Planets and Rings
Published on Nov 26, 2006 at 6:40 pm.
3 Comments.
Filed under planets.
Saturn is famous for its rings. Since the middle of the 17th Century, it has been known that Saturn is surrounded by a ring system. Saturn’s rings are so obvious that even a small department store type telescope can see the rings. For years, these rings were considered an anomaly. After all, only Saturn was seen to have rings. When the Pioneer 10 and Pioneer 11 spacecraft were sent on their way out of the Solar system, they carried plaques depicting nine planets with the sixth one surrounded by a ring. The Voyager spacecraft showed that Saturn’s ring particles are mostly icy, which is why they are so bright.
But, Saturn isn’t the only planet with rings. When the Voyager 1 spacecraft passed Jupiter in 1979, it found that Jupiter, too, had a ring system. Jupiter’s rings are very small, and very dim, and have not been imaged from Earth. This image was taken by the Galileo spacecraft. Jupiter is blocked out in the middle of the photo. Any exposure deep enough to show the rings would totally overexpose Jupiter. The Galildo spacecraft showed that Jupiter’s rings are mostly rocky. That may explain why they are not as bright as Saturn’s rings. There is also far less material in Jupiter’s rings.
So, both of the two largest gas giants were found to have rings. But in 1977, as Voyager 2 was heading towards an encounter with Uranus, astronomers found that Uranus, too, had a ring system. Uranus’s rings are very dark — almost pitch black, in fact. These rings are so dark that they can never be seen from Earth. 
They were discovered not by seeing them, but rather by the fact that they blocked the light of a distant star as Uranus passed in front of that star. Just before the planet passed in front of the star, the star dimmed as the rings passed by. Then, just after the planet had finished passing in front of the star, it again dimmed as the rings once again blocked its light. The image here was taken in 1986 by Voyager 2 after the spacecraft had passed into the planet’s shadow. The rings are seen by scattered light. They are likely a mixture of dust and ice particles. They are very dark because they seem to be coated in carbon and carbon compounds, like many of the moons of the Uranus system.
With three of the four gas giants haning rings, astronomers eagerly watched Neptune as it, too, passed in front of a star to see if it had rings. Indications of a ring were seen on one side of the planet, but not on the other. This was a mystery until the Voyager 2 spacecraft passed Neptune in 1989. 
It turns out that Neptunes rings are assymetric. They are termed ring arcs to differentiate them from the more uniform rings of the other planets (Actually, the Cassini spacecraft has found ring arcs in some of Saturn’s rings.). The Voyagers spent less and less time in each planet’s vicinity the farther out that it went, and it required longer and longer exposures to record images. Thus, we know less and less about each planet from Voyager data. Also, no other spacecraft besides Voyager 2 has ever been to Uranus and Neptune, so we know the least about their rings.
So, why do all of the gas giants have rings? Astronomers for years tried to explain why only Saturn had rings, and then were suddenly faced with explaining why all the gas giants have rings, and none of the terrestrial planets. It isn’t really clear, but it is known that the larger the planet’s mass, the farther moons have to be from the planet in order to avoid being torn apart by the planet’s gravity. This minimum distance at which a moon held together by gravity is torn asunder by by tidal forces is called the Roche Limit. The gas giants have Roche Limits far enough out that stable orbits are possible within the Roche Limit. So, any moon that is somehow pushed into orbits closer than the Roche Limit would be torn apart. Likewise, material closer to the planet than the Roche Limit would be unable to come together to form a moon. The terrestrial planets’ Roche Limits are so close that atmospheric drag would likely affect the ring particles. This may be part of the answer, but it is only a part of the answer. We really don’t fully understand rings, and their study is an area of active research.
-Astroprof
(Images courtesy of NASA)
A Ler…-- Rastos de Luz on November 27, 2006 at 8:13 am: 1
[…] “PM and Light Pollution“, no Astronomy Blog. A poluição luminosa é uma parte do problema de um desenvolvimento urbano mal estruturado. Se em Portugal, apenas há alguns anos se despertou para este problema, em Inglaterra, combate-se de uma forma diferente…mais sobre este tema no “UK floodlighting petition“, no Davep’s Astronomy; “Planets and Rings“, no Astroprof’s Page. […]
sabina on December 15, 2007 at 3:59 pm: 2
thank you this is veary helpfull
to me because my project i have in
school
rings on March 15, 2008 at 3:04 am: 3
Thank you for your information.