Lots of strange things about Saturn can be explained by a destroyed moon

Simulations appear to tie up lots of loose ends.

NASA, ESA, A. Simon, M.H. Wong, and the OPAL Team

Saturn is an unusual planet in some obvious ways, most notably its extensive ring system. But it's also strange in some less obvious ways: The rings appear to be far, far younger than the planet, and they stay stably in the plane of Saturn's orbit while the planet's axis of rotation wobbles around dramatically.

A new modeling study released in yesterday's edition of Science suggests that these oddities have the same explanation. It hypothesizes that Saturn had an additional moon that enabled gravitational interactions that explains the planet's large wobble. In the process of those interactions, however, the moon spun out of orbit, got close to Saturn, and was destroyed, creating the ring material. While the models don't tell us this is definitively what happened, they can provide some indications of what we need to look for to determine how probable these events were.

Explaining the oddities

The Solar System is over four billion years old. If we're to assume that it has always looked much like it currently does, that would seem to put a premium on stability. Yet, the Saturn system is very dynamic. The largest moon, Titan, is moving away from the planet; geysers on another, Enceladus, feed material into one ring; small moons are condensing out of the materials of other rings. So there are reasons to think that Saturn hasn't always looked like it currently does.

One of the things that isn't likely to have been stable is the rings. Scientists have estimated their age as about 100 million years, based on interactions with the nearby moons and the color changes that accumulate over time in a high-radiation environment. While there's some disagreement regarding the 100 million year figure, explaining their presence at this point in the Solar System's history remains a challenge.

Not content with that challenge, the team behind the new work threw in a second: Saturn's large rotational wobble, where its axis of rotation is over 25° from being perfectly vertical relative to the plane of Saturn's orbit. That's too large to have been produced during the planet's formation.

In this case, we have some ideas regarding how this might come about later in the planet's history. Saturn can enter what's called an "orbital resonance" with Neptune. Normally, gravitational interactions among planets average out over millions of years. But in a resonance, orbital periods line up so that specific configurations of bodies appear repeatedly. In this case, some gravitational interactions can end up being reinforced rather than averaging out, causing effects that build over time. In the case of Saturn and Neptune, a resonance could potentially influence the orientation of Saturn's poles.

While the math works out, we haven't known enough about some of the details of the Saturn system to determine whether it actually has a resonance with Neptune. But thanks to decades of data from the Cassini spacecraft, we now have the data we need to narrow down uncertainties.

Models on top of models

To determine whether the orbital resonance is possible, the team first had to build a model of Saturn itself. To do so, they used data on Saturn's gravitational field, built by tracking small changes in the movement of Cassini itself during its many orbits. In fact, they built three different models of the planet's interior to account for uncertainties; the three produced similar results.

All of the results showed one thing clearly: Saturn was very close to being in resonance with Neptune, but wasn't—it was off by about 1 percent. Even when including the changing location of Titan's orbit, there was no way to get the two bodies into resonance. And, if the system was placed in resonance with Neptune, then the changing orbits of Saturn's moons could not generate enough force to break the resonance and get Saturn to its current state.

So, on its own, the models seemed to suggest that this is extremely close to an explanation, but not quite there.

This is where the rings came back into play. One possible explanation for their existence is the breakup of a moon that wandered too close to Saturn and was disrupted by the giant planet's gravity. The existence of an additional moon at earlier points in Saturn's history could potentially generate enough force to get Saturn into resonance. And its later destruction could change gravitational interactions enough to break the resonance. Obviously, testing this idea required yet another model, one that included an additional moon that the researchers are calling Chrysalis.

The researchers started Chrysalis fairly far out, orbiting between the existing moons Titan and Iapetus. Assuming it was similar in size to Iapetus (one of Saturn's larger moons), this orbit would be stable until the change in Titan's orbit brought the two moons into their own gravitational resonance. At that point, the orbit of Chrysalis becomes chaotic, and its behavior is very sensitive to the precise configuration of the Saturn system—the researchers ran hundreds of simulations with various outcomes.

In some of them, Chrysalis would collide with another large moon and create a correspondingly large orbital mess. In others, it would be ejected out into interplanetary space. But in about 5 percent of the models, Chrysalis would end up in an eccentric orbit that caused it to graze Saturn's surface, causing the moon to break up and allowing part of its contents to enter orbit as Saturn's rings (the rest would be swallowed by the planet).

Their model also showed that the presence of Chrysalis created enough additional forces to allow Saturn to enter resonance with Neptune, producing its orbital wobble. And the loss of Chrysalis, either through destruction or ejection, would then break the resonance and enable the current almost-but-not-quite resonance.

Obviously, with models built on top of models, there's a lot of potential for things to be a bit off, perhaps off enough to raise questions about the idea. But it's also likely to spur other researchers to devise more precise models and to get astronomers to consider whether a process like this might have left its mark on some of the existing bodies orbiting Saturn. In any case, the concept has the appeal that it may explain two of Saturn's oddities at once.

Science, 2022. DOI: 10.1126/science.abn1234 (About DOIs).

Lots of strange things about Saturn can be explained by a destroyed moon