The Cassini spacecraft has revealed that Enceladus, the icy moon of Saturn, has a significant atmosphere, the US space agency NASA said Wednesday.

Using Cassini's magnetometer instrument for their studies, scientists at NASA's Jet Propulsion Laboratory (JPL) indicated source of this atmosphere may be volcanism, geysers, or gases escaping from the surface or the interior.

When Cassini had its first encounter with Enceladus on Feb. 17 at an altitude of 1,167 km, the magnetometer instrument saw a striking signature in the magnetic field. On March 9, Cassini approached to within 500 km of Enceladus' surface and obtained additional evidence.

The observations showed a bending of the magnetic field, with the magnetospheric plasma being slowed and deflected by the moon. In addition, magnetic field oscillations were observed.

These are caused when electrically charged (or ionized) molecules interact with the magnetic field by spiraling around the field line. This interaction creates characteristic oscillations in the magnetic field at frequencies that can be used to identify the molecule.

The observations from the Enceladus flybys are believed to be due to ionized water vapor.

"These new results from Cassini may be the first evidence of gases originating either from the surface or possibly from the interior of Enceladus," said Michele Dougherty in a JPL statement. She is the principal investigator for the Cassini magnetometer.

This is the first time since Cassini arrived in orbit around Saturn last summer that an atmosphere has been detected around a moon of Saturn, other than its largest moon, Titan.

Enceladus is a relatively small moon. The amount of gravity it exerts is not enough to hold an atmosphere very long. Therefore, at Enceladus, a strong continuous source is required to maintain the atmosphere.

The need for such a strong source leads scientists to consider eruptions, such as volcanoes and geysers. If such eruptions are present, Enceladus would join two other such active moons, Io at Jupiter and Triton at Neptune.

In 1981, NASA's Voyager spacecraft flew by Enceladus at a distance of 90,000 km without detecting an atmosphere. It's possible detection was beyond Voyager's capabilities, or something may have changed since then, scientists concluded.

Since the Voyager flyby, scientists have suspected that this moon is geologically active and is the source of Saturn's icy E ring. Enceladus is the most reflective object in the solar system,reflecting about 90 percent of the sunlight that hits it.

If Enceladus does have ice volcanoes, the high reflectivity of the moon's surface might result from continuous deposition of icy particles originating from the volcanoes.

Enceladus' diameter is about 500 km. Yet despite its small size,it exhibits one of the most interesting surfaces of all the icy satellites, the JPL said.