Of all the rings in the Solar System, Saturn's are, inarguably, the most spectacular.
Unlike the thin circles of dust and rock encircling Jupiter, Uranus, Neptune and even a Kuiper Belt dwarf planet named Haumea, Saturn's particolored ring system is a gloriously extensive thing. Dotted with tiny moons that sculpt its edges, it is a complex structure, with each of its seven rings moving at different speeds.
Interestingly, the very disc that Saturn is famed for appears to be shockingly young. Evidence suggests the ring system is just 100 million years old, give or take (and further evidence suggests that they'll be gone in less than 100 million years from now).
Why the rings appeared so long after the rest of the Solar System formed is challenging to explain, which has led some scientists to question whether the interpretation of that evidence is correct.
Now, as a by-product of an investigation into some of Saturn's other peculiarities, a team has come up with a plausible answer. If Saturn's rings are made out of a pulverized moon, it could explain not just its recent acquisition of dusty discs, but two other strange features of the ringed planet: its axial tilt, and the strange, rapidly expanding orbit of its largest moon Titan.
Every planet in the Solar System has an axial tilt, which is the angle between the rotational axis and orbital plane. Every one happens to be different, too. Saturn clocks in at 26.7 degrees, which is similar to the tilt of Earth, Mars and Neptune. However, the gas giant's lean is only becoming more extreme, which scientists have attributed to Titan's outward migration.
According to previous research, a chain of gravitational interactions from Saturn to Titan to Neptune have had a significant influence the ringed world's tilt. The rate at which Saturn wobbles on its rotational axis (rotational precession) is very close to the rate at which Neptune's entire orbit wobbles (orbital precession), a phenomenon known as resonance, suggesting a relationship.
This prior research found that, contrary to earlier studies, these gravitational stepping-stones would have linked up relatively recently, given Titan is migrating away from Saturn too quickly for them to have arisen in the early Solar System.
The new work, by a team led by astronomer Jack Wisdom of the Massachusetts Institute of Technology, found something odd, though. They used gravitational data from NASA's Cassini spacecraft, and a model of the planet's interior structure, to see if Saturn is still in resonance with Neptune. The answer? Not quite.
That was curious, so they dug deeper to try to figure out why. If Titan's orbit was first migrating outward as expected, but then unexpectedly changed, that could have pulled Saturn into and pushed it back out of resonance, resulting in the current not-quite resonance with Neptune.
The next step would be to figure out what could have changed Titan's orbit so dramatically. Well, there's something we know Saturn has a heck of a lot of: moons.
Currently, at a count of 82 (including some yet to be confirmed), it has the biggest known number of moons in the Solar System. If Saturn had an additional satellite that became destabilized, that could have altered the planet's precession and helped it escape its resonance with Neptune.
The team conducted hundreds of simulations, each with slightly different starting conditions, of the Saturn system including this hypothetical moon, named Chrysalis. And they found that this scenario neatly explained it all – the axial tilt, the orbit of Titan, and even Saturn's baby rings.
Specifically, the scenario proposes, the presence of Chrysalis could have caused Saturn to tilt to a greater degree than we see currently, around 36 degrees, through a resonance with Neptune. During this time, it would have had gravitational interactions with Titan.
Then, around 160 million years ago, the orbit of Chrysalis destabilized. This caused it to veer entirely too close to Saturn, whose gravity pulled the moon apart.
From their simulations, the researchers estimate that some 99 percent of Chrysalis ended up crashing into Saturn, but enough material remained suspended in orbit to form the planet's rings.
If the moon was icy, like some Solar System moons, this could have produced the observed abundance of ice in Saturn's rings today.
This violent encounter could also have pushed Saturn out of resonance, decreasing its axial tilt. However, of the 390 simulations, only 17 produced the conditions where the rings of Saturn formed.
So, as Cornell University astronomer Maryame El Moutamid explains in a commentary about the new study, while the disruption of Chrysalis is possible and plausible, although it's likely to have been a rare, one-off occurrence. We're unlikely to see another such event any time soon.
"We propose," the researchers write in their paper, "that Saturn once had an additional satellite, Chrysalis; that the system was previously in the spin-orbit precession resonance with Neptune; that Saturn's obliquity increased as the precession rate changed because of the migration of Titan; that it escaped the precession resonance because of an instability of the orbit of Chrysalis; and that a close encounter of this hypothesized satellite with Saturn led to the formation of its rings."
"It's a pretty good story," says Wisdom, "but like any other result, it will have to be examined by others."
Nevertheless, it does explain things that have heretofore been tricky to understand. Future research into the fascinatingly complex Saturn system, and other systems with rings, could help determine the rate at which events like the destruction of Chrysalis are expected to occur.
The research has been published in Science.