Today we’re going to talk about some very famous rings, the rings of Saturn. Almost everything we know about Saturn and its rings and moons, we learned from the Cassini spacecraft which orbited Saturn from 2004 to 2017 when it was deliberately burned in Saturn’s atmosphere to prevent the dead spacecraft from contaminating any of Saturn’s moons.
One of the things that Cassini focused on was Saturn’s famous rings. From Earth, the rings look very peaceful, but Cassini discovered that the rings are a rough and tumble roller derby.
In a paper published in the journal Science, the NASA researchers describe how Cassini showed us that collisions are routine and chunks of ice leave trails of debris in their wakes. Spacecraft data also revealed how small moons play tug-of-war with ring material and how bits of rubble that would otherwise join to become moons are ultimately ripped apart by the gravitational pull that Saturn exerts.
During the period when sunlight hits the rings exactly edge-on, Cassini witnessed rings that are normally flat and perhaps 50 feet thick, being flipped up as high as the Rocky Mountains. The spacecraft also showed us that the rings are made mostly of water ice, with a mysterious reddish contaminant that could be rust or small organic molecules like those found in red vegetables on Earth.
We learned most of what we know about Saturn’s rings from Cassini, and the work still goes on. Scientists are still processing the reams of data gathered by the spacecraft. One of the things they’ve finally learned is just how fast Saturn rotates.
If you look at the planet Saturn, you’ll see that, unlike Jupiter, it has virtually no surface features. That’s because it’s a gas giant and it has no solid surface with landmarks that can be tracked as it rotates. Saturn is also unusual because its magnetic axis is nearly perfectly aligned with its rotational axis. Jupiter’s magnetic axis isn’t aligned with its rotational axis, which means the magnetic pole swings around as the planet rotates. This enabled astronomers to measure a periodic signal in radio waves and calculate Jupiter’s rotation rate.
In a paper published in the Astrophysical Journal, the astronomers turned to the rings to help them determine Saturn’s rotation rate. The researchers studied wave patterns created within Saturn's rings by the planet's internal vibrations. In effect, the rings act as an extremely sensitive seismograph by responding to vibrations within the planet itself.
Most of the waves observed in Saturn's rings are caused by the gravitational effects of the moons that orbit outside the rings, but Cassini scientists discovered some of the ring features are due to the oscillations of the planet itself. These helped the astronomers understand the planet's internal oscillations and internal structure.
The Cassini data was used to probe Saturn’s interior and to obtain the first precise determination of its rotation rate. The length of a day on Saturn, according to their calculations, is 10 hours 33 minutes and 38 seconds. This is several minutes faster than previous estimates based on radiometry from the Voyager and Cassini spacecraft.
The scientists say that they were able to calculate the length of Saturn’s day when most astronomers thought it was impossible. They used the rings to peer into Saturn's interior and discovered this long-sought, fundamental quality of the planet. The rings held the answer.
Turns out you can learn a great deal from rings.