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 much smaller than the planet, remaining fixed in the plane of Saturn’s orbit while the planet’s axis of rotation oscillates dramatically.
A new modeling study published in yesterday’s edition of Science suggests that these anomalies have the same explanation. Saturn is presumed to have an extra moon that enabled gravitational interactions that explain the planet’s great oscillation. However, in the process of these interactions, the Moon came out of orbit, approached Saturn, and was destroyed, resulting in the formation of ring material. While the models don’t conclusively tell us that this is what happened, they can provide some indication of what we need to look for to determine how likely these events are.
Explanation of anomalies
The age of the solar system is more than 4 billion years. Assuming it always looks the same as it currently does, this might seem to put a premium on stability. However, the Saturn system is very dynamic. The largest moon, Titan, is moving away from the planet; Geysers on another surface, Enceladus, feed the material in one loop; Small moons are condensed from other ring materials. Therefore, there are reasons to believe that Saturn has not always looked like it currently does.
One of the things that was not likely to be stable were the rings. Scientists have estimated their age to be around 100 million years, based on interactions with nearby moons and color changes that accumulate over time in a highly radioactive environment. While there is some dispute over the 100 million-year number, explaining their existence at this point in the history of the solar system remains a challenge.
Not satisfied with the challenge, the team behind the new work threw in at one second: Saturn’s great rotation oscillation, in which its axis of rotation is more than 25 degrees from being perfectly perpendicular to the plane of Saturn’s orbit. This is too large to be produced during planet formation.
In this case, we have some ideas about how this might have happened later in the planet’s history. Saturn can enter what is called an “orbital resonance” with Neptune. Normally, the average gravitational interactions between planets over millions of years. But in the case of resonance, the orbital periods are lined up so that certain configurations of objects appear repeatedly. In this case, some of the gravitational interactions can end up enhancing rather than averaging, causing effects to accumulate over time. In the case of Saturn and Neptune, the resonance can affect the direction of Saturn’s poles.
While doing the calculations, we didn’t know enough about some of the details of Saturn’s system to determine if it actually resonated with Neptune. But thanks to decades of data from the Cassini spacecraft, we now have the data we need to narrow the uncertainty.