Saturn's rings and tilt could be the product of an ancient, missing moon
Saturn is a blatant indicator that the planet is rotating tilted. The belted giant rotates with respect to the plane of its orbit around the sun at a 26.7-degree inclination. Since Saturn's tilt precesses, like a spinning top, at almost the same rate as Neptune's orbit, astronomers have long hypothesized that this tilt results from gravitational interactions with its neighbor Neptune.
Saturn may have once been in sync with Neptune, but according to a recent modeling research by scientists at MIT and other institutions, Saturn has since eluded Neptune's influence. What was responsible for this planetary realignment? The one carefully examined theory the team has is a missing moon.
The team claims in a paper published in Science that Saturn, which currently has 83 moons, originally had at least one additional satellite that they have named Chrysalis. The researchers hypothesize that Chrysalis, together with its siblings, orbited Saturn for several billion years while exerting constant pressure on it to maintain the planet's tilt, or "obliquity," in resonance with Neptune.
Chrysalis, however, is thought to have become unstable some 160 million years ago and came too near to its planet in a grazing encounter, which tore the satellite apart. The loss of the moon was sufficient to free Saturn from Neptune's influence and give it the current tilt.
Furthermore, the team hypothesizes that while the majority of Chrysalis' body fragments may have collided with Saturn, some of them may have remained suspended in space and later disintegrated into tiny frozen pieces to form the planet's distinctive rings.
Therefore, the missing satellite could shed light on two enigmas that have persisted for a long time: Saturn's current tilt and the age of its rings, which were formerly thought to be roughly 100 million years old — far younger than the planet itself.
Jack Wisdom, professor of planetary sciences at MIT and principal author of the new study, explains that this satellite was long inert until suddenly becoming active and the rings appeared.
Rola Dbouk from MIT, Burkhard Militzer from Berkeley University, William Hubbard from the University of Arizona, Francis Nimmo and Brynna Downey from Santa Cruz University, and Richard French from Wellesley College are the study's other co-authors.
A period of advancement
Scientists proposed the theory that Saturn's tipped axis results from the planet being entrapped in a resonance, or gravitational relationship, with Neptune in the early 2000s. However, Cassini, a NASA spacecraft that orbited Saturn from 2004 to 2017, made discoveries that gave the issue a fresh angle. Titan, Saturn's largest satellite, was discovered to be departing from Saturn at a rate of roughly 11 millimeters per year, which was faster than anticipated. Scientists came to the conclusion that Titan was probably responsible for tilting Saturn and maintaining it in resonance with Neptune because of its rapid migration and gravitational attraction.
This explanation, however, is dependent on a crucial unknown: Saturn's moment of inertia, which describes how mass is distributed inside the planet. Depending on whether matter is more concentrated at Saturn's core or at its surface, the tilt of the planet may react differently.
Wisdom explains that in order to move further with the issue, it was necessary to ascertain Saturn's moment of inertia.
Using some of the last observations made by Cassini during its "Grand Finale," a phase of the mission during which the spacecraft made an extremely close approach to precisely map the gravitational field around the entire planet, Wisdom and his colleagues sought to determine Saturn's moment of inertia in their new study. The distribution of mass on a planet can be calculated using the gravitational field.
When Wisdom and his coworkers recreated Saturn's interior, they found a mass distribution that matched the gravitational field that Cassini had detected. Surprisingly, they discovered that Saturn was situated near, but just outside, the resonance with Neptune due to the newly discovered moment of inertia. The planets may have been in synchronicity previously, but not now.
Then, according to Wisdom, "we looked for ways to pull Saturn out of Neptune's resonance."
To determine whether any natural instabilities among the then-existing satellites would have had an impact on the planet's tilt, the researchers first ran simulations to evolve the orbital dynamics of Saturn and its moons backward in time. This search turned up nothing.
In order to better understand how a planet's axis of rotation shifts over time, known as precession, the researchers reexamined the mathematical equations that describe it. All the satellites contribute to one term in this equation. The group reasoned that the planet's precession might be impacted if one satellite were removed from this total.
How huge would such satellite need to be and what dynamics would it need to experience in order to remove Saturn from Neptune's resonance?
Simulations were performed by Wisdom and his colleagues to ascertain a satellite's mass, orbital radius, and the orbital dynamics necessary to remove Saturn from the resonance.
They come to the conclusion that Saturn's current tilt is a result of its resonance with Neptune and that its escape from the resonance was made possible by the loss of the satellite Chrysalis, which was about the size of Saturn's third-largest moon Iapetus.
Chrysalis entered a turbulent orbital zone between 200 and 100 million years ago, had several close encounters with Iapetus and Titan, and then came too close to Saturn in a grazing encounter, ripping the satellite to pieces and left a small portion to circle the planet as a debris-strewn ring.
They discovered that the disappearance of Chrysalis explains Saturn's precession, its current tilt, and the late development of its rings.
It's a fairly good story, but like any other outcome, it needs to be verified by other people, says Wisdom. However, it appears that the missing satellite was really a chrysalis waiting for instability.
The National Science Foundation and NASA both provided some funding for this study.
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