Saturn is not only unique in the solar system because of its numerous, astonishingly young rings, it is also amazing because of the significant tilt of its axis of rotation compared to its orbit around the sun. Planetary researchers could now have found an explanation for these peculiarities. Using data from NASA's Cassini spacecraft and model simulations, they conclude that the ringed planet once had another, large moon. Around 160 million years ago, this satellite, christened Chrysalis, lost its orbit and was torn apart by Saturn's enormous gravity. The debris that fell on the planet changed its axis, the remaining chunks could have become the rings of the gas planet - and thus explain their surprisingly young age.
The gas planet Saturn is unique in the solar system. No other planet has such a massive and complex ring system as it does. Around 400 years ago, Galileo Galilei observed strange "arms" on both sides of the planet, which he could not explain at the time. It is now clear that Saturn is surrounded by multiple belts of chunks of ice and dust, broken by gaps and moons. Unlike the planet, however, these do not come from the beginnings of the solar system, but are only around 100 million years old according to recent data. This raises the question of how Saturn's rings formed. And there is another peculiarity: The rings and the axis of rotation of Saturn are inclined by 26.7 degrees compared to its orbit around the sun. "This tilt is too large to have arisen from known formation processes in the protoplanar disk or from large planetary collisions," explains lead author Jack Wisdom of the Massachusetts Institute of Technology (MIT). "Various explanations for this have been proposed, but none are entirely convincing."
Is a resonance the reason?
One popular theory is that Saturn's gravitational interactions with the planet Neptune are responsible for its axial tilt. Because the precession of the Saturn axis - a circular wobble - corresponds in its period approximately to the orbital period of Neptune. Thus, Saturn's axis motion could be in resonance with Neptune's orbit. This resonance, in turn, could have tilted the planet's axis of rotation to its present position over time - so the previous assumption. However, whether it is correct depends crucially on a parameter that has not been determined precisely enough for a long time: Saturn's angular momentum. This results from the rotation rate and the inertia of the planet, which is influenced by the internal mass distribution. Because angular momentum also affects how easily a planet is thrown out of alignment by external influences, it can reveal whether Neptune's gravitational influence is strong enough to have shifted Saturn's spin axis and whether the two factors are in resonance.
"If the angular momentum is very large, then the system is in resonance and Neptune could explain why Saturn is spinning sideways," explains co-author Burkhard Militzer of the University of California, Berkeley. "But if the angular momentum is smaller, then the whole scenario falls apart and you have to look for another theory that explains Saturn's axial tilt." To clarify this question, Wisdom, Militzer and their team have measurement data from NASA's Cassini spacecraft used, which measured Saturn's gravitational field shortly before the end of its mission in 2017. This data allowed the team to more precisely determine the planet's mass distribution and angular momentum and use model simulations to verify that these values meet the conditions for resonance with Neptune.
Did a moon loss lead to a resonance break?
The analyzes revealed something surprising, because contrary to what was previously assumed, the two planets are not in resonance at all. "In all model assumptions and for all rotation periods, the system is close to the resonance region, but just outside it," report Wisdom and his colleagues. According to their data, the discrepancy is around 1 percent, leading them to believe that while Saturn and Neptune were originally in resonance, it was recently broken. But by what? "We then looked for ways in which Saturn might have left this resonance," says Wisdom. To do this, they used their models to reconstruct the past 200 million years of the development of the ringed planet and its neighbors and first checked whether changes in the orbits of Saturn's moons and the resulting shifting gravitational influences could have led to the resonance break. It found that while Saturn's moon Titan has indeed moved outward over time, that alone was not enough to break Saturn's and Neptune's resonance.
A second possible explanation remains: "The system could escape from resonance if Saturn once had an additional moon, which was then lost," explains Militzer. This lost satellite could either have been thrown out of the system by gravitational turbulence, or it came too close to Saturn and was torn apart by Saturn's tidal forces. According to the researchers' calculations, the orbit of this moon should originally have been between that of Saturn's largest moon Titan and that of the third largest moon Iapetus. The moon itself, which researchers dubbed "Chrysalis," should have been about as big and heavy as Iapetus. Using another model simulation, Wisdom and his team determined if and how the hypothetical Chrysalis moon might have been ejected or destroyed and how this would have affected Saturn's tilt and resonance.
Lunar destruction explains young rings
The simulations revealed that Saturn may indeed have lost a moon. Thus, Titan's slow outward drift may have been enough to destabilize the orbit of the hypothetical Chrysalis moon 100 to 200 million years ago. In one part of the simulation runs this resulted in the ejection of the moon from the system, but in another part in the destruction of Chrysalis near Saturn. "In both cases, Saturn would have fallen out of resonance as a result," the researchers report. However, if Chrysalis was torn apart by Saturn's tidal forces, it could also explain how the gas planet's rings formed: some of the lunar debris stayed in orbits around Saturn, creating its rings. "The cool thing is that our scenario helps explain the previously unexplained young age of Saturn's rings," says Wisdom. For Titan, this meant that its almost circular orbit was slightly deformed and assumed its current slight eccentricity.
According to the scientists, the loss of a previously existing moon could therefore explain several peculiarities of the ringed planet: "The loss of the hypothetical satellite Chrysalis can explain the obliquity of Saturn, the young age of its rings and the eccentricity of Titan," the researchers state. However, they also acknowledge that this scenario now needs further testing.
Source: Jack Wisdom (Massachusetts Institute of Technology, Cambridge) et al., Science, doi: 10.1126/science.abn1234