Mother Nature is a notorious tease, who seems to delight in confusing those who attempt to solve her myriad, magnificent mysteries. The rings of the beautiful gas-giant planet Saturn are a case in point, and there is a longstanding, unresolved debate among planetary scientists who are trying to determine their true age, and who are maddeningly coming up with contradictory results. Sometimes the evidence suggests that the rings are younger than the dinosaurs that died off about 65 million years ago, while other lines of evidence indicate that they are as old as our 4.56 billion year old Solar System. In September 2019, a team of astronomers presented their new findings that reignited the debate about the age of Saturn’s rings. The new study proposes that the rings are most likely very ancient, and probably formed early in the history of our Solar System.
In a paper published in the September 16, 2019 edition of Nature Astronomy and presented at the joint meeting of the European Planetary Science Congress (EPSC) and the American Astronomical Society’s Division of Planetary Sciences (AAS-DPS), held in Geneva, Switzerland, the authors suggest that processes that preferentially eject dusty and organic material out of Saturn’s rings could make the rings deceptively look much younger than the really are.
The findings also have important implications that go beyond Saturn. “The rings around Saturn and other giant planets are witnesses to our Solar System history. Determining how and when they formed, whether it was 4.5 billion or 200 million years ago, allows us to look back in time to understand the evolution of the Solar System, and other planetary systems around other stars,” explained Dr. Hsiang Wen (Sean) Hsu in a September 16, 2019 University of Colorado Press Release. Dr. Hsu is a research associate at the University of Colorado (Boulder).
When the Cassini spacecraft took its final, fatal dive down into the clouds of the gas-giant Saturn, during that successful mission’s Grand Finale in 2017, it provided data that planetary scientists interpreted as evidence that Saturn’s rings are young, and formed a mere tens of millions of years ago. This means that the rings formed about the same time that dinosaurs still roamed the Earth, before they received their final coup de grace when a large comet or asteroid crashed into our planet, resulting in mass extinctions. Gravity measurements obtained during Cassini’s fatal plunge gave a more precise estimate of the mass of the rings. The rings themselves are about 95 percent water ice and less than 5 percent rocks, metals, and organic materials. The mass estimate was then used to calculate how long the pristine ice of the rings would have to be exposed to dust and micrometeorites in order to attain the level of other “pollutants” observed in the rings today.
For many plantary scientists, Cassini’s findings resolved the puzzle of the age of the Saturnian rings. However, Dr. Aurelien Crida, lead author of the new study, proposes that the debate isn’t over.
“We can’t directly measure the age of Saturn’s rings like the rings on a tree stump, so we have to deduce their age from other properties like mass and chemical composition. Recent studies have made assumptions that the dust flow is constant, the mass of the rings is constant, and that the rings retain all the pollution material that they receive. However, there is stilll a lot of uncertainty about all these points and, when taken with other results from the Cassini mission, we believe that there is a strong case that the rings are much, much older,” Dr. Crida noted in the September 16, 2019 University of Colorado Press Release. Dr. Crida is of the Observatoire de la Cote d’Azur, National Center for Space Studies (CNRS), in Nice, France.
Saturn Is The True “Lord of the Rings”
The highly successful Cassini-Huygens space mission involved a collaboration between NASA, the European space Agency (ESA), and the Italian space Agency (ASI) that sent a probe to study Saturn and its many moons, especially its mysterious golden-orange, fog-enshrouded large moon, Titan. Titan is the second largest moon in our Solar System, after Ganymede of Jupiter. The robotic spacecraft was composed of NASA’s Cassini probe, with ESA’s Huygens lander riding to its destination piggyback. The Huygens lander was specifically sent to study Titan, and it is responsibe for finally lifting the moon’s heavy veil of smog, at last revealing its face. Cassini was the fourth space probe to soar to Saturn, as well as the first to enter orbit around its enormous target.
Cassini was launched aboard a Titan IVB/Centaur on October 15, 1997. The spacecraft remained active for almost two decades, with 13 of those years spent in orbit around Saturn. Cassini studied the gas-giant and its system after entering orbit on July 1, 2004. The mission ended on September 15, 2017, when Cassini’s trajectory caused it to crash into Saturn’s upper atmosphere, where it burned up. This was an intentional fatal plunge, that had been planned by mission scientists. The scientists destroyed the spacecraft in order to prevent any risk of contaminating Saturn’s moons, which might have provided habitable environments for stowaway Earth-born microbes aboard Cassini.
At the end of its mission, the Cassini spacecraft performed its Grand Finale, which involved a number of dangerous close passes through the gaps between Saturn and Saturn’s inner rings. This phase was performed to maximize Cassini’s scientific cornucopia of valuable information before the spacecraft was intentionally destroyed. Even though the atmospheric entry of Cassini ended the mission, analyses of the returned data will go on for many years to come.
Saturn is the second largest planet in our Sun’s family, after the banded behemoth Jupiter. It is orbited by a multitude of icy tidbits, sparkling frozen moonlets, and primarily icy moons. Saturn’s hydrocarbon-tormented large moon Titan is thought to resemble Earth, the way that it was long ago, before life emerged here out of non-living substances (prebiotic). Saturn’s mid-sized icy moon, Enceladus, is thought to have a subsurface ocean of liquid water hidden beneath its frozen crust–and this makes the moon an intriguing small world because where liquid water exists, life can possibly (though not certainly) exist as well.
Saturn is probably most famous for its bewitching, bewildering, and mesmerizing system of gossamer rings. The rings themselves are a collection of a multitude of icy objects that range in size from exquisitely tiny smoke-sized particles to boulders as large as some small buildings on Earth. The small, orbiting, twirling icy denizens of the Saturnian rings, interact with one another, and dance around together in a celestial waltz–and they are influenced by their gigantic parent-planet’s powerful magnetosphere, which is the region of Saturn’s magnetic influence. The little icy denizens of the rings are also subject to the influence of Saturn’s larger moons.
The primary rings of Saturn create a wide, but very thin, ethereal expanse of approximately 250,000 kilometers across. However, they are less than tens of hundreds of meters deep! Historically, the ages and origins of Saturn’s rings have presented a long-standing mystery for astronomers to solve.
How old are the rings? Are they as old as our ancient Solar System, or younger than the dinosaurs?
Bewitching, Bewildering Rings
Dr. Crida and his team propose that the mass measured during the Cassini mission’s Grand Finale is in excellent agreement with models of the dynamical evolution of massive rings dating all the way back to the primordial Solar System.
The rings are composed of blocks and particles that range in size from meters all the way down to micrometers. Viscous interactions occurring between the blocks result in the rings spreading out and carrying material along for the ride in a way that has been compared to a conveyor belt. This eventually leads to mass loss from the innermost edge, which is where particles tumble down into their parent-planet. It also results in mass loss from the outer edge, where material travels across the outer boundary into a region where moonlets and moons are being born.
The more massive rings broaden more quickly, and also lose mass faster, than the rings of lesser mass. The new models indicate that whatever the beginning mass of the rings, there is a tendency for the rings to meet up and converge on a mass measured by the Cassini spacecraft after approximately 4 billion years. This agrees with the timescale of the formation of our Solar System.
“From our present understanding of the viscosity of the rings, the mass measured during the Cassini Grand Finale would be the natural product of several billion years of evolution, which is appealing. Admittedly, nothing forbids the rings from having been formed very recently with the precise mass and having barely evolved since. However, that would be quite a coincidence,” Dr. Crida commented in the September 16, 2019 University of Colorado Press Release.
As Dr. Hsu also commented, “Despite the end of the mission, more Cassini results as well as inspired modeling and laboratory work will continue to shed light on Saturn, the real Lord of the Rings, in the years to come.”