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The newspaper of The Johns Hopkins University December 17, 2007 | Vol. 37 No. 15
APL Team Uses Cassini Images to 'See' Saturn's Invisible Ring

Saturn's ring current, captured on March 19, 2007.

By Michael Buckley
Applied Physics Laboratory

Scientists have gotten their best "look" ever at the invisible ring of energetic ions trapped in Saturn's giant magnetic field, finding that it is asymmetric and dynamic, unlike similar rings that appear around Earth.

Using the Magnetospheric Imaging Instrument on NASA's Cassini spacecraft, a team led by Stamatios Krimigis of Johns Hopkins' Applied Physics Laboratory discovered that Saturn's ring of energetic ions — called a "ring current" — is a warped disc that is deflected by the solar wind out of the equatorial plane on the planet's night side and thickens dramatically on the day side. The images were obtained by a unique camera that Krimigis says "visualizes the invisible" and show the plasma and radiation belts in Saturn's environment.

In the Dec. 13 issue of the journal Nature, Krimigis' team describes how Saturn's ring current changes over time; it's a dynamic system, doughnut-shaped but sometimes appearing like someone took a bite out of it. The scientists also found that Saturn's ring current is persistently asymmetric — unlike Earth's — and rotates closely in step with Saturn itself. Ring currents form when hot ionized gas, known as plasma, becomes trapped on a planet's magnetic field lines. The main source of the plasma that forms Saturn's ring current is material from the gas vented by geysers on the moon Enceladus.

At Earth, ring currents form during large solar wind-driven magnetic storms, although they fade quickly as the driving solar wind disturbance recedes into deep space. At Saturn, the Magnetospheric Imaging Instrument observed that the ring current's intensity seemed only weakly related to solar activity.

"We might get a more intense reading when a solar wind pressure spike passes by," said APL's Donald Mitchell, a MIMI co-investigator, "but the surprise is that Saturn's ring current didn't become symmetric or dissipate as it does at Earth. It stayed lumpy and rotated around the planet several times. We don't know exactly why that happens, but we have seen it exhibit this behavior repeatedly."

The presence of a ring current around Saturn was first suggested in the early 1980s from magnetic anomalies observed by NASA's Pioneer 11 and Voyager 1 and 2 spacecraft. But Saturn's ring current had never been mapped on a global scale; only small areas were mapped previously, and not in this detail. MIMI was designed for just this purpose; developed by an APL-led international team, MIMI has three distinct sensors, one of which contributed the images for this work.

False-color images accompanying the Nature article were taken by MIMI's ion and neutral camera and show the intensity of the energetic neutral atoms emitted from the ring current through a process called charge exchange. This happens when a trapped energetic ion steals an electron from a cold gas atom, becomes neutral and escapes the magnetic field. Scientists are using these images to create a map of the invisible ring current, which is roughly five times farther from Saturn than its famous icy rings.

MIMI gathered the images for the Nature paper in March 2007 as Cassini looped nearly 920,000 miles over Saturn's poles, giving the instrument a bird's-eye view of the magnetic activity swirling around the planet.

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, manages the mission for NASA's Science Mission Directorate. Krimigis is the principal investigator for MIMI, which was designed and built and is operated by an APL-led team.

For more information about the Cassini-Huygens mission, go to and the MIMI team's home page,


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