Saturn’s rings — thought to have formed a mere 100 million years ago — are one of the most distinctive planetary features in our Solar System. The gas giant also rotates at a 26.7-degree angle relative to the plane in which it orbits the Sun. In a new paper published in the journal Science, planetary scientists propose that Saturn previously had an additional satellite, which they name Chrysalis; together with its siblings, Chrysalis orbited Saturn for several billion years, pulling and tugging on the planet in a way that kept its tilt, or obliquity, in resonance with Neptune. But around 160 million years ago, Chrysalis became unstable and came too close to its planet in a grazing encounter that pulled the satellite apart; what’s more, while most of Chrysalis’ shattered body may have made impact with Saturn, a fraction of its fragments could have remained suspended in orbit, eventually breaking into small icy chunks to form the planet’s signature rings.
Composite of a true color image of Saturn, observed by Cassini in 2016, overlaid with a false color representation of the ultraviolet aurora in the northern hemisphere as observed on August 20, 2017. Image credit: NASA / JPL-Caltech / Space Science Institute / A. Bader, Lancaster University.
In the early 2000s, planetary researchers put forward the idea that Saturn’s tilted axis is a result of the planet being trapped in a resonance with Neptune.
But observations taken by NASA’s Cassini spacecraft, which orbited Saturn from 2004 to 2017, put a new twist on the problem.
Scientists found that Titan, Saturn’s largest satellite, was migrating away from Saturn at a faster clip than expected, at a rate of about 11 cm/year.
Titan’s fast migration, and its gravitational pull, led researchers to conclude that the moon was likely responsible for tilting and keeping Saturn in resonance with Neptune.
But this explanation hinges on one major unknown: Saturn’s moment of inertia, which is how mass is distributed in the planet’s interior.
Saturn’s tilt could behave differently, depending on whether matter is more concentrated at its core or toward the surface.
“To make progress on the problem, we had to determine the moment of inertia of Saturn,” said MIT Professor Jack Wisdom, lead author of the study.
In the study, Professor Wisdom and his colleagues looked to pin down Saturn’s moment of inertia using some of the last observations taken by Cassini in its Grand Finale phase.
The gravitational field can be used to determine the distribution of mass in the planet.
The authors modeled the interior of Saturn and identified a distribution of mass that matched the gravitational field that Cassini observed.
Surprisingly, they found that this newly identified moment of inertia placed Saturn close to, but just outside the resonance with Neptune. The planets may have once been in sync, but are no longer.
“Then we went hunting for ways of getting Saturn out of Neptune’s resonance,” Professor Wisdom said.
The scientists first carried out simulations to evolve the orbital dynamics of Saturn and its moons backward in time, to see whether any natural instabilities among the existing satellites could have influenced the planet’s tilt. This search came up empty.
So, they re-examined the mathematical equations that describe a planet’s precession, which is how a planet’s axis of rotation changes over time.
One term in this equation has contributions from all the satellites.
The team reasoned that if one satellite were removed from this sum, it could affect the planet’s precession.
The question was, how massive would that satellite have to be, and what dynamics would it have to undergo to take Saturn out of Neptune’s resonance?
The researchers ran simulations to determine the properties of a satellite, such as its mass and orbital radius, and the orbital dynamics that would be required to knock Saturn out of the resonance.
They conclude that Saturn’s present tilt is the result of the resonance with Neptune and that the loss of the satellite, Chrysalis, which was about the size of Iapetus, Saturn’s third-largest moon, allowed it to escape the resonance.
Sometime between 200 and 100 million years ago, Chrysalis entered a chaotic orbital zone, experienced a number of close encounters with Iapetus and Titan, and eventually came too close to Saturn, in a grazing encounter that ripped the satellite to bits, leaving a small fraction to circle the planet as a debris-strewn ring.
The loss of Chrysalis explains Saturn’s precession, and its present-day tilt, as well as the late formation of its rings.
“It’s a pretty good story, but like any other result, it will have to be examined by others. But it seems that this lost satellite was just a chrysalis, waiting to have its instability,” Professor Wisdom said.
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Jack Wisdom et al. 2022. Loss of a satellite could explain Saturn’s obliquity and young rings. Science 377 (6612): 1285-1289; doi: 10.1126/science.abn1234
