Vanishing Rings

One night in 1610, Italian astronomer Galileo Galilei was peering through a telescope at Saturn when he noticed something odd. The planet appeared to have an unusual shape, almost like a round head with two ears sticking out. A few decades later, astronomer Christiaan Huygens realized that these “ears” were actually the outer edges of Saturn’s most famous feature: its rings.

In modern telescope images, Saturn’s rings may look like a series of smooth, flat discs circling the planet. But observations in the late 1800s showed that the rings aren’t solid. They’re made up of billions of chunks of ice, along with some rocks and dust. All these pieces—some smaller than a grain of sand and others bigger than a school bus—orbit around Saturn.

For hundreds of years, people have wondered how the sixth planet from the sun got its rings (see Our Solar System). Scientists still aren’t 100 percent sure about their origins. The leading hypothesis is that tens of millions of years ago, rocky asteroids, icy comets, or possibly even a moon drifted too close to Saturn. The pull of the planet’s gravity may have torn the approaching object or objects to pieces. The resulting jumble eventually flattened into Saturn’s iconic rings.

Although the rings have been around for a long time, they won’t last forever. In fact, they’re disintegrating at this very moment. Tiny particles from the rings continually fall out of orbit, showering Saturn with debris called ring rain. Studying this phenomenon could help solve the mystery of how Saturn’s rings formed—as well as how long it will be before they’re gone for good.

One night in 1610, Italian astronomer Galileo Galilei peered through a telescope at Saturn. He noticed something odd. The planet appeared to have an unusual shape. It looked almost like a round head with two ears sticking out. A few decades later, astronomer Christiaan Huygens identified these “ears.” They were actually the outer edges of Saturn’s most famous feature: its rings.

In modern telescope images, Saturn’s rings may look like smooth, flat discs around the planet. But observations in the late 1800s showed that the rings aren’t solid. They’re made up of billions of chunks of ice, plus some rocks and dust. Some of these pieces are smaller than a grain of sand. Others are bigger than a school bus. All of them orbit around Saturn.

How did the sixth planet from the sun get its rings? People have wondered for hundreds of years (see Our Solar System). Scientists still aren’t 100 percent sure about the rings’ beginnings. The leading idea involves rocky asteroids, icy comets, or a moon. Tens of millions of years ago, one or more of these objects drifted too close to Saturn. The pull of the planet’s gravity may have torn the object or objects to pieces. Over time, the jumble flattened into Saturn’s famous rings. 

The rings have been around for a long time, but they won’t last forever. In fact, they’re falling apart at this very moment. Tiny particles from the rings constantly fall out of orbit. They shower Saturn with material called ring rain. Studying ring rain could help solve the mystery of how Saturn’s rings formed. It could also reveal how much time is left before they’re gone for good.

Ring rain was first spotted in the 1980s, when NASA scientists noticed strange lines in Saturn’s atmosphere—the layers of gases surrounding the planet. Researchers proposed that the lines were streaks of ice particles falling from the rings onto Saturn. Sean Hsu, a planetary scientist at the University of Colorado Boulder, compares the discovery to going outside in the morning and finding puddles and streaks of water on the ground: clear signs of a downpour. “You’d realize, ‘Oh! It rained last night,’” he says. “That’s essentially the way ring rain was discovered.”

The concept of ring rain makes sense. “Things in orbit slowly lose energy and spiral downward, so there’s going to be material from the rings falling to the planet,” says Bonnie Meinke, a planetary scientist with the National Ecological Observatory Network. But researchers couldn’t confirm this hunch right away. “We had this idea, but we didn’t have any clear evidence for it,” says Meinke.

Ring rain was first spotted in the 1980s. That’s when NASA scientists noticed strange lines in Saturn’s atmosphere—the layers of gases around the planet. Researchers proposed that the lines were streaks of ice particles. They were falling from the rings onto Saturn. Sean Hsu is a planetary scientist at the University of Colorado Boulder. He compares the discovery to going outside in the morning and finding puddles and streaks of water on the ground. Those are clear signs of a downpour. “You’d realize, ‘Oh! It rained last night,’” he says. “That’s essentially the way ring rain was discovered.”

The idea of ring rain makes sense. “Things in orbit slowly lose energy and spiral downward, so there’s going to be material from the rings falling to the planet,” says Bonnie Meinke. She’s a planetary scientist with the National Ecological Observatory Network. But researchers couldn’t confirm this hunch right away. “We had this idea, but we didn’t have any clear evidence for it,” says Meinke.

Then in 1997, NASA launched a probe named Cassini to study Saturn. The spacecraft spent 13 years orbiting the planet and sending information back to scientists on Earth. For the final leg of its mission, Cassini zipped around Saturn’s rings before descending into the atmosphere, taking direct measurements of ring rain. Based on Cassini’s observations, scientists now estimate that up to 10,000 kilograms (22,000 pounds) of ring rain pour down onto Saturn every second. At this rate, the rings could disappear in 100 million years. That may sound like a long time from now, but for a 4.5-billion-year-old planet like Saturn, 100 million years is right around the corner.

Then in 1997, NASA launched a probe named Cassini to study Saturn. For 13 years, the spacecraft orbited the planet. It sent information back to scientists on Earth. For the final leg of its mission, Cassini zipped around Saturn’s rings. Then it descended into the atmosphere and took direct measurements of ring rain. Based on Cassini’s observations, scientists estimate the amount of ring rain. Up to 10,000 kilograms of it (22,000 pounds) pours down onto Saturn every second. At this rate, the rings could disappear in 100 million years. That may sound like a long time from now. But for a 4.5-billion-year-old planet like Saturn, 100 million years is right around the corner.

Scientists continue to study the trove of incredible data that Cassini collected from Saturn. For example, NASA researchers are currently trying to see if ring rain can help them better estimate the age of the rings. They also want to learn more about the processes driving the rings to break down and rain onto the planet.

Cassini collected incredible data from Saturn, and scientists continue to study it. For example, NASA researchers are trying to better estimate the age of the rings. They’re seeing if ring rain can help them. They also want to learn more about the reasons the rings break down and rain onto the planet.

Scientists know that micrometeoroids cause some of Saturn’s ring rain. Micrometeoroids are tiny pieces of rock that whiz through space at superfast speeds. When they slam into small particles in Saturn’s rings, some particles can become electrically charged. Once charged, the particles become attracted to the magnetic field surrounding the planet and get pulled toward it. As they get closer to Saturn, gravity takes over, and they continue their descent. But electrically charged particles account for only a small fraction of the total amount of ring rain. What’s causing the rest isn’t fully understood.

For Meinke, ring rain is a reminder that our solar system is constantly changing. “Change is what science is about,” says Meinke. And although Saturn may ultimately lose its magnificent rings, perhaps someday a distant moon might break up around another planet in our solar system, forming a new set of rings there, she says: “You never know!”

Scientists know that micrometeoroids cause some of Saturn’s ring rain. Micrometeoroids are tiny pieces of rock. They whiz through space at superfast speeds. When they slam into small particles in Saturn’s rings, some particles can become electrically charged. These charged particles become attracted to the magnetic field around the planet and get pulled toward it. As they get closer to Saturn, gravity takes over. The particles continue to fall. But electrically charged particles are only a small fraction of the total amount of ring rain. What’s causing the rest? Scientists don’t fully understand.

Ring rain reminds Meinke that our solar system is constantly changing. “Change is what science is about,” says Meinke. Someday, Saturn may lose its magnificent rings. But perhaps a distant moon will break up around another planet in our solar system. Then a new set of rings will form there. She says, “You never know!”