BioEd Online

Enceladus has joined the small but select band of moons known to have an atmosphere. The Cassini spacecraft, currently orbiting Saturn, has found a layer of water vapour surrounding the icy moon, which is likely to be issuing from its surface or interior.

The probe found that electrically charged molecules around Enceladus are wobbling Saturn's magnetic field as the moon orbits the ringed giant. The frequency of this jitter, measured by Cassini's on-board magnetometer, is characteristic of ionized water molecules spiralling through the planet's magnetic field.

"It was a complete surprise to find these signals at Enceladus," says Michele Dougherty of Imperial College London, who leads the magnetometer team.

At just 500 kilometres across, the moon's gravity is insufficient to keep hold of the water vapour for long. This means that a strong flow of water must continually replenish the atmosphere, suggesting that Enceladus may be volcanically active or possess steamy geysers.

Topped up

Two other volcanically active moons are already known in the Solar System: Io at Jupiter, and Triton at Neptune. In both cases, gases released during eruptions keep their thin atmospheres topped up. "Enceladus could be Saturn's more benign counterpart to Jupiter's dramatic Io," says Fritz Neubauer of the University of Cologne, Germany, a member of the magnetometer team.

Jupiter's other large moons (Europa, Ganymede and Callisto) have tenuous atmospheres too, but Enceladus is by far the smallest moon to boast a gaseous shroud. Neubauer says it may extend hundreds of kilometres from the surface.

Saturn's largest moon, Titan, is the only satellite in the Solar System with a truly thick atmosphere: its surface air pressure is 50% higher than Earth's. Further calculations are needed to determine Enceladus's surface pressure, says Neubauer, but he thinks it is likely to be similar to that of Jupiter's large satellites.

Rain of ice

The magnetometer measurements were made during Cassini's last two fly-bys of Enceladus, on 17 February and 9 March this year, when the probe got within 500 kilometres of the moon.

The Voyager spacecraft flew past Enceladus in 1981 at a distance of about 90,000 kilometres without detecting an atmosphere. However, since then, scientists have begun to suspect that Enceladus could be the source of Saturn's E ring, made of tiny ice particles.

A continuous rain of ice on the moon itself would also keep the surface fresh and clean, possibly explaining why Enceladus is the most reflective body in the Solar System.