Currently, Saturn hosts 83 moons. However, research from the Massachusetts Institute of Technology (MIT) suggests that it could have harbored at least one more and that its demise may have proved fundamental to the gas giant’s development. In their study, which appeared in the journal Science, the MIT team believes that the former moon, dubbed “Chrysalis”, could have given Saturn its current tilt as well as its famous rings.
The rings of Saturn revolve around the planet’s equator, a telltale sign that the planet spins on a tilted axis. The belted giant’s axis of rotation is tilted by an angle of 26.7 degrees with respect to its orbital plane around the sun. Since Saturn’s tilt moves like a spinning top, at nearly the same rate as Neptune’s orbit, astronomers have long suspected that this results from gravitational interactions with its celestial neighbor.
While Neptune might have previously had an enormous gravitational interaction, Chrysalis could have put an end to that after an impact with Saturn. The team estimates that around 160 million years ago, Chrysalis became unstable and had a grazing encounter with its planet that tore the satellite apart. Saturn’s current tilt is a direct result of its moon’s demise, which also freed the planet from Neptune’s grip.
Additionally, the impact of the icy moon could have created the large set of rings that the planet is most famous for. It could also explain why the rings are relatively young compared to the planet itself. The researchers speculate that while most of Chrysalis’ shattered body collided with Saturn, some of the fragments may have remained in orbit and eventually broken into small icy chunks to form Saturn’s distinctive rings.
“Just like a butterfly’s chrysalis, this satellite was long dormant and suddenly became active, and the rings emerged,” says Jack Wisdom, professor of planetary sciences at MIT and lead author of the new study.
Saturn’s current tilt and the age of its rings, previously estimated to be about 100 million years old, much younger than the planet itself, are best explained by the missing satellite. All other alternative hypotheses fall short.
“The tilt is too large to be a result of known formation processes in a protoplanetary disk or from later, large collisions,” said Jack Wisdom, a professor of planetary science at MIT. “A variety of explanations have been offered, but none is totally convincing. The cool thing is that the previously unexplained young age of the rings is naturally explained in our scenario.”
In the early 2000s, researchers proposed that Saturn’s resonance with Neptune, or gravitational association, could explain the planet’s oblique spin axis. However, the issue was given a new spin (pun intended) thanks to observations made by NASA’s Cassini spacecraft as it orbited Saturn from 2004 to 2017. Scientists were surprised to discover that Titan, the planet’s largest moon, was moving away from the planet at a rate of about 11 centimeters per year, much quicker than they had anticipated. Scientists believe that Titan’s rapid migration and gravitational pull caused Saturn to tilt and maintain resonance with Neptune.
Saturn’s moment of inertia, which describes the distribution of mass within the planet, is a key variable in this explanation, but it remains a mystery. The tilt of Saturn may act differently depending on whether its mass is concentrated closer to its center or its exterior.
For this new study, Wisdom and coworkers used data from Cassini’s “Grand Finale,” when the spacecraft came extremely close to Saturn in order to precisely map the gravitational field around the entire planet in order to determine Saturn’s moment of inertia.
Together, Wisdom and his team ran computer models of Saturn’s interior and determined a distribution of mass that agreed with Cassini’s measurements of the planet’s gravitational field. Surprisingly, they discovered that Saturn’s newly identified moment of inertia put it close to, but just outside, the resonance with Neptune.
In order to determine if natural instabilities among Saturn’s existing satellites could have influenced the planet’s tilt, the team first ran simulations to run the orbital dynamics of Saturn and its moons backward in time. However, that search came up empty.
Back to the drawing board, the researchers reevaluated the equations used to describe a planet’s precession, or the gradual wobbling of its axial tilt over time. All the moons contribute to a single term in this equation. Now, the question was how big that moon would have to be and what kind of motion it would have to make to break Saturn’s resonance with Neptune. Turns out, pretty darn big. Chrysalis would have had to be roughly the size of Iapetus, Saturn’s third-largest moon.
“It’s a pretty good story, but like any other result, it will have to be examined by others,” Wisdom says. “But it seems that this lost satellite was just a chrysalis, waiting to have its instability.”