Nic Fleming
Source

Saturn's moon Enceladus

Space scientists said yesterday that they were baffled and excited at the discovery of a mysterious heat source beneath the surface of Enceladus, one of Saturn's moons.

Readings taken by the Cassini spacecraft and unveiled yesterday unexpectedly showed the 311 mile-wide moon had an atmosphere composed mostly of water vapour.

The most detailed images yet of the moon show a series of long and intriguing fault lines around Enceladus's south pole.

Cassini's instruments identified an unexplained source of heat below the moon's surface in this region that appears to be shooting out jets of gas, ice and dust particles.

Scientists are intrigued because neither radioactive decay nor gravitational tidal forces, thought to be the only two potential sources of internal heating of planetary bodies, should be able to generate the effects measured by Cassini.

Prof Michele Dougherty, of Imperial College London, and principal investigator for Cassini's magnetic field measuring equipment, said: "It was a complete surprise to find these signals at Enceladus.

"These new results from Cassini may be the first evidence of gases originating either from the surface or possibly from the interior of Enceladus."

Dr Torrence Johnson, from Nasa's Jet Propulsion Laboratory in Pasadena, California, said at a news conference in London yesterday: "We've been here before. These are early days in understanding the data. In a sense, it's nice to be baffled at this point.

"At the moment the maths does not add up. We did not expect what we know to be the available power sources here to be able to produce this kind of heat.

"I think we can expect some of our clever colleagues with models of evolutionary history of orbits and interior structure to come up with ways to explain this." Mission scientists made their first surprising discoveries about the moon after examining data collected by Cassini during two flybys, at 725 miles and 310 miles above the moon, on Feb 17 and March 9.

The spacecraft's magnetometer showed Saturn's magnetic field was being bent around Enceladus, suggesting it had an atmosphere, or layer of gas bound to it.

That an atmosphere is able to persist around a body with such low gravity indicated a level of geological activity sufficient to produce a constant supply of gas and water vapour. On July 14 the spacecraft got down to 108 miles above the moon's surface.

This time magnetometer readings showed the atmosphere was concentrated at the south pole. Cameras captured detailed images, showing the variability of its surface.

While the area to the north of the equator is pockmarked with impact craters, indicating geological inactivity, large areas around the south pole have a smooth surface, suggesting the terrain is much younger.

Of interest was a series of large cracks, dubbed "tiger stripes", centred around the south pole.

Also unexpected was the detection of frozen methane and other simple organic chemicals on the moon and in its atmosphere.

Infrared measurements showed an unexpected temperature distribution with a patch of "warm" temperatures, around -188C (-307F), near the southern fault lines.

Enceladus is believed to be losing material from its interior at the rate of around half a ton a second, probably settling an old debate about whether it is the source of material for Saturn's "E-ring", the outermost of the planet's famous rings.

Scientists do not know what is creating the heat source but believe is has to be a combination of radioactive decay of rock and tidal heating - frictional heating of the moon's interior caused by the gravitational pull of Saturn.

Cassini, a £2 billion joint European Space Agency and Nasa mission, has been exploring Saturn and its large family of moons since July last year.

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