How Were Saturn’s Rings Formed?

From the lecture Series: A Field Guide to the Planets

By Sabine Stanley Ph.D., Johns Hopkins University

Saturn’s rings are masterpieces of gravity, spiraling dust, and ice with enormous empty spaces in and out. These rings are big enough to host moons inside while others are too narrow to host anything and just particles. The rings will disappear one day. Ever thought, why?

Image showing Saturn, the sixth planet from the Sun and the second-largest in the Solar System.
Saturn’s rings are made of dust and ice particles under the effect of gravity from different directions. (Image: NASA images/Shutterstock)

What are Saturn’s Rings Made of?

Saturn’s rings are not the only ones in the solar system, yet they are unique. They are not made of material emitted from moons. So, what are they made of?

The common theory is that the particles came from a moon, at least 500 kilometers in diameter, which broke up into pieces as it approached Saturn. As a moon gets closer to a giant planet like Saturn, the planet’s tidal forces surpass the moon’s gravity and break it apart. This happens in the Roche region, where the disruptive tidal forces or the attractive gravitational forces are strongest.

It is the Roche limit that presses particles together to form moons. Except for the outermost E ring and the Phoebe ring, the other rings are within the Roche limit. These two rings are composed of micron-sized ice and dust particles from two distant moons. Apart from the composition, how long will it take a ring to be formed?

How Old are the Rings?

There is no definite answer to the question, but the rings are perhaps only 10 to 100 million years old. The supporting evidence came from the Cassini mission: it measured the mass of the rings by detecting how much the spacecraft was gravitationally attracted to the rings. They contain around 40% of the mass of Saturn’s 7th largest moon, Mimas.

Combining the mass with the rings’ cleanness shows that they are perhaps even younger than dinosaurs! But like dinosaurs, they will also die out someday.

3D rendering of Enceladus, Saturn's moon.
The rings were formed, perhaps from a big moon, about 10 to 100 million years ago. (Image: Stephane Masclaux/Shutterstock)

This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.

When Will Saturn’s Rings Disappear?

The rings’ particles ‘rain’ on Saturn. Microscopic-sized water and dust grains get trapped in Saturn’s magnetic fields, descending onto the planet at mid-latitudes. Every 30 minutes, about an Olympic-sized swimming pool’s worth of water rains from the rings.

Scientists initially estimated that the rings will vanish in 300 million years from now. However, the Cassini mission found out the amount of rain is more than expected. Around the equator, the rings rain about 10,000 kilograms of material per second. Other than water ice, there are also methane, nitrogen, carbon dioxide, silicates, and organics in the rain.

By new estimates, the rings will disappear in 10 to 100 million years from now. What is the role of Saturn itself in killing the rings?

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Saturn’s Size and Composition

Saturn is a yellow-shaded gas giant that can fit 764 Earths inside and is 95 times more massive than Earth. It is mainly composed of hydrogen and helium and is significantly larger at the equators than the poles.

There are zonal winds in Saturn’s atmosphere, and the clouds have layers of ammonia ice, ammonium hydrosulfide ice, and water ice. The wind speed can reach 1100 miles per hour, as a result of Saturn’s interior cooling and solar heating. Saturn has seasons due to its 27° tilt as the Earth does, but as the Saturn’s year is 29.5 Earth years, each Saturn season is around 7.5 years long. There are storms too.

Storms on Saturn

Saturn, like Jupiter, produces more heat than it receives from the sun. Unlike Jupiter, the giant storms on Saturn do not last very long. For example, the ‘Great White Spot’ formed in 2010 and ended in 2011. The storm spread out horizontally until it encircled the whole planet and turned into a ring!

The storms disappear shortly after turning into rings. The first storm of this type was observed in 1876. Storms seemed to be seasonal, appearing every one Saturn year. Smaller storms appear now and then, but if another giant one forms in 2020, the seasonal storm theory gets much stronger. What happens at the poles?

Image showing the planet Saturn.
Apparently, everything tends to take a ring form around Saturn, even the storms that disappear relatively quickly. (Image: Merydolla/Shutterstock)

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Saturn’s Poles

Saturn’s north pole has a hexagon about 14,000 kilometers long, rotating at the same speed as Saturn’s interior. The hexagon is still a mystery, but it could be a kind of standing wave. The south pole, on the other hand, has a vortex.

The southern vortex is as big as the Earth with wind speed reaching up to 340 miles per hour. The Cassini mission changed orbit from the equatorial plane to a polar orientation, to discover the poles and Saturn’s atmosphere. After 22 orbits, its signal was lost about 950 miles above the cloud tops.

Thus, the data from the 17th century up to now combined to answer questions about Saturn’s rings. There are still mysteries in and around the planet, but some are solved.

Common Questions about Saturn’s Rings

Q: Why does Saturn have rings?

Saturn’s rings are particles of different sizes orbiting the planet. They are formed as a result of the gravitational forces of small moons around Saturn.

Q: How are the rings around Saturn called?

The rings around Saturn are named alphabetically in the order of discovery. The three closest ones are A, B, and C.

Q: Will Saturn lose its rings?

Evidence shows that Saturn’s rings will eventually be lost. That will take from 10 to 100 million years from now.

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