Cassini has found another Saturnian moon with an atmosphere: this time, it is Enceladus. The discovery was made during two recent flybys by the NASA space craft.

On the first fly-by, the team noticed a striking signature in Saturn's magnetic field. On the second pass, they noticed that the field was actually deflected by the moon. They also saw magnetic field oscillations, caused by ionised molecules interacting with the field lines, at a frequency consistent with ionised water vapour.

Professor Michele Dougherty, of Imperial College, London and Principal Investigator for the Cassini magnetometer described the results as "a complete surprise".

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

Enceladus is interesting for a number of reasons: it is a small icy world, thought to be the source of Saturn's E ring; it also is the most reflective object in the solar system, reflecting 90 per cent of the light that hits it.

But at just 500km in diameter, it is also too small to hold on to an atmosphere for very long, meaning it must be the source of its own atmosphere. This source needs to be continuous and very strong, the research team says, adding to speculation that the moon might be geologically active; generating its atmosphere either though volcanic activity, geysers or gases escaping from the surface or interior of the moon.

Scientists began to suspect that the moon is an active world after the Voyager fly-by in 1981, when it was determined that the moon was the source of the E ring. Ice volcanoes could also explain the moon's high reflectivity, as new eruptions would cover it continually in fresh ice. ®

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