ENCELADUS, one of Saturn’s innermost moons, is one of only three in the solar system to display geothermal or volcanic activity, scientists believe.

Observations sent back by Cassini from Saturn show a surprise hotspot and uneven atmosphere around the moon’s south pole. Such activity has been observed only on Jupiter’s moon Io and Neptune’s Triton.

Professor Dennis Matson, of Nasa’s Jet Propulsion Laboratory in Pasadena, California, said the findings produced “stunning excitement”. The observations were made in a third fly-past of Enceladus by Cassini.

Professor Michele Dougherty, of Imperial College, London, said: “On the basis of the results . . . from the first fly-bys we persuaded the Cassini team to take a much closer look. We got within 106 miles of the surface.” Data collected showed “a really surprising picture of the processes at work”.

Temperatures at the south pole could not be produced by heat from the Sun, as the shiny surface of the planet reflects over 80 per cent of incident rays. The finding is as bizarre as if Antarctica had been found to be hotter than Earth’s equator.

Parallel 80-mile “warm” crevasses were also found around the pole, which researchers have dubbed “tiger stripes” because of their appearance. The series of bluish stripes are spaced at 25-mile intervals and warmer than the surrounding ice surface, although they are still very cold at around minus 180C (minus 292F). The cracks act like vents and spew out fine ice crystals which then evaporate into clouds.

Dr David Parker, Director of Space Science at the Particle Physics and Astronomy Research Council said: “This little tiny world turns out to have, perhaps, a boiler within it.”

Terrain at the south pole also appears to be much smoother than elsewhere, indicating recent geologic activity. Simple organic molecules have also been detected, although this does not suggest the presence of any life.

Enceladus, the eighth of Saturn’s known satellites, is only 314 miles wide - half the length of Britain. Discovered in 1789, it is named after a Titan in Greek mythology who was killed in battle and buried under Mount Etna by Athena. It is the shiniest object in our solar system.The Cassini spacecraft, a four-year collaboration between Nasa, the European Space Agency and the Italian Space Agency, departed in 1997 and reached Saturn on July 1, 2004. On January 14, the European Space Agency’s Huygens probe descended to the surface of the moon Titan.

The research team has two further fly-bys of Enceladus planned for later this year to gather more data.

Professor Matson said: “Icy satellites were earlier written off as dull and boring. We don’t know what the whole story is yet, but it’s going to be interesting.”

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