Cassini Captures Best View Yet Of Saturn's Ring Currents

Applied Physics Laboratory
Johns Hopkins University
Laurel, Maryland

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For Immediate Release: December 12, 2007

Cassini Captures Best View Yet Of Saturn’s Ring Currents

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 Dr. S. Krimigis of the Johns Hopkins University Applied Physics
Laboratory (APL) 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. They also
found that Saturn’s ring current is persistently asymmetric – unlike
Earth’s – and it 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 (MIMI)
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,” says Dr. D. Mitchell, a MIMI co-investigator from APL. “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 1.5 million kilometers (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 in Pasadena, manages the
mission for NASA’s Science Mission Directorate, Washington, D.C. APL’s
Krimigis is the principal investigator for MIMI, which was designed and
built and is operated by an Applied Physics Laboratory-led team. The
University of Maryland and the Max Planck Institute for Solar Physics in
Germany contributed two of the three sensors. Part of the analyses for this
work was performed as a collaborative effort with the Academy of Athens in

For more information about the Cassini-Huygens mission, visit
and the MIMI instrument team’s home page,

The Applied Physics Laboratory (APL) is a not-for-profit laboratory and
division of The Johns Hopkins University. APL conducts research and
development primarily for national security and for nondefense projects of
national and global significance. APL is located midway between Baltimore
and Washington, D.C., in Laurel, Md. For information, visit

[NOTE: Images and a movie supporting this release are available at ]