This artist's concept shows a glowing patch of ultraviolet light near Saturn's north pole that occurs at the 'footprint' of the magnetic connection between Saturn and its moon Enceladus. The footprint and magnetic field lines are not visible to the naked eye, but were detected by the ultraviolet imaging spectrograph and the fields and particles instruments on NASA's Cassini spacecraft.
Credit: NASA/JPL/JHUAPL/University of Colorado/Central Arizona College/SSI
LONDON: An interaction between Saturn's magnetic field and its moon Enceladus, known as the moon's 'auroral footprint', has been explained with the discovery of an electric link between the two.
The report of these strong beams of electrons and ions emitted by Enceladus towards Saturn, published in the current issue of Nature, was made by a team of scientists working on NASA's Cassini mission. A similar coupling exists between Jupiter and one of its moons, Io.
"Our results provide the first conclusive confirmation that our global magnetic field model of Saturn is accurate," said lead author Abigail Rymer from the John Hopkins Applied Physics Laboratory in Maryland, U.S. "That's a vital piece of information when you are trying to construct a 3D picture from single spacecraft observations."
Saturn's restless moon
Enceladus is Saturn's six-largest moon, an icy satellite with a diameter of just 500 km (our Moon has a diameter of nearly 3,500 km).
However, despite its relatively small size, Enceladus is of great interest to scientists because it is one of the few outer solar system bodies on which volcanic activity has been observed.
A previous success of the Cassini orbiter was the discovery of cryovolcanism on Enceladus; plumes of icy particles are spewed out of vents on the moon's surface. The auroral footprint of Enceladus arises when ionized particles associated with these volcanic plumes interact with Saturn's magnetic field.
The mechanisms involved in this interaction are similar to those that generate Earth's northern lights, that is accelerated electrons gyrating around high-latitude magnetic field lines.
Dynamic interaction
In Saturn's case, the electrons represent a huge current system looping through Enceladus all the way back to Saturn. Enceladus behaves somewhat like a dynamo, generating electrical currents that form the auroral footprint in Saturn's upper atmosphere.
Abigail Rymer and colleagues have found that the Saturn-Enceladus interaction is dynamic; the auroral emission varies by almost an order of magnitude. They suggest that these variations are related to changes in the volcanic plume activity on Enceladus.
The strong ion and electron beams were detected by instruments aboard the Cassini space probe, a NASA project that has been in orbit around Saturn since July 2004. However, Cassini is usually pointed away from Saturn when it passes a moon.
To make this reading, the Cassini orbiter was turned to point North, in order to look along the magnetic field towards Saturn. "This was a relatively fortuitous observation", said Rymer.
