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Saturn’s
faint rings share some of their secrets
NASA
Cassini spacecraft images of Saturn’s diaphanous G and E
rings are yielding new clues about their structure and
formation.
A sequence of recent Cassini images, which has
been made into a brief movie, shows an arc of bright material
looping around the inside edge of the G ring, a tenuous
7,000-kilometer-wide (4,400 miles) band of dust-sized icy
particles lying beyond the F ring by 27,000 kilometers (16,800
miles). Cassini passed between the F and G rings during its
insertion into orbit in June 2004.
Dramatic
edge-on Cassini views of Saturn’s E ring, like these
side-by-side images, reveal for the first time a
double-banded structure similar to that of Jupiter’s
gossamer ring and to the bands of dust found within the Sun’s
asteroid belt.
Credit:
CICLOPS/Space Science Institute
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The G ring arc is the same
feature identified in images of this ring taken in May 2005. “We
have seen the arc a handful of times over the past year,”
said Dr. Matt Hedman, Cassini imaging team associate working at
Cornell University in Ithaca, New York. “It always appears
to be a few times brighter than the rest of the G ring and very
tightly confined to a narrow strip along the inside edge of the
‘normal’ G ring.”
Imaging team members
now believe this feature is long-lived and may be held together
by resonant interactions with the moon Mimas of the type that
corral the famed ring arcs around Neptune. “We’ve
known since the days of Voyager that we had Jovian-type and
Uranian-type rings within the rings of Saturn,” said
Cassini imaging team leader Dr. Carolyn Porco in Boulder, Colo.,
who was the first to work out the dynamics of the Neptunian arcs
in Voyager observations. “Now it appears that Saturn may be
home to Neptunian-type rings as well. Saturn’s rings have
it all!”
The researchers do not know exactly how the
bright arc formed. One possibility is that a collision between
small, perhaps meter-sized icy bodies orbiting within the G ring
set loose a cloud of fine particles that eventually came under
the influence of Mimas. But this new observation suggests that
the remainder of the G ring itself may be derived from particles
leaking away from this arc and drifting outwards. Future Cassini
imaging observations are being planned to take a closer look at
the G ring arc.
Results from Cassini’s previous
encounters with Enceladus indicated its south polar geysers as
the primary source of the E ring particles. Now, images of the E
ring with finer resolution than has ever been obtained before
show telling details that appear to confirm this
relationship.
The new images, taken when Cassini was in
the ring plane and consequently showing an edge-on view, reveal a
double-banded appearance to the ring, created because the ring is
somewhat fainter close to the ring plane than it is 500-1,000
kilometers (300-600 miles) above and below. This appearance can
result if the particles comprising the ring circle Saturn on
inclined orbits with a very restricted range of inclinations. (A
similar effect is seen in the Jupiter’s gossamer ring and
in the bands of dust found within the Sun’s asteroid
belt.)
This special condition might arise for two reasons.
First, the particles being jetted out of Enceladus and injected
into Saturn orbit may begin their journey around Saturn with a
very restricted range of velocities and therefore inclinations.
Second, the particles may begin with a large range of
inclinations but those orbiting very close to the ring plane get
gravitationally scattered and removed from that region.
Future
studies of the E ring, including observations and dynamical
models, should decide this issue. Cassini imaging team member Dr.
Joseph Burns, also of Cornell, said, “We’ll want
images from a few other vantage points to be sure of the
structure, and then we can test several models to see why these
ring particles end up in such a distinct configuration.”
The
Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. The Jet
Propulsion Laboratory (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 and its two onboard cameras were designed,
developed and assembled at JPL. The imaging team consists of
scientists from the U.S., England, France, and Germany. The
imaging operations center and team leader (Dr. C. Porco) are
based at the Space Science Institute in Boulder, Colo.
Source
/ Credit: CICLOPS/Space Science Institute
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