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Cassini Finds an Atmosphere on Saturnís Moon Enceladus

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Issued by the Particle Physics and Astronomy Research Council

For Immediate Release
16th March 2005

The Cassini spacecrafts 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. image: Artist's impression of the atmosphere on Saturn's moon Enceladus

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 Voyagers 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 Saturns more benign counterpart to Jupiters 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 moons 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.

Notes for Editors

For images, including those from the two flybys of Enceladus, and information on the Cassini mission visit Opens in new window and Opens in new window . The magnetometer team homepage is Opens in new window . An artists impression showing the atmosphere on Enceladus is available from the above websites. It is also available on the PPARC website Opens in new window or by contacting Gill Ormrod in the PPARC press office (contact details below).

Image Caption
This artist concept shows the detection of an atmosphere on Saturn's icy moon Enceladus. The Cassini magnetometer instrument is designed to measure the magnitude and direction of the magnetic fields of Saturn and its moons. During Cassini's two close flybys of Enceladus - Feb. 17 and March 9 - the instrument detected a bending of the magnetic field around Enceladus.

The graphic shows the magnetic field observed by Cassini along its trajectory plotted in a vector form. Even though the spacecraft altitude was almost 500 kilometers (310 miles) at closest approach and the flyby was upstream of the moon (where the interaction is expected to be weaker) Cassini's magnetometer observed bending of the magnetic field consistent with its draping around a conducting object, which indicates that the Saturnian plasma is being diverted away from an extended atmosphere.
Credit: NASA/JPL


Gill Ormrod, PPARC Press Office
Tel +44 1793 442012. Mobile: 0781 8013509.

Professor Michele Dougherty Principal Investigator, Magnetometer
Imperial College London
Tel: +44 20 7594 7757. Mobile: +44 7990 973761.

Carolina Martinez - Jet Propulsion Laboratory, Pasadena, Califonia, USA
Tel: +1 (818) 354-9382.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL. The magnetometer team is based at Imperial College in London, working with team members from the United States and Germany.

The Particle Physics and Astronomy Research Council (PPARC) is the UKs strategic science investment agency. It funds research, education and public understanding in four broad areas of science - particle physics, astronomy, cosmology and space science.

PPARC is government funded and provides research grants and studentships to scientists in British universities, gives researchers access to world-class facilities and funds the UK membership of international bodies such as the European Particle Physics Laboratory, CERN, the European Space Agency and the European Southern Observatory. It also contributes money for the UK telescopes overseas on La Palma, Hawaii, Australia and in Chile, the UK Astronomy Technology Centre at the Royal Observatory, Edinburgh and the MERLIN/VLBI National Facility.