Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.
People have coveted Saturn’s rings ever since they were first spotted hundreds of years ago. They are beautiful, and as the Cassini-Huygens mission found to be, more than meets the eye.
Moonlets and Rings
Cassini saw irregularities in Saturn's F ring up to 650 feet in length that are not uniformly distributed in the ring, likely due to gravitational pulls from the moon Prometheus, which is just outside the Roche limit and thus plays havoc on any potential moons forming.
As a result of the gravitational interactions of this and other small moons in the ring, tons of half-mile-sized objects are paving their way through it. The collisions happen at relatively slow speeds (about four miles per hour) because the objects are moving around the ring at roughly the same pace. The paths of the objects look like jets as they travel through the ring.
The collisional theory would help explain why so few irregularities have been spotted since Voyager, which witnessed much more in its short visit than Cassini has. As the objects collide, they break up and thus cause less and less visible collisions to be seen (STSci, Weinstock Oct. 2004, NASA "Cassini Sees," JPL "Bright").
But because of Prometheus's orbital alignment with the rings every 17 years, the gravitational interactions are strong enough to create new moonlets, and a fresh cycle of collisions begins. Fortunately, this alignment happened again in 2009, so Cassini kept an eye on the F ring over the next few years to gather more data.
For the B Ring, not only were gravitational interactions with Mimas at play along the edge of the ring, but also some resonant frequencies were being hit. As many as three additional different wave patterns can travel through the ring at once (STSci, Weinstock Oct. 2004, NASA "Cassini Sees," JPL "Bright").
Another interesting development in our understanding of Saturn's rings came in the discovery of S/2005 S1, now known as Daphnis. It resides in the A Ring, is 5 miles wide, and is the second moon to be found in the rings. Eventually, Daphnis will disappear, for it slowly erodes and helps sustain the rings (Svital Aug 2005).
Aging the Rings
And how old are the rings? Scientists were not sure because models show the rings should be young, but that would mean a constant source of replenishment. Otherwise, they would have faded away a long time ago. Yet initial Cassini measurements show the rings to be about 4.4 billion years old, or just slightly younger than Saturn itself!
Using Cassini's Cosmic Dust Analyzer, they found that the rings usually receive little contact with dust, meaning that it would have taken a long time for the rings to accumulate the material they see. Sascha Kempf, from the University of Colorado, and coworkers found that over seven years, only 140 large dust particles were detected whose paths can be backtracked to show that they did not come from the local area (Wall, Witze, Klesman, Haynes).
The majority of the ring rain comes from the Kuiper Belt, with small traces of the Oort cloud and interstellar dust possible. It is unclear why dust from the inner solar system isn't a larger factor, but size and magnetic fields may be a reason. The potential for dust to come from destroyed moons is still a possibility too. But data from Cassini's death dive into the inner rings showed that the mass of the rings matches that of the moon Mimas, meaning that the earlier findings were contradicted because rings shouldn't be able to hold onto so much mass over a long span of time. The new findings point to an age of 150 to 300 million years old, considerably younger than the prior estimate (Wall, Witze, Klesman, Haynes).
And with all that dust, objects can sometimes form in the rings. In June 2004, data indicated that the A ring had moonlets. Images from Cassini taken on April 15, 2013, show an object at the edge of the same ring. Nicknamed Peggy, it is either a moon forming or an object falling apart.
After this discovery, scientists looked back at over 100 past images and saw interactions in the area of Peggy. Other objects near Peggy were spotted and could be a result of gravitational forces pulling ring material together. Janus and Epimetheus also happen to orbit near the A ring and could contribute to the bright clumps on the edge of the A ring. Unfortunately, Cassini will not be in a viewing position to follow up until late 2016 (JPL "Cassini Images", Timmer, Douthitt).
Feeding the Rings
Though it was long thought to be true, scientists did not have observational evidence for Enceladus feeding Saturn's E ring until recent observations showed the material leaving the moon and entering the ring. Such a system is unlikely to last forever though as Enceladus loses mass each time it ejects the plumes (Cassini Imaging Central).
Sometimes the rings of Saturn fall into shadow during eclipses and offer a chance to be studied in detail. Cassini did this in August of 2009 with its Infrared Spectrometer and found that, as expected, the rings cooled off. What scientists did not expect was how little the A ring did cool off. In fact, the middle of the A ring stayed the warmest during the eclipse. Based on the readings, new models were built to try and explain this.
The most likely reason is in a reevaluation of the size of the particles, with the likely diameter of the average A ring particle being 3 feet in diameter and with a small coating of regolith. Most models predicted a heavy layering of this around the icy particles, but these would not be as warm as needed for the observations seen. It is not clear what is causing these particles to grow to this size (JPL "At Saturn).
Measuring a Day
Interestingly, the rings were key to getting a precise fixing on the length of Saturn's day. Normally, one could use a fixed feature on a planet to find the rate, but Saturn doesn't have that feature. If one understands the interior below, then one could use the magnetic field to help piece it together.
This is where the rings come into the picture, for changes to Saturn's interior caused gravity shifts that manifested themselves in the rings. By modeling how those changes could have arisen using Cassini data, scientists could understand the interior's distribution and find a length of 10 hours, 33 minutes, and 38 seconds (Duffy, Gohd).
Cassini Imaging Central Lab. "Icy tendrils reaching into Saturn ring traced to their source." Astronomy.com. Kalmbach Publishing Co., 20 Apr. 2015. Web. 07 May 2015.
Douthitt, Bill. "Beautiful Stranger." National Geographic Dec. 2006: 50. Print.
Duffy, Alan. "Giving Saturn the time of day." cosmosmagazine.com. Cosmos. Web. 06 Feb. 2019.
Gohd, Chelsea. "What time is it on Saturn? We finally know." Astronomy.com. Kalmbach Publishing Co., 22 Jan. 2019. Web. 06 Feb. 2019.
Haynes, Korey. "Propellers, waves, and gaps: Cassini's last looks at Saturn's rings." Astronomy.com. Kalmbach Publishing Co., 13 Jun. 2019. Web. 04 Sept. 2019.
JPL. "At Saturn, One of These Rings Is Not Like the Others." Astronomy.com. Kalmbach Publishing Co., 03 Sept. 2015. Web. 22 Oct. 2015.
---. "Bright Clumps in Saturn Ring Now Mysteriously Scarce." Astronomy.com. Kalmbach Publishing Co., 16 Sept. 2014. Web. 30 Dec. 2014.
---. "Cassini Images May Reveal Birth of a New Saturn Moon." Astronomy.com. Kalmbach Publishing Co., 15 Apr. 2014. Web. 28 Dec. 2014.
Klesman, Alison. "Saturns Rings Are a Recent Addition." Astronomy, Apr. 2018. Print. 19.
NASA. "Cassini Sees Objects Blazing Trails in Saturn Ring." Astronomy.com. Kalmbach Publishing Co., 24 Apr. 2012. Web. 25 Dec. 2014.
STSci. "Cassini uncovers galactic behavior, explains long-standing puzzles in Saturn’s rings." Astronomy.com. Kalmbach Publishing Co., 02 Nov. 2010. Web. 28 Jun. 2017.
Timmer, John. "Cassini May Be Witnessing the Birth (or Death) of a Moon of Saturn." ars technica. Conte Nast., 16 Apr. 2014. Web. 28 Dec. 2014.
Wall, Mike. "Age of Saturn's Rings Estimated at 4.4 Billion Years." HuffingtonPost.com. Huffington Post, 02 Jan. 2014. Web. 29 Dec. 2014.
Witze, Alexandra. "Saturn's Rings are 4.4 Billion Years Old, New Cassini Findings Suggest." HuffingtonPost.com. Huffington Post, 20 Aug. 2014. Web. 30 Dec. 2014.
© 2022 Leonard Kelley