Cassini Finds an Atmosphere on Saturn’s Moon Enceladus

The Cassini spacecraft’s two flybys of the icy moon Enceladus have revealed that the moon has a significant atmosphere. Scientists using Cassini’s magnetometer instrument for their studies, say the source may be due to volcanism, geysers, or gases escaping from the surface or its interior.

When the Cassini had its first encounter with Enceladus on 17th February 2005 at an altitude of 1,167 kilometres (725 miles), the magnetometer instrument saw a striking signature in the magnetic field. On 9th March 2005 Cassini approached to within 500 km (310 miles) 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 ionised) molecules interact with the magnetic field by spiralling 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 ionised water vapour.

“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,” said Professor Michele Dougherty, of Imperial College, London and Principal Investigator for the Cassini magnetometer. In 1981 the Voyager spacecraft flew by Enceladus at a distance of 90,000 kilometres (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.

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 from the surface, 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. “Enceladus could be Saturn’s more benign counterpart to Jupiter’s dramatic Io”, said Professor Fritz Neubauer, co-investigator for the Cassini magnetometer from the University of Cologne, Germany.

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 kilometres or 310 miles (the equivalent distance between London and Penzance). Yet despite its small size Enceladus exhibits one of the most interesting surfaces of all the icy satellites.