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High
Energy Electron Holes Reveal Unseen Rings
Tuesday, February 19, 2008
Credit:
ESA
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 that 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 km across, while Anthe, discovered in
Cassini images in 2007, is about 2 km wide. 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 as when 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 1000 to 3000 km 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."
Source:
ESA
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