Saturn’s rings orbit around the planet’s equator, a dead giveaway that the planet rotates obliquely. The giant belt rotates at an angle of 26.7 degrees with respect to the plane around the Sun. Astronomers have long suspected that this tilt comes from gravitational interactions with its neighbor Neptune, since Saturn’s tilt, like a spinning top, is about the same rate as Neptune’s orbit.
But a new modeling study by astronomers at MIT and elsewhere finds that while the two planets may have once coincided, Saturn has since escaped the clouds of Neptune. What is responsible for planetary realignment? The team has one thoroughly tested hypothesis: a missing moon.
In a study appearing in Sciences, the team suggests that Saturn, which today hosts 83 moons, at least once harbored an additional satellite they call Chrysalis. Researchers, along with its siblings, suggest that Chrysalis has orbited Saturn for several billion years, attracting and dragging the planet in such a way that it maintains its tilt, or “skew,” in alignment with Neptune.
But about 160 million years ago, the team estimates, Chrysalis became unsettled and got too close to its planet in a grazing encounter that tore up the satellite. The loss of the Moon was enough to remove Saturn from the grip of Neptune and leave it with the current tilt.
What’s more, the researchers believe, while most of Chrysalis’ fractured body may have had an impact with Saturn, a portion of its fragments may remain suspended in orbit, eventually breaking into small ice pieces to form the planet’s distinctive rings.
Therefore, the missing satellite can explain two ancient mysteries: the present-day tilt of Saturn and the age of its rings, which were previously estimated at about 100 million years – much younger than the planet itself.
“Just like a butterfly cocoon, this satellite had been dormant for a long time and suddenly became active, and the rings appeared,” says Jack Wisdom, professor of planetary sciences at MIT and lead author of the new study.
Study co-authors include Rola Dubok of the Massachusetts Institute of Technology, Burkard Melitzer of UC Berkeley, William Hubbard of the University of Arizona, Frances Nimo and Brenna Downey of UC Santa Cruz, and Richard French of Wellesley College.
moment of progress
In the early 2000s, scientists put forward the idea that Saturn’s tilted axis is the result of the planet being trapped in a resonance, or gravitational link, with Neptune. But observations 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-than-expected clip, at a rate of about 11 cm per year. Titan’s rapid migration and gravitational pull have led scientists to conclude that the Moon is most likely responsible for tilting Saturn and maintaining a resonance with Neptune.
But that 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 can act differently, depending on whether the material is more concentrated in its core or toward the surface.
“To make progress on the problem, we had to identify the moment of Saturn’s insufficiency,” Wisdom says.
In their new study, Wisdom and his colleagues looked at determining Saturn’s moment of inertia using some of the latest observations taken by Cassini in “Grand Finale,” a phase of the mission during which the spacecraft has taken a very close approach to accurately mapping the gravitational field around the entire planet. The gravitational field can be used to determine the distribution of mass in a planet.
Wisdom and colleagues modeled the interior of Saturn and determined a mass distribution consistent with the gravitational field observed by Cassini. Surprisingly, they discovered that the recently determined moment of inertia put Saturn close to, but outside of, Neptune’s echo. The planets may have once been synchronized, but they are no longer.
Wisdom says, “Then we went to look for ways to get Saturn out of the echo of Neptune.”
The team first ran simulations to develop the orbital dynamics of Saturn and its moons back in time, to see if any natural instabilities between current satellites could have affected the planet’s tilt. This search came up empty.
So, the researchers re-examined the mathematical equations that describe the planet’s motion, which is the way the planet’s rotation axis changes over time. One term in this equation has contributions from all satellites. The team concluded that if one satellite was removed from that total, it could affect the planet’s motion.
The question was, how massive would this satellite be, and what dynamics would it have to undergo to get Saturn out of Neptune’s resonance?
Wisdom and colleagues ran simulations to determine the characteristics of a satellite, such as its mass and orbital radius, and the orbital dynamics that would be required to knock Saturn out of resonance.
They concluded that Saturn’s current tilt is the result of a resonance with Neptune and that the loss of the satellite, Chrysalis, which was 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 region, experienced a number of close encounters with Iapetus and Titan, and eventually got too close to Saturn, encountering a grazing that tore the satellite into bits, leaving a tiny bit to rotate the planet as a filled ring with debris.
They found that the loss of the cocoon explains the beginning and current tilt of Saturn, as well as the late formation of its rings.
“It’s a very good story, but like any other outcome, it should be scrutinized by others,” Wisdom says. “But it looks like this missing satellite was just a cocoon waiting to be destabilized.”
This research was supported, in part, by NASA and the National Science Foundation.