NASA's Cassini
spacecraft has found two new, partial rings around Saturn that each accompany a
small moon, shedding light on what determines whether a partial or complete
ring forms with the moon.
The partial rings, called ring arcs,
extend ahead of and behind the small Saturnian moons
Anthe and Methone in their orbits.
Both Anthe and Methone
orbit Saturn in locations called resonances, where the gravity of the nearby
larger moon Mimas disturbs their orbits. Mimas provides a regular gravitational tug on each moon,
which causes the moons to skip forward and backward within an arc-shaped region
along their orbital paths, said Nick Cooper of Queen Mary, University of London
an a member of the Cassini
imaging team.
"When we realized that the
Anthe and Methone ring arcs were very similar in
appearance to the region in which the moons swing back and forth in their
orbits due to their resonance with Mimas, we knew we had a possible cause-and-effect
relationship," Cooper said.
Scientists believe that the faint
ring arcs likely consist of material knocked off the small moons by
micrometeoroid impacts. The material doesn't spread all the way around Saturn
to form a complete ring because the interactions of the moons with Mimas confine the material to a narrow region along the
moons' orbits.
The recent
Cassini images were the first detection of arc
material near Anthe. The images confirmed the presence of the Methone arc, which was previously detected by Cassini's Magnetospheric Imaging
Instrument.
Previous Cassini
images have also shown faint rings connected with other small moons within or
near the outskirts of Saturn's main ring system, such as Pan, Janus, Epimetheus and Pallene. Cassini has also
previously observed an arc in the G ring,
one of Saturn's fain, major rings.
"This is probably the same
mechanism responsible for producing the arc in the G ring," said Matthew Hedman of Cornell
University and another Cassini imaging team member.
Hedman and his colleagues previously
determined that the G-ring arc is also formed by gravitational resonance with Mimas.
"Indeed, the Anthe arc may be
similar to the debris we see in the G-ring arc, where the largest particles are
clearly visible," Hedman said. "One might
even speculate that if Anthe were shattered, its debris might form a structure
much like the G ring."
The material that orbits with Pallene, Janus and Epimetheus, however, isn't subject to the same powerful
resonant forces and is free to spread out around the planet, forming a complete
ring.
advertisement
