August 24, 2007

C.U. Team Discovers Ring Origin

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The Cornell Cassini imaging team, in collaboration with other astronomers from around the world, has discovered the possible origin of the G ring around Saturn. Their discoveries bring science one-step closer to understanding the formation of Saturn’s ring system, one of space’s many mysteries.
The rings of Saturn are very large structures that interact with other rings, moons and Saturn itself. Some of the rings are very bright and can be seen from telescopes on Earth and others are very dim and can only be seen by spacecraft visiting Saturn.
They have been alphabetically named in the order of discovery, but from the innermost ring to the outermost they are named D, C, B, A, F, G and E. The bright rings that are most familiar from photographs are the C, B and A rings.
Each ring has unique features but all are primarily made from small particles of water ice. The more distant rings are fainter and have been discovered more recently.
“Cassini is the last of the great Flagship missions, designed to answer many different science questions with multiple instruments,” said Prof. Philip Nicholson, astronomy.
Cassini arrived in orbit around Saturn in 2004 and has been sending back information from many different onboard instruments including several cameras and radar as well as a detachable probe that explored Titan, one of Saturn’s moons.
One of Cassini’s mission goals was to better characterize and explain Saturn’s ring system.
While observing the G ring by use of Cassini, astronomers noted a bright arc of material that only appeared in some images.
“To find an arc is kinda weird in a ring, it’s difficult to keep matter confined to a certain longitude [portion of the ring],” said Matthew Hedman, a member of the Cornell Cassini imaging team with Prof. Joe Burns, astronomy, and lead author of a paper on the subject for the journal Science.
Normally matter tends to be spread out along the full length of the ring and not confined to a particular area. However, the Cornell astronomers discovered that the bright arc was in a stable resonance with the nearby moon Mimas.
For every six times Mimas orbited Saturn, the bright arc of material went around Saturn seven times. Stable resonances can trap material, such as this bright arc, and prevent it from spreading out evenly throughout the G ring.
Cassini also has a charged particle detector that can detect the flux of electrons. When Cassini flew over the G ring, there was a significant drop in the number of electrons near the arc. By measuring the drop in the number of electrons, Cornell astronomers were able to find the amount of material in the arc.
If the mass in the arc was collected into a single object it would be about the size of a small moonlet, about 100 meters in diameter. Yet, since the mass is dispersed into small objects ranging from centimeters to meters in size, each piece is difficult to see individually and they appear as a bright arc.
The large objects trapped in this arc are bombarded by meteorites creating smaller particles. These smaller particles are captured by Saturn’s magnetic field and pulled out of resonance and then spread around in a ring, creating the G-ring.
“It’s important that we know where a specific ring came from, since the rings have always been a mystery,” Burns explained.
The next big step according to Hedman will be understanding how the arc of material was created originally.
“That is an open question,” he said.