Saturn’s rings are one of the most beautiful and breathtaking sights in the Solar System – but it hasn’t always been that way. New evidence in data sent back by the now-defunct Cassini spacecraft shows that they are much younger than the planet itself, and that Saturn was actually ringless for most of its existence.
Saturn formed about 4.5 billion years ago, along with the other planets in the Solar System, but the new findings show that the rings have only been around for the past 10-100 million years – evolving during the age of dinosaurs on Earth. Although that still sounds ancient, it is much younger than the planet itself.
The findings – from a team led by Luciano Iess at the Sapienza University of Rome in Italy – were published online on Jan. 17, 2019 in the journal Science.
The conclusions are based on measurements taken by Cassini during its final orbits of Saturn in 2017, just before the mission ended (as planned) in September. Cassini coasted between the innermost rings and the planet itself, acting as a sort of gravity probe as it got “tugged” around by the gravity of both the rings and the planet.
By knowing how much gravity affected Cassini, which caused it to accelerate – down to a fraction of a millimeter per second – scientists were able to calculate the mass of both the rings and the planet.
“Only by getting so close to Saturn in Cassini’s final orbits were we able to gather the measurements to make the new discoveries,” said Iess. “And with this work, Cassini fulfills a fundamental goal of its mission: not only to determine the mass of the rings, but to use the information to refine models and determine the age of the rings.”
It turns out that the mass of the entire rings is only about 40% the mass of Saturn’s moon Mimas, which itself is 2,000 times smaller than Earth’s moon.
Once the masses were known, scientists could figure out the approximate age of the rings. Previous studies had already pointed to a younger age, and the new study supports that. A lower mass of the rings would indicate a younger age, and that is what was found. The ring particles are mostly ice, and bright, which means they are relatively young. If they were much older, they would have been contaminated and darkened by interplanetary debris.
Even Saturn’s clouds played a role in determining the masses with greater accuracy.
“The first time I looked at the data I didn’t believe it, because I trusted our models and it took a while to sink in that there was some effect that changed the gravity field that we had not considered,” said Burkhard Militzer, a professor of earth and planetary science at the University of California, Berkeley, who models planetary interiors. “That turned out to be massive flows in the atmosphere at least 9,000 kilometers deep around the equatorial region. We thought preliminarily that these clouds were like clouds on Earth, which are confined to a thin layer and contain almost no mass. But on Saturn they are really massive.”
So what about how the rings formed? Two theories in particular now stand out – a comet passed close to Saturn and was torn apart by gravity or something happened to cause earlier icy moons to meet the same fate. Either scenario could explain the resulting debris that would begin to form the rings millions of years ago.
The new results also helped scientists to better understand Saturn’s interior and estimate the mass of its inner core – between 15-18 times the mass of Earth – similar to previous estimates.
“The discovery of deeply rotating layers is a surprising revelation about the internal structure of the planet,” said Cassini project scientist Linda Spilker of NASA’s Jet Propulsion Laboratory in Pasadena, California. “The question is what causes the more rapidly rotating part of the atmosphere to go so deep and what does that tell us about Saturn’s interior.”
Cassini’s mission ended during the Grand Finale in September 2017, in which it plunged into Saturn’s atmosphere after completing the last of its close orbits of the planet. This was done to minimize the possibility of any microbes on the spacecraft contaminating any of the moons after Cassini ran out of fuel. Cassini provided unprecedented views and data about Saturn and its moon during its mission which started in 2004.
Cassini may be gone now, but the data it sent back will keep scientists busy for many decades to come. Thanks to the Cassini mission, we now have a much better understanding of one of the most significant mysteries of Saturn – how old its rings are and how they formed.
“These measurements were only possible because Cassini flew so close to the surface in its final hours,” Militzer said. “It was a classic, spectacular way to end the mission.”