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Astronomers long assumed that Saturn’s distinctive rings formed about the same time as the planet some 4.5 billion years ago, in the earliest days of our solar system. That assumption was given a serious challenge by a 2019 analysis of data collected by NASA’s Cassini spacecraft, which suggests the rings were only 10 million to 100 million years ago — just a blink of an eye on cosmic timescales. Now, a new analysis of data on how much dust has accumulated on the rings confirms that controversial finding, according to a new paper published in the journal Science Advances.
“In a way, we’re done with a question that started with James Clerk Maxwell,” said study co-author Sascha Kempf, an astronomer at the University of Colorado, Boulder. In 1610, Galileo Galilei was the first to observe the rings, although his telescope was too coarse to identify them as true rings. He described them as “Saturn’s ears” because they looked like two smaller planets on either side of Saturn. Galileo was stunned when the “ears” disappeared in 1612 as the Earth passed through the ring plane, especially when they reappeared the following year.
Christopher Wren conjectured that Saturn had a ring in 1657, although Christiaan Huygens preceded him in its publication and suggested in his 1659 treatise that the ring had detached from the planet System Saturn, who also reported his discovery of Saturn’s moon, Titan. Robert Hooke saw shadows on the rings. In 1675, Giovanni Cassini had discovered that the ring was a series of smaller rings with gaps between them. More than a century later, Pierre-Simon Laplace would show mathematically that any massive ring would be unstable. Maxwell determined that the “ring” must be made up of many small particles, each independently orbiting Saturn, which was confirmed by observations in 1859. We now know that those particles are almost entirely made up of water ice.
The Space Age made it possible to send probes to explore our solar system, and Pioneer 11, Voyager 1 and Voyager 2 all returned increasingly detailed images of the ringed planet. Then, in 1997, NASA launched the Cassini orbiter, a joint venture with the European Space Agency to explore Saturn, its moons and its ring system. Cassini spent 13 years orbiting the gas giant and did just that, yielding stunning images of unprecedented resolution, as well as a host of scientific insights, including evidence that the so-called “ring rain” falling on the planet can cause that the rings gradually disappear in less than 100 million years.
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One of the instruments aboard Cassini was the Cosmic Dust Analyzer, whose data showed that the rings are being slowly but steadily polluted by a mix of rock dust and other organic compounds, mostly from micrometeoroids in the Kuiper Belt. “Think of the rings as the carpet in your home,” Kempf said. “If you have a clean carpet lying around, you just have to wait. Dust will settle on your carpet. The same goes for the rings.” That’s relevant because one argument for a young age is that the water ice in Saturn’s rings is remarkably clear and pure for structures believed to be 4.5 billion years old. Accumulated layers of dust should have made them much darker.
During its spectacular 2017 “Grand Finale,” Cassini made 22 dives between Saturn and its rings, allowing scientists to determine the masses of both before the spacecraft plummeted to its fiery death in the gas giant’s atmosphere. Luciano Iell of Sapienza University in Rome and his co-authors relied on that data from Cassini for their 2019 paper, as it allowed them to determine the amount of soot on the rings, the rate at which it falls, and the age of the dust. . They concluded that the rings were no older than 100 million years and formed at a time when dinosaurs still roamed the Earth — a vivid detail that generated a thousand headlines and helped the idea gain a stronghold in the popular imagination. They also found that Saturn’s B ring was massive enough to dilute incoming dust, which would explain the relative purity of the icy particles.
Those results were met with skepticism by some, given the many uncertainties. Among the skeptics was Aurelien Crida, a planetary scientist at the Cote d’Azur Observatory, who published a refutation of Iell et al. later that year in Nature Astronomy. To explain the lack of dust deposition on the rings, Crida suggested that some kind of planetary “scrubber” preferentially removed dust from the rings via the ring rain. Cassini’s data showed that this rain contained only 24 percent ice, compared to the rings themselves, which are 95 percent ice. Crida found a possible candidate for this scrubbing mechanism in a 2017 paper by the Kempf group (published in the same special issue of Science), noting the presence of nanobeads coalescing from the main rings flowing toward Saturn.
