A CU Boulder study shows that Saturn’s rings are much younger than the planet itself, about 400 million years old, but their origin remains a mystery. A new study suggests that Saturn’s rings are probably no older than 400 million years, and much younger than the planet itself, which is about 4.5 billion years old. Using data from the Space Dust Analyzer on NASA’s Cassini spacecraft, the research team analyzed the dust accumulation in the rings and concluded that the rings had been collecting dust for only a few hundred million years. However, the origin of Saturn’s rings is still unknown, and preliminary research suggests that they may gradually disappear due to icy rain falling on the planet.

A new study led by physicist Sasha Kempf of the University of Colorado at Boulder has provided the strongest evidence yet that Saturn’s rings are extremely young, potentially answering a question that has plagued scientists for more than a century. The study, published May 12 in the journal Science Advances, determines the age of Saturn’s rings at 400 million years. This makes the rings much younger than Saturn itself, which is about 4.5 billion years old.

“In a way, we’ve completed a question that started with James Clerk Maxwell,” said Kempf, an associate professor at the University of Boulder Laboratory for Atmospheric and Space Physics (LASP).

The researchers came to this conclusion by examining what seemed like an unusual topic: dust. Kempf explained that tiny grains of rocky material flow almost continuously into Earth’s solar system. In some cases, this flow can leave a thin layer of dust on planetary bodies, particularly the ice that makes up Saturn’s rings.

In the new study, he and his colleagues decided to put a date on Saturn’s rings by examining how quickly this layer of dust accumulates—like how old a house is when you run your finger across its surface.

“Think of rings like rugs in your home,” Kempf said. “If you have a clean carpet, you just have to wait. Dust settles on the carpet. The same goes for rings.”

It’s been a painstaking process: From 2004 to 2017, the team used an instrument called the Cosmic Dust Analyzer aboard NASA’s latest spacecraft, Cassini, to analyze dust particles flying around Saturn. During those 13 years, the researchers collected only 163 from outside the planet. But that was enough. According to his calculations, Saturn’s rings probably collected dust for only a few hundred million years.

In other words, a planet’s rings are new phenomena that appear (and potentially even disappear) in cosmic terms in the blink of an eye.

“We know roughly how old the rings are, but that doesn’t solve our other problems,” Kempf said. “We still don’t know how these rings formed.”

From Galileo to Cassini

Researchers have been fascinated by these seemingly translucent rings for over 400 years. In 1610, Italian astronomer Galileo Galilei first observed these features with a telescope, although he did not know what they were. (Galileo’s original drawings make the rings look like the handles of a jug). In the 1800s, Maxwell, a scientist from Scotland, concluded that Saturn’s rings could not be solid, but made up of many separate parts.

Scientists know today that Saturn has seven rings, many of which are made up of countless chunks of ice no bigger than a rock on Earth. In total, this ice weighs half as much as Saturn’s moon Mimas and extends about 275,000 miles from the planet’s surface. Kempf added that for much of the 20th century, scientists assumed that the rings probably formed at the same time as Saturn.

But this idea raised several issues: Saturn’s rings shine with purity. Observations show that these elements consist of about 98% by volume pure water ice and only a small amount of rocky matter.

“It’s nearly impossible to get something this clean,” Kempf said.

Cassini has proposed an exact age for Saturn’s rings. The spacecraft first reached Saturn in 2004 and collected data until it deliberately crashed into the planet’s atmosphere in 2017. A space dust analyzer in the shape of a small bucket collected tiny flying particles.

LASP engineers and scientists have designed and built a much more sophisticated dust analyzer for NASA’s Europa Clipper mission, scheduled for launch in 2024. The team estimated that this interplanetary dirt would add much less than a gram of dust per square foot of Saturn’s rings each year; Previous studies have also suggested that the rings may be young, but did not include precise measurements of dust accumulation.

Chance

The rings may have already disappeared. In previous studies, NASA scientists had reported that ice gradually fell on the planet and could disappear completely after 100 million years.

These transient features seem too good to be true to exist at a time when the Galileo and Cassini spacecraft were able to observe them, and require an explanation for how the rings arose in the first place, Kempf said. For example, some scientists have suggested that Saturn’s rings may have formed when the planet’s gravity shattered one of its moons.