Saturn’s rings might be shredded moons
NEW ORLEANS, LA. — Saturn’s iconic rings are a recent addition. That’s what astronomers now conclude after analyzing the final data collected last year by the Cassini spacecraft. The craft flew between the planet and its rings last year before plunging to its death within the gas giant’s atmosphere, last September. The rings are likely young, just a few hundred million years old. And they also appear far less massive than previously thought.
Such findings suggest the rings probably are the remnants of at least one now deceased moon. Earlier estimates had suggested those rings might have been ancient leftovers from the stuff that had formed the planet.
Scientists shared their new assessments on December 12 and 13, here, at the fall meeting of the American Geophysical Union.
For decades, scientists puzzled over the age and origins of Saturn’s rings. The planet formed some 4 billion years ago. If its rings had formed at that time, a constant bombardment of debris from the more distant solar system should have made the icy bands appear darker than they are. But scientists thought the rings were too heavy to have formed much more recently. That’s because there would have been less material available than in the solar system’s youth (from which Saturn could pull together those rings).
Cassini’s final orbits may have settled the issue. In the lead-up to the end of its mission, Cassini swooped between Saturn and its rings 22 times. Those daredevil moves let astronomers measure the difference in the gravitational tug on the probe from Saturn alone and from the rings-and-planet together.
These measurements revealed that the B ring — which makes up 80 percent of the total mass of the rings — is about 15 billion billion kilograms (33 billion billion pounds). That’s 0.4 times the mass of Saturn’s innermost moon Mimas (My-mus), reported Luciano Iess at the meeting. Iess is a planetary scientist in Italy at Sapienza University of Rome.
That’s lightweight enough to be young, agrees Larry Esposito. He had long proposed that the rings must be old. Esposito is a planetary scientist at the University of Colorado Boulder. He was not involved in the new work, but the topic has been an important one for him. In 1983, he estimated the rings’ mass — and got a similar answer to the new one — using data from the Voyager spacecraft. “But I always thought that was an underestimate,” he says of those earlier data. “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. That means the B ring could actually be as massive as two Mimases. But that’s still lighter than Esposito had hoped.
Dust raining down on the rings also supports the rings’ youth, Sascha Kempf reported on December 13. He’s a planetary scientist at the University of Colorado Boulder. Kempf and his colleagues used all of the debris that Cassini’s dust-counting instrument had detected since the spacecraft arrived at Saturn in 2004. These showed that the still-bright rings collect too much dust pollution to have retained their youthful shine for billions of years.
“Our data implies that the ring can only have a pollution age of a few hundred million years or so.” Concludes Kempf: “The rings are young.”
Taken together, the two results “really argue for young rings,” Esposito agrees. “That’s sent me back to square one.”
Riddle of the rings
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, Nev., 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., worked with Kempf on the new analysis. He thinks ring formation might not be a one-off event. Instead, Saturn might go through cycles of moons and rings. Matija Ćuk also works at the SETI Institute. In 2016, he and his colleagues calculated that if a former outermost moon of Saturn had moved inward a bit, that motion could have destabilized the whole moon system.
If that happened, it could have forced the moons into orbits where Saturn’s gravity would have shredded them into the dust now orbiting as rings. Those rings might one day collect to form new moons. Eventually, they would go through the whole process again. Indeed, Estrada notes, this “could have happened many times.”
astronomy The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.
atmosphere The envelope of gases surrounding Earth or another planet.
Cassini A space probe sent by NASA to explore the planet Saturn. Cassini was launched from Earth in 1997. It reached Saturn in late 2004. The craft included a variety of instruments meant to study Saturn’s moons, rings, magnetic field and atmosphere.
colleague Someone who works with another; a co-worker or team member.
constant Continuous or uninterrupted.
debris Scattered fragments, typically of trash or of something that has been destroyed. Space debris, for instance, includes the wreckage of defunct satellites and spacecraft.
force Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.
gas giant A giant planet that is made mostly of the gases helium and hydrogen. Jupiter and Saturn are gas giants.
gravity The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.
icon (adj. iconic) Something that represents another thing, often as an ideal version of it.
mass A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.
Mimas The smallest and innermost moon of Saturn. Its cratered surface covers an oval-shaped body with a radius of less than 198 kilometers (123 miles). Being only a little more 186,000 kilometers (115,000 miles) above Saturn, Mimas orbits its planet once every 22 hours and 36 minutes. English astronomer William Herschel discovered Mimas on September 17, 1789. It remained little more than a dot to ground-based viewers until the Voyager I and II spacecraft sent back photos of this moon in 1980.
orbit The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood.
remnant Something that is leftover — from another piece of something, from another time or even some features from an earlier species.
Saturn The sixth planet out from the sun in our solar system. One of the four gas giants, this planet takes 10.7 hours to rotate (completing a day) and 29 Earth years to complete one orbit of the sun. It has at least 53 known moons and 9 more candidates awaiting confirmation. But what most distinguishes this planet is the broad and flat plane of seven rings that orbit it.
SETI An abbreviation for search for extraterrestrial intelligence, meaning life on other worlds.
solar system The eight major planets and their moons in orbit around our sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.
Voyager spacecraft Two NASA missions to conduct close-up explorations of Jupiter, Saturn, Saturn's rings and the larger moons of the both large planetary gas giants. Despite their names, the Voyager 2 craft launched Aug. 20, 1977; Voyager 1 launched 16 days later. Both are near the edge of the solar system and still flying on into space.
Meeting: L. Iess et al. The dark side of Saturn’s gravity. American Geophysical Union meeting, New Orleans, December 12, 2017.
Meeting: P. Estrada et al. Ballistic transport: After the Cassini grand finale, is there a final consensus on ring origin and age? American Geophysical Union meeting, New Orleans, December 12, 2017.
Meeting: S. Kempf et al. The age of Saturn’s rings constrained by the meteoroid flux into the system. American Geophysical Union meeting, New Orleans, December 13, 2017.
Journal: M. Ćuk et al. Dynamical evidence for a late formation of Saturn’s moons. The Astrophysical Journal. Vol. 820, March 24, 2016. doi: 10.3847/0004-637X/820/2/97.
Journal: L.W. Esposito et al. The structure of Saturn's rings: Implications from the Voyager stellar occultation. Icarus. Vol. 56, December 1983, p. 439. doi: 10.1016/0019-1035(83)90165-3.