The loss of the moon may have created the rings of Saturn

Saturn is not only unique in the solar system because of its many amazingly young rings, but it is also amazing because of the greater inclination of its axis of rotation compared to its orbit around the sun. Planetary researchers can now find an explanation for these special properties. Using data from NASA’s Cassini spacecraft and simulations, they conclude that the ringed planet once had another large moon. About 160 million years ago, this satellite, called Chrysalis, lost its orbit and was torn apart by Saturn’s immense gravity. The debris that fell on the planet changed its axis, and the remaining fragments could become the planet’s gas rings – thus explaining its surprisingly young age.

The gaseous planet Saturn is unique in the solar system. No other planet has such a huge and complex ring system. About 400 years ago, Galileo Galilei noticed strange “arms” on both sides of the planet, which he could not explain at that time. It is now clear that Saturn is surrounded by multiple belts of masses of ice and dust, broken up by gaps and moons. Unlike the planet, these do not come from the beginnings of the solar system, but are only about 100 million years old according to recent data. This raises the question of how Saturn’s rings are formed. Another peculiarity: the rings and the axis of Saturn’s rotation are tilted by 26.7 degrees compared to its orbit around the Sun. “This tendency is too large to arise from known formation processes in the protoplanetary disk or from large planetary collisions,” explains lead author Jack Wisedom of the Massachusetts Institute of Technology (MIT). “Various explanations for this have been proposed, but none have been completely convincing.”

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Is resonance the cause?

One popular theory is that Saturn’s gravitational interactions with Neptune are responsible for its axial tilt. Because the front of Saturn’s axis – a circular oscillation – corresponds in its period approximately to the orbital period of Neptune. Thus, the motion of Saturn’s axis can be identical to that of Neptune’s orbit. This resonance, in turn, can tilt the planet’s rotation axis to its current position over time – so the previous assumption. However, whether it is correct depends critically on a parameter that has not been determined with sufficient precision for a long time: the angular momentum of Saturn. This results from the planet’s rotation rate and inertia, which are affected by the internal mass distribution. Since angular momentum also affects how easily a planet can be pushed out of alignment by external influences, it can reveal whether Neptune’s gravitational influence is strong enough to shift Saturn’s spin axis and whether the two factors resonate.

“If the angular momentum is too large, the system will resonate and Neptune could explain why Saturn is rotating sideways,” explains co-author Burkhard Melitzer of the University of California, Berkeley. “But if the angular momentum is smaller, the whole scenario will collapse and you have to look for another theory that explains Saturn’s axial tilt.” To clarify this question, Hikma, Melitzer and their team have measurement data from NASA’s Cassini spacecraft, which measured Saturn’s gravitational field shortly before the end of its mission in 2017. This data allowed the team to more accurately determine the planet’s mass distribution and angular momentum and use simulation models to verify that These values ​​meet the conditions for resonance with Neptune.

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Did the loss of the moon lead to the interruption of the ringing?

The analyzes revealed something surprising, because contrary to what was previously assumed, the planets are not in resonance at all. “In all model assumptions and for all rotation periods, the system is close to, but just outside of, the resonance region,” Wisdom and colleagues say. According to their data, the discrepancy is about 1 percent, leading them to believe that while Saturn and Neptune were originally in resonance, they were recently broken. But what? “We next looked for ways in which Saturn might leave this resonance,” Wisdom says. To do this, they used their models to reconstruct the past 200 million years of evolution of the ringed planet and its neighbors and first examined whether changes in the orbits of Saturn’s moons and the resulting shifting gravitational influences led to the interruption of the resonance. It found that while Saturn’s moon Titan did indeed move outward over time, this alone was not enough to break the resonance of Saturn and Neptune.

The second possible explanation remains: “The system could escape resonance if Saturn had one extra moon, which was then lost,” Melitzer explains. This missing satellite could have been thrown out of the system by gravitational perturbation, 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 have originally been between the orbit of Saturn’s largest moon Titan and the third largest moon Ipetus. The moon itself, which the researchers called the “cocoon,” must have been the size and weight of Iapetus. Using other model simulations, Wisdom and his team determined whether Chrysalis’ hypothetical moon had been ejected or destroyed and how that would have affected Saturn’s tilt and resonance.

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The destruction of the moon explains the young episodes

Simulations revealed that Saturn may already have lost a moon. Thus, Titan’s slow outward drift may be enough to destabilize the orbit of the hypothetical moon Chrysalis 100 to 200 million years ago. In one part of the simulation, this resulted in the moon being expelled from the system, but in another part it resulted in the destruction of the cocoon near Saturn. “In both cases, Saturn’s echo could have been emitted as a result,” the researchers said. However, if Chrysalis was torn apart by tidal forces at Saturn, it could also explain how the gas planet’s rings formed: some moon debris remained in orbits around Saturn, forming its rings. “The remarkable thing is that our scenario helps explain the previously unexplained young age of Saturn’s rings,” Wisdom says. For Titan, this meant that its semi-circular orbit was slightly distorted and assumed its slight current skew.

According to scientists, the loss of the previously existing moon could explain many of the distinctive characteristics of the ringed planet: “The loss of the hypothetical satellite Chrysalis could explain the eccentricity of Saturn, the young age of its rings, and the eccentricity of Titan.” Researchers say. However, they also acknowledged that this scenario now needs further testing.

Source: Jack Wisdom (MIT, Cambridge) et al., Science, doi: 10.1126/science.abn1234

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