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The rings of Saturn have fascinated skywatchers for centuries, and scientists are still finding tiny gems hidden in the icy dust. Using data from the Cassini spacecraft, researchers studying one of the rings recently uncovered gaps just a few tens of meters wide that they believe surround unseen mini moonlets. These regions of empty space might be smaller versions of structures spotted previously in one of Saturn’s larger rings.
In the predawn hours of 15 October 1997, a Titan rocket lifted off the launch pad at Florida’s Cape Canaveral. After nearly 7 years and multiple gravity assists from Venus, Earth, and Jupiter, the Cassini spacecraft and Huygens probe arrived at Saturn, a journey of more than 3 billion kilometers (2 billion miles). From 2004 to 2013, onboard instruments collected data about the Saturnian system and beamed the measurements back to Earth.
“We get a measurement of how much starlight passes through.”
In addition to capturing more than 450,000 images of the Saturnian system, the spacecraft inadvertently tracked distant stars poking through Saturn’s rings. These observations turned out to be pretty useful for studying the rings themselves.
A star viewed that way is basically functioning like a searchlight behind the rings, said Richard Jerousek, a planetary astrophysicist at the University of Central Florida in Orlando and a member of the Cassini Ultraviolet Imaging Spectrograph team from 2008 to 2017. “We get a measurement of how much starlight passes through,” he said.
Jerousek and his colleagues recently analyzed data from those “occultations” collected by the Ultraviolet Imaging Spectrograph. By aggregating those data, the researchers assembled a 2D map revealing the relative transparency of different parts of Saturn’s rings.
Such a map can reveal hitherto unseen details in Saturn’s rings, said Frank Spahn, a theoretical physicist at the University of Potsdam in Germany not involved in the research. “The dimming gives you an image of the structure,” he said.
There’s also a big advantage to using occultation data rather than simply taking a picture of Saturn’s rings, Jerousek said. Occultation observations can reveal features as small as about 100 square meters (1,100 square feet). That’s about one nine hundredth the area of a pixel in the highest-resolution images returned by Cassini, he said.
Spotting Aircraft
The researchers spotted dozens of places in Saturn’s C ring—one of its innermost rings—that appeared to be 100% transparent. Those regions were small, the team inferred: just a few tens of meters wide in the radial direction and 5–10 kilometers long in the azimuthal direction.
Their elongated geometry was a tip-off to their potential identity—similarly shaped structures, albeit much larger, have been spotted in the outer regions of Saturn’s A ring. Known as propellers, those features are big enough to show up in Cassini imagery rather than just occultation data, Jerousek said. “We have images of propellers in other ring regions.”
Propellers, as their name suggests, are shaped much like the blades used to propel aircraft and boats. The propellers in Saturn’s A ring carry names such as Blériot, Santos-Dumont, and Earhart. “They’re named after famous propeller pilots,” Jerousek said.
Scientists believe that propellers exist because of unseen moonlets measuring, at most, several hundred meters in diameter. Saturn’s rings are made up of an amalgam of icy particles, most of which range in size from centimeters to meters. Moonlets just happen to be a bit larger than average, and their extra gravity contributes to clearing out lobe-shaped regions of space ahead of and behind them in their orbits around Saturn. “You have two wings,” said Spahn. “In the middle, you have the object that causes the structure.”
“What we’re talking about here is much, much smaller scale.”
The features in the Cassini-Huygens occultation data are consistent with propellers caused by very small “mini moonlets,” Jerousek and his team suggested. Those mini moonlets would be no more than about 20 meters across, the researchers believe, which would make them far more diminutive than the objects that create the named propellers, Jerousek said. “What we’re talking about here is much, much smaller scale.”
These findings were published in Icarus.
It makes sense that propellers would exist over a wide range of scales, Spahn said. In fact, there are case studies of what happens when moonlets are too large to create propellers and instead create uninterrupted gaps extending 360° around Saturn, he said. “Two moons do that.” Those are Pan and Daphnis, and they are both embedded in Saturn’s A ring.
There are still lots more Cassini data left to analyze, but Jerousek and his colleagues are also looking to the future. Uranus, located nearly 1.5 billion kilometers (900 million miles) beyond Saturn, has very faint and thin rings of its own. Exploring Uranus’s rings would be a logical next step, Jerousek said. Scientists are rallying behind a flagship mission to Uranus, which has been visited only once before, by Voyager 2 in 1986.
—Katherine Kornei (@KatherineKornei), Science Writer