NEW ORLEANS — Saturn’s iconic rings are a recent addition. Final data from the Cassini spacecraft, which flew between the planet and the rings this year before plunging into the gas giant’s atmosphere, show the rings are around a few hundred million years old and less massive than previously thought.
Those findings suggest the rings are probably the remnants of at least one moon, rather than ancient remains of the stuff that formed the planet. The results were presented at the fall meeting of the American Geophysical Union on December 12 and 13.
For decades, scientists puzzled over the age and origins of Saturn’s rings (SN: 11/12/16, p. 10). If the rings had formed with Saturn some 4 billion years ago, a constant bombardment of debris from the more distant solar system should make the icy bands appear darker than they do. But scientists thought the rings were too heavy to have formed relatively recently, when there was less material available than in the solar system’s youth for Saturn to pull into the rings.
Cassini’s final orbits may have settled the issue. In the lead-up to the end of its mission in September, Cassini swooped between Saturn and its rings 22 times (SN Online, 9/15/17). Those daredevil moves let astronomers measure the difference in the gravitational tug the probe experienced from Saturn alone and from the rings and the planet together.
Those measurements reveal that the B ring, which makes up 80 percent of the total ring mass, is about 15 billion billion kilograms, or 0.4 times that of Saturn’s moon Mimas, planetary scientist Luciano Iess of Sapienza University of Rome said at the meeting on December 12.
That’s lightweight enough to be young, says planetary scientist Larry Esposito of the University of Colorado Boulder, a longtime old rings proponent who wasn’t involved in the new work. In 1983, Esposito used data from the Voyager spacecraft to estimate the rings’ mass and got a similar answer. “But I always thought that was an underestimate,” he says. “I’m disappointed that they’re not more massive.”
Iess noted that there was an extra gravitational force nudging Cassini that is still not explained, so the B ring could actually be as massive as two Mimases. But that’s still lighter than Esposito had hoped.
The last look at the dust raining down on the rings supports the rings’ youth, too, planetary scientist Sascha Kempf of the University of Colorado Boulder reported on December 13. Using all the measurements from Cassini’s dust-counting instrument since the spacecraft arrived at Saturn in 2004, Kempf and colleagues showed that the still-bright rings collect too much dust pollution to have maintained their youthful shine for billions of years. “Our data implies that the ring can only have pollution age of a few hundred million years or so,” Kempf said. “The rings are young.”
Taken together, the two results “really argue for young rings,” Esposito says. “That’s sent me back to square one.”
How the rings formed remains a mystery. Esposito’s best guess is that a single moon about half the mass of Mimas was ripped up around 200 million years ago. That perfect timing is about as likely as hitting the jackpot in Las Vegas, he says. “We’re just really lucky to have developed intelligent life on Earth and launched a spacecraft to Saturn during the 200 million years when it happens to have rings around it,” he says.
Paul Estrada of the SETI Institute in Mountain View, Calif., one of Kempf’s coauthors, thinks ring formation might not be a one-off event. Instead, Saturn might go through cycles of moons and rings. In 2016, Matija Ćuk, also of the SETI Institute, and colleagues calculated that if a former outermost moon of Saturn had moved inward a bit, that motion could have destabilized the whole moon system and forced the orbs into orbits where Saturn’s gravity would have shredded them into rings. Those rings could accrete into new moons and eventually go through the whole process again. “It could have happened many times,” Estrada says.