It ain’t St. Louis without the Gateway Arch, it ain’t Mount Rushmore without the Presidents, and it sure ain’t Saturn without the rings. That, at least, is how it’s always seemed—but things are apparently changing. According to a new study published in the journal Icarus, Saturn could be largely ringless in as little as 100 million years.
Among astronomers, the smart betting has never been that Saturn’s rings would live forever. The diameter of the ring system is huge: 170,000 miles, or almost three-quarters of the distance from the Earth to the moon. But it’s also as little as 30 feet thick. The rings are made of billions of particles of ice and rock, some boulder-sized, but others microscopically tiny. In the dynamic, high-energy environment of Saturn, it was always a question whether so diaphanous a structure could survive. And as long ago as 1980 and 1981, when Voyager 1 and Voyager 2 reconnoitered Saturn, the evidence began suggesting that the answer was no.
Those spacecraft observed suspicious variations in both the electrical charge in Saturn’s ionosphere and the thickness of its rings, as well as dark-colored bands running around the planet at higher latitudes. A 1986 paper in the journal Geophysical Research Letters sought to explain this, theorizing that the ring particles were becoming entrained in Saturn’s magnetic field, plunging toward the planet and creating what amounted to a “ring rain,” which cleared away haze and created the signature lines. The rings, the thinking at that point went, would likely be gone in a maximum of 300 million years or, in a worst case scenario, just 100 million.
That theory was just a theory, but the new findings—based on ground-based observations by Keck Observatory telescopes on Hawaii’s Mauna Kea Mountain—may confirm it.
The research, led by post-doctoral fellow James O’Donoghue of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, relied principally on scanning Saturn’s atmosphere for the infrared signature of a three-atom hydrogen ion known as H3+. O’Donoghue and his colleagues theorized that the ring rain begins when ring particles become electrically charged, either by ultraviolet energy from the sun or by plasma—hot, charged gas generated when micrometeoroids bombard Saturn’s atmosphere. Once the particles are energized, they react to Saturn’s magnetic field and ride the high-latitude lines of attraction down into the atmosphere. Such a bombardment should energize H3+ ions in the atmosphere, and that energy should be visible in the infrared spectrum. The Keck instruments detected precisely that.
Worse for Saturn, the ring erosion may be even more severe than the Hawaii research suggests. In October, NASA released findings from the hair-raising dive its Cassini spacecraft made between the innermost edge of Saturn’s rings and the uppermost reaches of its atmosphere, shortly before its planned suicide plunge into the planet in September of 2017. The spacecraft detected ring rain not only where the Keck study did, but at the equator too.
“We estimate that [higher-latitude] ‘ring rain’ drains an amount of water products that could fill an Olympic-sized swimming pool from Saturn’s rings in half an hour,” said O’Donoghue in a statement that accompanied the release of the new study. “From this alone, the entire ring system will be gone in 300 million years, but add to this the Cassini-spacecraft measured ring-material detected falling into Saturn’s equator, and the rings have less than 100 million years to live. This is relatively short, compared to Saturn’s age of over 4 billion years.”
Both findings help answer another mystery: whether Saturn was born with its rings or acquired them later in its life, possibly by the collision of icy moons that were orbiting the planet. The transient nature of the rings established by the recent studies suggests that they were indeed a later accessory, since their very fragility makes it unlikely they could have the lasted the roughly 4.5 billion years from Saturn’s birth until now.
If O’Donoghue is right about the 100 million-year timeline, humanity should be grateful that we came along now, in time to see the rings before they’re gone. Of course, it’s also possible we came along too late: Jupiter, Uranus and Neptune all have faint and fragmentary ring systems, suggesting that they may once have been as gaudily decorated as Saturn is today—a solar system spectacle that came and went before we ever got a glimpse of it.
