[meteorite-list] High Energy Electron Holes Reveal Unseen Rings (Cassini)

[Ron Baalke]

http://www.jpl.nasa.gov/news/features.cfm?feature=1611

High Energy Electron Holes Reveal Unseen Rings
Jet Propulsion Laboratory
February 19, 2008

Gaps in the soup of high energy particles near the orbits of two of
Saturn's tiny moons indicate that Saturn may be surrounded by
undiscovered, near-invisible partial rings. A paper in the February
issue of the journal Icarus suggests the larger Saturnian moons may not
be the only ones contributing material to Saturn's ring system.

A team of scientists has detected two peculiar breaks in the
near-constant rain of high energy electrons that bombard Cassini when
near Saturn. They made the discovery using Cassini's Low Energy
Magnetospheric Measurement System, a part of the Magnetospheric Imaging
Instrument. The gaping holes fall along the orbits of two newly
discovered moons, Methone and Anthe. Methone, discovered by Cassini in
2004, is about 3 kilometers across (2 miles), while Anthe, discovered in
Cassini images in 2007, is about 2 kilometers wide (1 mile). Both moons
are located between the orbits of Mimas and Enceladus.

"These observations tell us that even Saturn's smallest moons could be a
source of dust in the Saturnian system," said Elias Roussos, the paper's
lead author from the Max Planck Institute for Solar System Research in
Katlenburg-Lindau, Germany.

If the tiny moons are indeed feeding dust into the rings, Roussos says
possible future detection and characterization of these rings by more
Cassini sensors could provide information about the surface of the moons
Methone and Anthe, which are difficult to observe due to their small size.

Moons are known to absorb high energy particles. The fact that particles
are missing is sensed by Cassini in the same way there are brief moments
without rain falling on the windshield when driving under a bridge.
These gaps in the flow of electrons showed that something wide was
absorbing the charged particles. However, the gaps Cassini saw at
Methone and Anthe are so wide, about 1,000 to 3,000 kilometers (600 to
1,900 miles) across, that they cannot be explained solely by the
presence of such tiny moons. Instead, the measurements may indicate that
the two moons are losing dust from their surface, building up one or
more arcs of material along their orbits. Each ring arc is expected to
be a few thousand kilometers wide and to comprise large dust grains or
dust clumps.

"The released material may develop into ring arcs due to the
gravitational 'tug of war' between Saturn's larger moons, such as
Mimas," added Roussos. "A similar process has been found to take place
at the arc within Saturn's G-ring."

Meteoroid impacts on Methone and Anthe are the most likely cause of the
release of this material from their surfaces. The same process is
thought to have formed Jupiter's faint rings at the orbits of the moons
Amalthea, Thebe, Metis and Adrastea. The same situation might be
happening at Saturn. In fact, rings of similar origin have also recently
been detected in Cassini images along the orbits of the Saturnian moons
Janus, Epimetheus and Pallene.

"What's odd is that these inferred ring arcs still remain undetected in
Cassini images, while the rings at Janus, Epimetheus and Pallene orbits,
thought to form under the same process, are visible," said Roussos.
"This means the dust grains making up these two different classes of
rings have different characteristics and sizes. However the reason
behind this difference is a mystery."

The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. JPL, a division of
the California Institute of Technology in Pasadena, manages the
Cassini-Huygens mission for NASA's Science Mission Directorate,
Washington. The Cassini orbiter was designed, developed and assembled at
JPL.

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