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A new study led by University of Colorado Boulder physicist Sascha Kempf provides the strongest evidence yet that Saturn’s rings are remarkably small — potentially answering a question that has puzzled scientists for more than a century.
The research will be published May 12 in the journal advance science, He relates the age of Saturn’s rings to no more than 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 finished with a question that started with James Clerk Maxwell,” said Kempf, associate professor in the Laboratory for Atmospheric and Space Physics (LASP) at CU Boulder.
Researchers arrived at this closure by studying what might seem like an unusual subject matter: dust.
Kempf showed that small grains of rocky material seep through Earth’s solar system on an almost constant basis. In some cases, this outflow can leave behind a thin layer of dust on planetary bodies, including the ice that makes up 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 — like knowing the age of a house by running your finger across its surfaces.
“Think of rings like the rug in your home,” said Kempf. “If you have a clean carpet, just wait. The dust will settle on your carpet. The same goes for the loops.”
It was a painstaking process: From 2004 to 2017, the team used an instrument called the Cosmic Dust Analyzer aboard NASA’s late Cassini spacecraft to analyze specks of dust flying around Saturn. Over those 13 years, the researchers collected only 163 grains that originated from outside the planet’s immediate vicinity. But it was enough. Based on their calculations, Saturn’s rings have likely only been gathering dust for a few hundred million years.
In other words, planet rings are new phenomena that arise (and possibly disappear) in what amounts to a blink of an eye in the cosmic sense.
“We know about how old the rings are, but they don’t solve any of our other problems,” said Kempf. We still don’t know how these rings formed in the first place.
From Galileo to Cassini
These seemingly transparent rings have captivated researchers for more than 400 years. In 1610, the Italian astronomer Galileo Galilei first noticed the features through a telescope, although he didn’t know what they were. (Galileo’s original drawings make the rings look a bit like the handles of a water jug.) In the 19th century, Maxwell, a scientist from Scotland, deduced that Saturn’s rings could not be solid, but instead consisted of many individual pieces.
Today, scientists know that Saturn hosts seven rings made up of countless pieces of ice, most of which are no larger than a rock on Earth. Altogether, this ice weighs about half that of 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 the rings likely formed at the same time as Saturn.
But that idea raised some problems—namely, that Saturn’s rings are quite shimmery. Observations indicate that these features consist of about 98% pure water ice by volume, with only a trace amount of rocky matter.
“It’s almost impossible to end up with something so clean,” said Kempf.
Cassini offered an opportunity to establish 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. The cosmic dust analyzer, shaped like a bucket, picked up small particles as it passed.
LASP engineers and scientists have designed and built a more advanced dust analyzer for NASA’s upcoming Europa Clipper mission, which is scheduled to launch in 2024.
The team estimated that this interplanetary dirt would contribute far less than one gram of dust per square foot of Saturn’s rings each year — a light sprinkling, but enough to build up over time. Previous studies also indicated that the rings could be small but did not include definitive measures of dust accumulation.
A stroke of luck
The rings may already be gone. In a previous study, NASA scientists reported that ice is slowly raining down on the planet and could disappear completely in another 100 million years.
The existence of these ephemeral features at a time when Galileo and the Cassini spacecraft could have observed them seems too good to be true, Kempf said — and warrants an explanation for how the rings formed in the first place. Some scientists have hypothesized, for example, that Saturn’s rings may have formed when the planet’s gravity tore off one of its moons.
“If episodes are so short-lived and dynamic, why do we see them now?” He said. “That’s a lot of luck.”
Co-authors on the new study include Nicholas Altobelli of the European Space Agency. Jürgen Schmidt of the Free University of Berlin; Jeffrey Causey and Paul Estrada of NASA Ames Research Center; and Ralph Srama of the University of Stuttgart.
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