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The Question
(Submitted October 27, 1998)
What keeps planetary rings in place?
The Answer
You can think of rings as a bunch of tiny moons, all in a similar orbit
around the planet. Each particle is in free-fall, like the space shuttle
in orbit --- so one could ask why would a ring not stay there forever
instead.
They don't because they collide with each other, and are subject to forces
other than the gravity of its parent planet (gravity of the bigger moons,
magnetic field etc.). Such collisions and extra forces tend to spread
rings out, and at the lowest altitude, friction with the tenuous outer
atmosphere will cause particles to drop onto the planet.
Sometimes planetary rings are kept in place by the gravitational
force of shepherd moons. Saturn has a very intricate ring system with
lots of moons helping to keep its rings together. According to
http://ringmaster.arc.nasa.gov/neptune/neptune.html,
Neptune's rings are probably confined by one of its moons. In the case of
Jupiter's rings, particles in them probably don't stay there
for long (due to atmospheric and magnetic drag). Galileo (the spacecraft)
found clear evidence that the rings are continuously resupplied by dust
formed by micrometeor impacts on the four inner moons, which are very
energetic because of Jupiter's large gravitational field. The inner halo
ring is broadened by interactions with Jupiter's magnetic field.
(from
http://www.seds.org/nineplanets/nineplanets/jupiter.html)
Uranus, like the other gaseous giant planets in our solar system, also
has
a ring system.
You can read more about planetary rings here:
http://www-space.arc.nasa.gov/displaypage.cfm?page=Cuzzi_rings&branch=sst
And more about the planets here:
http://www.seds.org/nineplanets/nineplanets/
Hope this helps!
Maggie Masetti & Koji Mukai
for Ask an Astrophysicist
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