Saturn’s rings are about 100 million years old, but it’s unclear how they could have formed in such a short time. The belted giant rotates at an angle of 26.7 degrees relative to the plane in which it orbits the sun.
Because Saturn’s tilt precesses like a top at almost the same speed as Neptune’s orbit, astronomers have long hypothesized that this tilt results from gravitational interactions with its neighbor Neptune.
While the two planets may have once been in sync, astronomers from MIT and other universities have discovered that Saturn has since eluded Neptune’s gravitational pull.
What caused this realignment of the planets?
The team comes up with a new hypothesis. They suggested that the Saturn system previously contained an additional moon called Chrysalis. Along with other moons, Chrysalis orbited Saturn for several billion years, pulling and tugging at the planet in a way that kept its tilt, or “skewness,” in resonance with Neptune.
However, it is believed that around 160 million years ago, Chrysalis became unstable and got too close to its planet in a grazing encounter that tore the satellite apart. The loss of the moon was enough to free Saturn from Neptune’s pull and give it its tilt today. In addition, a fraction of Chrysalis’s fragments may have gotten stuck in orbit and eventually shattered into small icy pieces to form the planet’s distinctive rings.
Astronomers suggest this missing moon could explain two long-standing mysteries: Saturn’s present-day tilt and the age of its rings, which have been estimated at around 100 million years — much younger than the planet itself.
Jack Wisdom, a professor of planetary sciences at MIT and lead author of the new study, said: “Just like a butterfly’s chrysalis, this satellite was dormant for a long time and suddenly became active and the rings appeared.”
In the early 2000s, scientists hypothesized that Saturn had a tilted axis due to its gravitational resonance or association with Neptune. However, Cassini, a NASA spacecraft that orbited Saturn from 2004 to 2017, made discoveries that gave the problem a new perspective. Titan, Saturn’s largest satellite, was discovered to be drifting away from Saturn at a rate of about 11 centimeters per year, which was faster than expected. This rapid migration and its gravitational pull allowed scientists to conclude that the Moon was likely responsible for tilting Saturn and keeping it in resonance with Neptune.
Jack Wisdom, a professor of planetary sciences at MIT and lead author of the new study, said: “To move forward with the problem, we needed to determine Saturn’s moment of inertia.”
In this new study, scientists attempted to determine Saturn’s moment of inertia using some of Cassini’s latest observations in its “grand finale,” a phase of the mission during which the spacecraft approached extremely closely to accurately capture the gravitational field map around the whole planet. The mass distribution on the planet can be determined from the gravitational field.
They modeled Saturn’s interior and identified a mass distribution consistent with the gravitational field observed by Cassini. Surprisingly, this newly identified moment of inertia placed Saturn near, but just off resonance with Neptune. The planets may once have been in sync, but they are no longer.
They began running simulations to develop the orbital dynamics of Saturn and its moons backward in time. They wanted to determine whether natural instabilities between the existing satellites could have affected the planet’s tilt. This search was empty.
To better understand how a planet’s axis of rotation shifts over time, known as precession, the scientists re-examined the mathematical equations that describe it. One term in this equation has contributions from all satellites. The team argued that the removal of a satellite from this total could affect the planet’s precession.
Through simulations, scientists determined Chrysalis’ properties, including its mass and orbital radius, as well as the orbital dynamics that would be required to knock Saturn out of resonance. They conclude that the resonance between Saturn and Neptune caused the planet’s current tilt and the loss of the satellite Chrysalis, which is about the same size as Saturn’s third largest moon Iapetus, allowed Saturn to avoid the resonance.
Scientists found “Sometime between 200 and 100 million years ago, Chrysalis entered a chaotic orbital zone, experienced several close encounters with Iapetus and Titan, and eventually came too close to Saturn in a grazing encounter that ripped the satellite apart, leaving a small fraction orbiting the planet as a ring strewn with debris.”
“The loss of Chrysalis explains Saturn’s precession and tilt today, as well as the late formation of its rings.”
wisdom said “It’s a pretty good story, but like any other finding, it needs to be investigated by others. But it seems this lost satellite was just a puppet waiting to experience its instability.”
- Jack Wisdom et al. The loss of a satellite could explain Saturn’s tilt and young rings. SCIENCE. DOI: 10.1126/science.abn1234