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 What happened around Saturn in September 1983?

 Here’s a hint: it was pretty much the same thing happening to Jupiter in the July 1994.

This cosmic detective story was explained by Dr. Matt Hedman of Cornell University at the Northwestern University weekly Astrophysics seminar on 29 March 2011.

Hedman’s talk was titled, "What's Going on Around Saturn? New Findings on Saturn's Rings from the Cassini Mission"

The Cassini spacecraft has been in orbit around Saturn for about seven years, making a wide variety of measurements of the planet, its moons and its rings. This flood of information gives us a new picture of the diverse range of dynamic phenomena occurring in the complex ringed planet system.

Hedman is one of the Cassini team studying how the rings of Saturn respond to outside influences. For the first time, researchers can watch detailed reactions in the denser regions in Saturn's main rings, which are very hard to theoretically analyze or model.

Cassini is the size of a school bus. It also carried the Huygens Probe when went to the surface of Saturn’s moon Titan in 2005.

Launched 1997, Cassini it flew by Venus twice and Earth once to gather enough momentum to reach the outer part of the solar system, doing a Jupiter flyby on the way to Saturn, which it reached in 2004. It will be working until 2017.

Saturn's rings are a natural laboratory for studying the physics off disk systems. They are nearby examples of astrophysical disks, like spiral galaxies.

Hedman explained how in 2009 the team’s attention was caught by a pattern of brightness variations in Saturn's C ring. The faint pattern appeared when the light from the Sun illuminated the rings edge-on for the first time in Cassini’s mission.

The ring material appeared in a pattern of waves similar to vertical “corrugations” previously seen in the D ring. The C ring waves were 30 to 80 kilometers apart, with an amplitude of only two to 20 meters.

“In the C ring we see a very characteristic brightness band pattern that we didn't see before 2009,” Hedman said. “It is a vertical corrugation……The amplitude is very low, and that's why we only saw it when we did.”

The goal of the Cassini experiment includes studies of the electromagnetic environment, the rings and moons.

Like Earth, the rotation axis of Saturn is tilted slightly, about 5.5 degrees, to the plane of the orbit around the Sun. Saturn's year is 29 Earth years long.

The vertical features were not seen before August 2009, the first day of northern spring in Saturn. In 2004, when Cassini arrived in orbit around Saturn, it was deep winter. “When the sunlight passes through the equatorial ring the rings are illuminated edge on, so we can look for vertical structures,” Hedman explained.

These shadows disappeared to Cassini’s sensors after being visible for about a year.

The pattern of corrugations cover the entire C ring, penetrating the D ring as well. The D ring corrugations have a much larger amplitude, 500 meters. “The Hubble telescope saw this in 1996 in the D ring long before Cassini arrived, but we didn't realize what it is,” Hedman noted.

The ring plane is very thin and it doesn't take much to kick things out of the rings and cast shadows, even from a few hundred meters above the plane. The Cassini teams suspected the ring material was deflected by some outside impact, and the corrugations record the ring particles’ oscillating recovery to the equatorial plane.

By analyzing and modeling the dissipation of these waves in the rings, the Cassini team can estimate how long ago something passed through the rings and caused this disturbance.

So when did this happen? “We can calculate the ring became tilted in late 1983, in September,” Hedman said.

But, Hedman continued, we see the same corrugated ring patterns on Jupiter's rings. We can calculate a better date for Jupiter, the summer of 1994, and every astronomer knows what happened to Jupiter in July 1994, when it was hit by pieces of the comet Shoemaker-Levi 9.What happened in September 1983? No one on earth knows because Saturn and Earth were on opposite sides of the sun then.

“The comet broke up in 1992,” Hedman explained. “We calculated when particles of certain sizes would have returned, and they would have crashed through the ring with enough momentum to tilt the ring by a few degrees.”

Could a comet have caused the corrugations seen by Cassini in Saturn’s rings? Just any comet won't do it. A single object will not produce a broad-scale tilt. You need a lot of debris hitting lots of ring particles. “This still our best guess on what happened to Saturn in 1983,” Hedman concluded. “A one-kilometer comet would do it, like Shoemaker-Levi 9,” as long as it was broken up into a diffuse cloud of debris and extending over a larger area of the rings.

The record of Shoemaker-Levi 9 suggests comets are broken up by close encounters with Jupiter and Saturn, and then crash into the planets on later orbits.

The orbit of Cassini is very elliptical, and goes far outside the rings in a very complex orbital system. It's all done by flying past Titan, which has enough mass to torque the orbit around.

Cassini will run until 2017, when the last of its propellant will be used to steer it into a run by Titan so that the spacecraft passes close by the cloud-tops of Saturn, then a second pass by Titan will change the orbit and crash the spacecraft into the planet when another research probe is flying by to record the event.

“Why crash Cassini into the planet?” Hedman was asked.

“ We do not want a chunk of Cassini crashing into one of the moons potentially capable of having life, of contaminating the moon with earth organisms and plutonium from the power plant,” he explained.

All Photos from NASA-JPL-Space Science Institute