A new study led by University of Colorado Boulder physicist Sascha Kempf has provided the strongest evidence yet that Saturn’s rings are remarkably young, potentially answering a question that has puzzled scientists for more than a century. . The research, to be published May 12 in the journal Science Advances, pegs the age of Saturn’s rings at no more than 400 million years. That makes the rings much younger than Saturn itself, which is about 4.5 billion years old. “In a way, we’ve closed a question that started with James Clerk Maxwell,” said Kempf, an associate professor in the Laboratory for Atmospheric and Space Physics (LASP) at CU Boulder.
The researchers arrived at that closure by studying what might seem like an unusual subject: dust. Kempf explained that small grains of rocky material are washed through Earth’s solar system almost constantly. In some cases, this flow can leave a thin layer of dust on planetary bodies, including the ice that forms Saturn’s rings. In the new study, he and his colleagues set out to date Saturn’s rings by studying how quickly this layer of dust accumulates, a bit like knowing the age of a house by running your finger across its surfaces. “Think of the rings as the rug in your house,” Kempf said. “If you have clean carpet, you just have to wait. Dust will settle on your carpet. The same goes for the rings.
It was an arduous process: From 2004 to 2017, the team used an instrument called the Cosmic Dust Analyzer aboard NASA’s latest Cassini spacecraft to analyze dust specks flying around Saturn. During those 13 years, the researchers collected just 163 grains that originated beyond the immediate neighborhood of the planet. But it was enough. Based on their calculations, Saturn’s rings have likely been collecting dust for only a few hundred million years. The planet’s rings, in other words, are new phenomena that arise (and potentially even disappear) in what amounts to the blink of an eye in cosmic terms. “We know roughly how old the rings are, but it doesn’t solve any of our other problems,” Kempf said. “We still don’t know how these rings formed in the first place.”
Researchers have been captivated by these seemingly translucent rings for more than 400 years. In 1610, the Italian astronomer Galileo Galilei first observed the features through a telescope, although he did not know what they were. (Galileo’s original drawings make the rings look a bit like the handles of a water jug.) In the 1800s, Maxwell, a scientist from Scotland, concluded that Saturn’s rings could not be solid, but were made up of many individual pieces. Today, scientists know that Saturn is home to seven rings made up of countless chunks of ice, most no bigger than a rock on Earth. In total, this ice weighs about half as much as Saturn’s moon Mimas and extends nearly 175,000 miles from the planet’s surface. Kempf added that for most of the 20th century, scientists assumed that the rings probably formed at the same time as Saturn. But that idea raised some problems, namely Saturn’s rings are clean and shiny. Observations suggest that these features are made up of about 98% pure water ice by volume, with only a small amount of rocky matter. “It’s almost impossible to end up with something that clean,” Kempf said.
Cassini offered the opportunity to put a definitive age on Saturn’s rings. The spacecraft first arrived at Saturn in 2004 and collected data until it deliberately crashed into the planet’s atmosphere in 2017. Shaped similar to a bucket, the Cosmic Dust Analyzer picked up tiny particles as they whizzed by. LASP engineers and scientists designed and built a much more sophisticated dust analyzer for NASA’s upcoming Europa Clipper mission, scheduled for launch in 2024. The team estimated that this interplanetary debris would contribute much less than a gram of dust to each square foot of Saturn’s rings each year: a small amount, but enough to accumulate over time. Previous studies also suggested that the rings might be young, but did not include definitive measurements of dust accumulation.
Stroke of luck The rings may already be disappearing. In an earlier study, NASA scientists reported that ice is slowly falling on the planet and could disappear completely in another 100 million years. That these ephemeral features existed at a time when Galileo and the Cassini spacecraft could observe them seems almost too good to be true, Kempf said, calling for an explanation of how the rings formed in the first place. Some scientists, for example, have postulated that Saturn’s rings may have formed when the planet’s gravity tore at one of its moons. “If the rings are short-lived and dynamic, why are we seeing them now?” he said. “It’s too lucky.”