The Rings of Saturn

FAS Astronomers Blog, Volume 33, Number 1.

Saturn is without a doubt the most spectacular of the planets with its extensive ring system. All the outer planets have rings, but none are as impressive as Saturn’s. As a comparison, Astronomy magazine has a nice poster showing the rings of the four outer planets. 

Saturn’s main four rings (D, C, B, and A) are forty thousand miles across, but are as little as thirty feet thick. The rings are named in the order they were discovered. 

• The most visible outer A ring and the inner B ring are separated by the Cassini Division. 
• The fainter C and D rings are found inside the B ring closer to the planet itself.
• The F ring is just outside the A ring.
• The very faint G ring is found farther out from the F ring. 
• The wide diffuse E ring is even farther out.  

Galileo Galilee first observed Saturn’s rings in 1610. This was the same year he discovered the four large moons of Jupiter. At the time, Galileo didn’t know what the rings were. He thought they might be moons, although they looked more like “ears” on either side of the planet. In 1612, Galileo noticed that the rings had disappeared! A year or more later, they were back again. This time, Galileo described them as two half ellipses visible on either side. Christiaan Huygens eventually solved the mystery around 1655 (published in 1659) by identifying them as rings that wrapped around the planet. 

Huygens thought the rings were a solid disk. However, around 1676, Giovanni Cassini was the first to hypothesis that there were multiple rings and to discover a gap in the rings. This gap is now known as the Cassini Division and separates the two largest and brightest A and B rings. In 1787, Pierre-Simon de Laplace suggested that the rings were not one solid ring but were composed of smaller ringlets. Soon after, in 1789, James Clerk Maxwell demonstrated that the rings were composed of smaller particles that orbited Saturn together in a disk. 

Over time additional rings have been discovered from the C ring in 1850 to the G ring discovered by Voyager 2 in 1980. Although, there are seven major rings, close up views from spacecraft found that these rings were actually composed of millions of ringlets as seen in these false color images from NASA.

The rings do contain gaps. As noted above, the large gap between the A and B rings is the Cassini Division, which is some 2,900 miles wide. The smaller Encke gap (200 miles wide) and Keeler gap (21 miles wide) are found within the A ring. 

Once again, the rings are not solid. Saturn’s rings are mostly particles of water ice with some rock ranging in size from a grain of sand to a house or maybe even a mountain.

If you look closely, you can see that the rings contain mysterious spokes, which might be caused by electrically charged particles being lifted above the plane of the rings. The spokes were first observed by Voyager 2 in 1981 and later by the Cassini spacecraft. In December 2023, the Hubble Space Telescope captured a nice video of the spokes as they circle the planet. 

All of Saturn’s large moon orbit outside the main rings. The rings reside inside what is known as the Roche limit. This concept was introduced by Éduoard Roche in the late 19^th century. In general, objects within the Roche limit are torn apart by their planet’s gravity, and, as such, rings rather than moons form. Russell Deeks of the BBC Sky at Night Magazine referred to it this way. 

As the great Irish poet WB Yeats once wrote, “things fall apart”, and within the Roche limit is where they do it.

Russell Deeks. “Roche limit explained.” BBC Sky at Night Magazine. May 18, 2024. 

Some moons, called Shepherd Moons, are found within the rings. These moons hold the rings in place and create the gaps found within the rings. 

• The moon Pan orbits at the Encke gap within the A ring and keeps the gap free of particles.
• Daphnis is found in the Keeler gap within Saturn’s A ring. 
• Atlas has an orbit outside of the A ring and was initially thought to be the shepherd moon for the A ring. Although more recently, astronomers concluded that Atlas isn’t responsible for the A ring after all.  
• Epimetheus and Janus are a bit farther out from the main rings and share almost the same orbit. As such, they actually trade places every four years or so. Dust from these two moons creates a very faint ring and these two moons might be responsible for the gap between Saturn’s F and G rings. 
• Prometheus (inner edge) and Pandora (outer edge) were once thought to act as Shepherd moons for Saturn’s F ring – although now some studies suggest that only Prometheus is responsible for the F ring.
• Aegaeon (ee-jee-ən) is an extremely small moon and might be the source of Saturn’s G ring. However, others have attributed the G ring through interaction with the moon Mimas.
• Enceladus, although not a Shepherd Moon per se, has icy geysers at its southern pole. It is this material that is thought to be the source of Saturn’s E ring. 

Over the years there has been a debate over the exact age of the rings. Some think the rings are ancient and formed around the same time as Saturn itself. Others surmise they are the result of a comet or asteroid that ventured too close to Saturn and was torn apart by Saturn’s tidal forces. Two recent studies (2019 and 2022) imply that the rings might be very young (between 100 to 200 million years old) and were possibly formed from the breakup of a moon named Chrysalis. 

Watch out! The rings of Saturn will be going away. They really aren’t leaving, but they will disappear from our view in March 2025. On March 23, the Earth will cross the plane of Saturn’s rings – this is called the ring plane crossing and is when the rings will vanish from our perspective. However, at the time, Saturn will be at conjunction, which means it will be on the other side of the Sun from the Earth and hidden by the glare of the Sun. 

Don’t worry, the rings will reappear for a short time and then disappear again in November 2025. On November 23, you might see a very faint line crossing Saturn as the rings almost fade completely from view. Then as we move into 2026, the rings will slowly reappear eventually appearing the widest in 2032 when the south pole of Saturn points toward the Sun.  

Saturn completes one orbit around the Sun every 29.4 years. While doing so, it is tilted 26.7^o – a bit more than the Earth’s 23.5^o tilt. Because of Saturn’s tilt, the side facing the Sun (and Earth) appears to pivot up and down as it goes around the Sun. This is the same process that gives the Earth seasons. For Saturn, it results in the rings appearing to tilt up, then down, over a 29.4-year cycle. Therefore, every 14.7 years or so, the rings will appear straight on from our vantage point – and they seem to disappear.

On July 19, 2013, the Cassini spacecraft passed into the shadow of Saturn and looked back toward the giant planet revealing the rings. Down to the right below the main rings and between the bright F ring and dim G ring is a faint blue dot – The Earth. Also visible were several of Saturn’s moons. Did you look up and wave at Saturn that day?