In August 2009, NASA’s Cassini spacecraft became the first robotic emissary from Earth to witness an equinox at Saturn, when the sun was shining directly on the giant planet’s equator.
Saturn’s rings are among the most eye-catching sights in the solar system—and yet they haven’t always been there. A new analysis suggests the planet’s majestic bangles might be so young that if dinosaurs looked up, they could have witnessed an otherwise plain, straw-colored Saturn putting a ring on itself—one that would have been briefly enormous.
“Even with the unaided eye, Saturn would be as bright as Venus and would be noticeably elongated,” says Matija Cuk of the SETI Institute. “If the dinosaurs had birdlike vision, they would definitely be able to see it. Not clear they would care, though.”
Lucky for us, humans came along at the right time to be impressed. According to the new observations, gathered by NASA’s Saturn-exploring Cassini spacecraft, the rings may be just a hundred million to 200 million years old, making mammals the only group to have evolved after the rings appeared.
Everyone else—fish, reptiles, birds, plants—showed up first.
Cassini’s final observations suggest the rings are nowhere near as old the planet itself, which formed 4.5 billion years ago along with the rest of the solar system. Cassini’s team, which said goodbye to their spacecraft in September, presented the preliminary story at the American Geophysical Union’s annual meeting in December.
“It is interesting to realize how dynamic our solar system is,” says NASA’s Jeff Cuzzi. “Most scientists have always assumed that what we see out there in the cosmic realm is kind of permanent, and it’s always been that way.”
Forging a Ring
First glimpsed through a telescope by Galileo about 400 years ago, Saturn’s exquisite rings are unique among those in our neighborhood. Hanging around the other giant planets—Jupiter, Uranus and Neptune—are darker, ragged rings that are quite unlike the bright, luxurious loops fastened around Saturn.
But ever since the venerable Voyager spacecraft flew by Saturn in the early 1980s, scientists have wondered whether its iconic rings are primordial, dating back to the dawn of the solar system.
“The question here is, are the rings old or new?,” asks Bonnie Buratti of the Jet Propulsion Laboratory. “Most of us believe it was a collision that formed the rings, but did it happen a long time ago or did it happen recently?”
Data from Voyager suggested the rings weren’t hefty, a clue to their youth. Measuring just a fraction of the mass of Saturn’s small moon Mimas, the rings didn’t contain enough mass to absorb the dark dust that zooms through the outer solar system and would not have stayed bright for billions of years. What’s more, the tiny moons skirting the perimeters of the rings were moving away from the planet too quickly to have been there over the entirety of Saturn’s lifetime.
But no one could figure out how such a young system of rings would have formed around Saturn. On the other hand, crafting a primordial ring is relatively easy; way back in the day, planetary bodies shot around the solar system like billiard balls that accidentally ended up in a game of pinball, and there were more than enough collisions in Saturn’s neighborhood to kick up the icy rings that now decorate the planet.
“All the catastrophic dynamics happened back at the beginning,” Cuzzi says.
As the solar system aged, however, things calmed down considerably, and today, it’s harder to find a comet or asteroid on a Saturn-crossing orbit that’s large enough to be a ringbearer.
Matter of Timing
The mystery deepened as Cassini scientists began analyzing their data after the probe’s demise.
During its final act of Saturnian exploration, Cassini performed a daredevil series of orbits. While swooping through the space between Saturn and its rings, the spacecraft carefully measured the gravitational tugs of both the planet and the rings, which it used to determine the rings’ mass. Its data suggest, again, that the rings are rather paltry, containing roughly 40 percent the mass of Mimas. There’s a hitch, though: As Cassini attempted to peer into Saturn’s interior, it uncovered a strange gravitational anomaly that made it tricky for the team to definitively pin a mass on the rings.
“There’s something very strange about Saturn’s interior that nobody seems to understand very well,” Cuzzi says.
However, even though the ring mass is not completely known at this point, Cuzzi and others says there’s no way it could realistically be anywhere near the five to 10 Mimas masses needed for a bright ring to have formed in the early days of the solar system.
But that’s not all that points to young rings. During its 13 years in the Saturn system, Cassini discovered that more dusty particles are colliding with the Saturn system than scientists estimated 30 years ago. It also determined that most of those grains are coming from the dark, icy bodies that live beyond the orbit of Neptune—in short, Cuzzi says, “The pollution problem is a little worse.”
That makes it even more improbable for wimpy, ancient rings to stay bright because, put simply, the older the rings, the darker they should be. So, by measuring how many of these dark dust particles Cassini caught, scientists could determine how old the rings are. By rewinding the darkening dust clock, the team calculated that the rings were born between a hundred million and 200 million years ago.
“Ancient rings are truly dead,” says Larry Esposito of the University of Colorado Boulder, who has been a chief proponent of the primordial rings theory. “I have no good model to create the rings in the last 200 million years—it is highly improbable—so we must just be very lucky!”
But not everyone is convinced. Buratti notes that we really don’t know how many things are flying around in proximity to Saturn; without knowing that, it’s hard to estimate how probable a recent catastrophic collision could be.
“I don’t think it’s an open and shut case,” she says. “I’m still skeptical.”
And Julien Salmon of the Southwest Research Institute has models suggesting that the present-day ring mass doesn’t necessarily mirror the initial ring mass. As well, he’s not convinced the dark, polluting material would simply accumulate on the icy ring particles.
“There are a lot of things that need to be better understood before we can definitively claim that the rings are young, or old,” he says.
Putting a Ring on It
If Saturn did, in fact, start wearing its ring relatively recently, it’s tough to explain how that might have happened, given the relative dearth of large comets or asteroids that could collide with Saturn.
This is where Matija Cuk and his colleagues started. What if, they asked, instead of Saturn shredding something foreign, the planet ate one of its own moons?
According to Cuk’s model, Saturn and its collection of icy moons marched around the sun for billions of years, minding their own business until one day about a hundred million years ago when the sun’s gravity jostled some of the inner icy moons. Gravity works even over the millions of miles that separate Saturn and our star, and Cuk’s model shows that there’s a sweet spot in the planet’s orbit where these little icy moons can be perturbed by the sun. So the sun had some fun, the moons’ orbits shifted, they collided, and all icy hell broke loose.
“The moons were probably thoroughly smashed,” he says.
The resulting spectacle would have temporarily put a giant ring around Saturn—10 times bigger and brighter than we currently see.
One consequence of Cuk’s moon-smash theory would be that basically all the moons orbiting inside Rhea are young; they got busted up and put back together after Saturn settled down. The ones farther out, like Titan and Iapetus, are ancient. This means, though, that geyser-spewing Enceladus could be quite young—maybe much too young for life to have evolved in its sea.
Also problematic are the complex, cratered terrains marking the surfaces of moons such as Enceladus, Rhea, Dione and Mimas, which appear to be quite old.
“I think we have a problem having all of that geology happen in the last 200 million years,” Buratti says. “These surfaces look really old.”
Regardless of how old the rings are, what is clear is that they aren’t going anywhere any time soon—though they won’t always be as brilliant as they are now. In other words, humans are pretty lucky to have evolved brains capable of making telescopes at just the right time to see Saturn as it is.
Will the rings be gone in another hundred million years? “The answer is no,” Cuzzi says. “They’re stable enough to now remain for a very long time. They’re just going to keep getting darker.”