By Sabine Stanley Ph.D., Johns Hopkins University
Saturn has always been a bright object in the sky but was wrongly considered a star until 1655. Then, scientists realized they were looking at a gas giant with mysterious rings and a few moons. The rings attracted even more attention when their composition and structure were revealed.
Early Observations of Saturn
Galileo was the first astronomer to build a telescope for the specific purpose of observing the sky and its objects. In 1610, he discovered that Saturn was a ‘star’ composed of three, which, in his words, ‘almost touch each other, never change or move relative to each other and are arranged in a row’.
Galileo explained that Saturn had two massive moons, orbiting the planet very close and never changing. Two years later, the never-changing moons disappeared. Galileo correctly predicted that they would return. In 1616, he observed a new shape for Saturn, with a large central sphere surrounded by two arms.
More shapes were observed and sketched until 1655 when scientists realized Saturn was a planet with a ring system. Christiaan Huygens, a Dutch scientist, made a 50-times-magnification telescope and studied Saturn’s shapes by the plane of the solar system, the ecliptic. He found out that Saturn was surrounded by ‘a thin, flat ring, nowhere touching, and inclined to the ecliptic’.
Learn more about Saturn’s Moons: Titan to Enceladus.
Saturn’s Different Shapes
When the rings are perfectly aligned with the Earth-Sun line, they are too thin to be observed. They are about one kilometer thick and 70,000 kilometers wide, which makes them appear 20 times thinner than paper, from the edge.
The rings tilt 27° relative to Saturn’s orbital plane. The thinness and the tilt create Saturn’s different shapes. In 1675, Giovanni Cassini realized the rings were numerous, with lots of gaps. Thus, Saturn’s biggest mission and gap were named after him.
The Cassini mission orbited Saturn 294 times from 2004 to 2017, sending valuable information back to Earth. The rings were, of course, a big part of the mission.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.
The Story of the Rings
Saturn’s rings were named alphabetically in the order of discovery. The main rings are A, B, and C. Fainter rings, with lower density and smaller particles, were discovered from 1980 onward. The closest ring to Saturn is D, and the faintest ones are the outer rings F, G, and E. Ring E is be centered on Saturn’s moon, Enceladus, which erupts the material that forms E.
The rings can be composed of micron-sized particles, but 99.9% are of water ice, making them shine so bright. The particles are loosely scattered, and about 97% of the rings is space. Considering the average size of the particles, one meter in size, they are, on average, about a meter away from each other.
In 2009, the Phoebe ring was discovered. This significantly large ring is located very far from Saturn and is tilted 27° away from the plane of the other rings. With an average of 10 dust particles per cubic kilometer, it is the faintest ring. What keeps the rings with so many gaps together?
Gravitational Forces and the Rings
The gaps are formed due to gravitational interactions with small moons. For instance, the biggest gap, the Cassini division, results from an orbital resonance with Saturn’s moon Mimas. Mimas’s orbital period around Saturn is twice the orbital period that particles would have if they entered the Cassini division. Thus, periodic gravitational tugs are formed that push out material from the Cassini division.
Some other gaps are the result of small moons embedded in the gaps. One example is the Encke Gap in the A ring, caused by Pan – the moon, which is only 30 kilometers across. Pan also creates waves through the rings. Another example is Daphnis, the moon whose shadow is seen on the A ring while it orbits in the Keeler Gap. The resulted waves are about 1.5 kilometers high at the edges of the rings.
The Spokes in Ring B
Ring B has vertical structures up to 2.5 kilometers high. In 1980, the Voyager 1 mission first observed the ‘spokes’. They disappeared in 1998 but were spotted again in 2005 by the Cassini mission, in a Saturn season change. Perhaps, they result from electrostatic repulsion in tiny dust particles hovering about 80 kilometers above the rings. It is not known how they form, but each lasts a few hours. They might be meteoroid impacts into the rings or even lightning bolts in Saturn’s atmosphere.
The Narrow and Dynamic F Ring
The F ring is also unique. Prometheus and Pandora are two moons orbiting right inside and outside the F ring. In 2014, scientists realized that the moons create dynamic incursions and, hence, many kinks and spirals in the F Ring.
In 2006, the Cassini mission discovered the propeller moonlets, thousands of which lie in the A Ring. The moonlets are perhaps the result of bigger moon’s collisions, and they create the propeller-like structures, about 10 kilometers across.
Even though many things were discovered about Saturn since the 17th century, there are still mysteries and new things about it. Saturn’s story might not have an end, but it had the beginning described here.
Learn more about How Our Sun Defines Our Solar System.
Common Questions about Saturn and Its Rings
Saturn is a giant planet, a gas giant, known for its significant rings. Before the 17th century, it was thought of as a star.
Saturn’s rings are made up of ice, a significant amount of space, winds, and magnetic fields.
The most comprehensive mission to Saturn was named after Giovanni Cassini, an astronomer. The Cassini mission arrived at Saturn in 2004 and completed its mission in 2017. Thirteen years of traveling the Saturn system, completing 294 orbits of Saturn, visiting the rings and the moons, and sending a probe to land on one of them, have transformed our view of the ringed planet.