The collision could also help explain why Saturn is no longer in sync with Neptune.
Two of Saturn’s satellites – the largest and one of the strangest – may owe their current shape and orbit to a pileup of two moons about 400 million years ago.
A smashup between a doomed moon and the massive moon Titan could have given birth to the squishy-looking Hyperionsuggests a study submitted February 9 to arXiv.org. The ensuing chaos in the Saturn system could then have led to the formation of its rings.
The notion builds on a 2022 proposal from another team, which suggested the existence of an ancient moon to solve some long-standing mysteries about the Saturn system, including its relatively high inclination, its young rings, and the orbital relationships of some of its moons.
The first clues came from Saturn’s relationship with Neptune. For decades, planetary scientists assumed that Saturn and Neptune had what is called a spin-orbit resonance: Saturn’s spin axis and Neptune’s orbit around the sun appeared to wobble at almost the same rate. But data from NASA’s Cassini mission, which orbited Saturn from 2004 to 2017showed that Saturn is slightly out of sync with Neptune. Still, the oscillation rates are close enough to suggest that the planets canceled their resonant relationship relatively recently in cosmic terms – perhaps a few hundred million years ago.
“This tells us that there have been disturbances in Saturn’s outer system,” says planetary scientist Matija Ćuk of the SETI Institute in Mountain View, California.
Ćuk and colleagues suggest that this disruption occurred in two parts. First, a doomed moon collided with Titan, altering Titan’s tug on Saturn’s rotation axis enough to break the resonance with Neptune and produce debris that could later coalesce to form Hyperion. This collision could also have left Titan in a more extreme orbit – one that continued to slowly expand over the next hundreds of millions of years. Titan’s orbital evolution could ultimately have gravitationally triggered a slow-motion train wreck that led Saturn’s inner moons to collide and crash, ultimately giving rise to both the rings and a new crop of young inner moons.
Previously, MIT planetary scientist Jack Wisdom and colleagues suggested that the breakdown of the Saturn-Neptune relationship coincided with the formation of The famous rings of Saturnwhich, according to some planetologists, is only 150 million years old. Wisdom’s team proposed that an additional moon, named Chrysalis, could have pulled on Saturn’s rotation axis, breaking the resonance with Neptune, before coming dangerously close to the planet and being torn apart in the rings.
“Jack wanted to link these two,” says Ćuk. “But I thought the formation of Hyperion was a more direct clue.” Based on previous work, Ćuk calculated that Hyperion must have settled into its current orbital arrangement within the last 400 million years, a time frame comparable to that of Saturn’s presumed dissolution and Neptune’s resonance.
In their new work, Ćuk and his colleagues suggest that Saturn once had an extra moon, which they dub proto-Hyperion, about four times more massive than Chrysalis. Through computer simulations of a collision between the proto-Hyperion and Titan, the team discovered that Titan survives, while some debris from the collision accumulates in the present-day Hyperion, a porous egg-shaped body that tumbles chaotically through space.
But without Chrysalis, the rings would have had to have a different origin, says Ćuk. His team suggests that Titan and Hyperion could also explain this – if there were more moons missing. They postulate that Saturn may have originally had several inner moons more massive than those present today. Titan’s altered orbit after the collision could, over hundreds of millions of years, put it in resonance with one of these inner moons, changing that moon’s orbit until it collides with another.
Wisdom doesn’t think Ćuk’s scenario really works. On the one hand, the inner moons would all have to be a few hundred million years old. But Mimas, one of these moonshas enough craters to suggest it is much older.
“Their arguments do not invalidate our scenario,” he said, adding that the new proposal is “a very different and more complicated scenario.”
Ćuk thinks Mimas might still be young – its craters may have formed relatively quickly in Saturn’s chaotic system. Both planetary scientists agree that more detailed simulations of the Saturn system are needed to show which picture is more plausible.
“There could be a third variant of instability that combines mine and Jack’s,” says Ćuk – or something new.