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Enceladus presents a puzzle for Cassini team

Weird stuff afoot around Saturn

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Enceladus, a tiny moon orbiting Saturn, appears to be venting water into space from a series of fractures over its south pole.

The water vapour is pouring out of the body through an anomalous hotspot centred over its south pole, in only the second extra-terrestrial observation of thermal activity in the solar system.

According to the latest data from Cassini, the moon might have lost as much as five per cent of its mass since its formation, and figures suggest it is losing enough material to account for Saturn's E-ring.

The difficulty is, scientists can't explain how the south pole region might be getting so hot.

All the instruments on board Cassini point to something very unusual happening at Enceladus' south pole, the area towards the bottom of the moon in the image here, marked by four heavy crevasses, known as the 'tiger stripes'.

Enceladus: A baffling moon. Picture: CICLOPS

Speaking at a press conference in London yesterday, the researchers professed themselves "happy to be baffled" by the tiny moon. "If all you're doing is confirming your colleagues papers, it is no fun," said Torrence V. Johnson of NASA's Jet Propulsion Laboratory.

He explained that the two known sources of heat: radiogenic heating and tidal heating, are not enough, either alone or in combination to account for the behaviour. "Also, you would expect tidal heating to affect the whole body, or at least both poles. Its possible there is something special about Enceladus' structure that is focusing the tidal heating all in one area," he said.

Another possibility is that Enceladus was in a more eccentric orbit some time ago, and that this orbit provided enough tidal heating to kickstart the process we see the results of today. The surface of Enceladus's south pole is very young, so this could have been a relatively recent event.

Johnson says that the researchers have developed several 'cartoon sketches' of what might be causing the heating in the region, but stressed that he would not refer to their ideas as fully-formed hypotheses.

The first sign that something odd was going on was spotted by Professor Michele Dougherty. While reviewing magnetometer data, she noticed that Saturn's magnetic field lines were being deflected by the moon to a greater degree than its size could account for. She suspected the body might have an atmosphere - which would be unusual for a body just 500km in diameter, and would have to have an internal source - and petitioned mission planners for a closer fly by to see what was going on.

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