Atmosphere found on Saturn moon

Enceladus is the most reflective object in the Solar System

The Cassini orbiter has made yet another remarkable discovery: Saturn's moon Enceladus has an atmosphere.

The satellite has a diameter of about 500km (310 miles); its gravity is too weak to hold on to an atmosphere for long so it must be being replenished.

Scientists say the source may be volcanism, geysers, or gases escaping from the surface or the interior.

Cassini's magnetometer instrument made the detection on close flybys of the moon on 17 February and 9 March.

"It was a complete surprise to find these signals at Enceladus," said Professor Michele Dougherty, of Imperial College, London, UK, and principal investigator for the instrument.

We're seeing... part of a wider story which is concerned with how the rings are formed and dissipated Prof Stan Cowley, University of Leicester

In 1981, the Voyager spacecraft flew by Enceladus at a distance of 90,000km (56,000 miles) without detecting an atmosphere.

It is possible that detection was beyond Voyager's capabilities or something may have changed since that flyby.

The Cassini magnetometer instrument is designed to measure the magnitude and direction of the magnetic fields around Saturn and its moons.

Ice particles

It was able to infer the presence of a water vapour shroud around Enceladus from the way ionised gas molecules streaming away from the moon were interacting with, and dragging on, the local field lines.

"You see the field bending as though the lines are being draped over the moon," explained Cassini magnetometer team member Professor Stan Cowley, who said if there was no atmosphere the lines should pass straight through the moon.

"You can infer the amount of mass from the moon that is being loaded on to the field lines from the amount of bending that is seen. This signal has been measured twice, in the February and March encounters."

One alternative explanation might be sputtering - a phenomenon that sees energetic particles in the local space environment hit the moon, knocking off atoms and molecules from its cold surface. But the amount of mass detected by Cassini suggests something far more significant is occurring on Enceladus.

If Enceladus does have ice volcanoes they probably get their energy from the contorting tidal effects of Saturn's strong gravity.

The moon is the most reflective object in the Solar System, throwing back about 90% of the sunlight that hits it.

Scientists say it is possible the high reflectivity could result from continuous deposition of icy particles originating from volcanoes.

"If there are ice flakes coming off, these could be a source for the E ring," Professor Cowley, from the University of Leicester, UK, told the BBC News website.

"Enceladus sits in the middle of the outermost ring of Saturn. This is composed of tiny ice particles that only last for hundreds of years. There has to be a source of them and that source is probably Enceladus.

"So, we're seeing here part of a wider story which is concerned with how the rings are formed and dissipated as well."