Saturn's rings might have formed 100 million years ago when one of its icy moons was ripped apart by the planet's gravity.
A debate has raged for decades as to the age of Saturn's rings. Some planetary scientists thought that the rings could be as old as the planet itself, but in the early 1980s Peter Goldreich of Caltech and Scott Tremaine of the Massachusetts Institute of Technology (MIT) estimated a relatively young age of 100 million years based on the velocity of icy particles in the rings and how often they collide and wear each other down.
The end of the Cassini mission to Saturn in 2017 brought more evidence for a young age. The spacecraft's Cosmic Dust Analyzer measured the infall of interplanetary dust onto the rings, and then based upon Cassini's measurement of the mass of the rings, found that only one per cent of the rings consisted of dust, meaning that the rings could only be 100 million years old.
Though Cassini's findings were mostly welcomed, some planetary scientists urged caution, pointing out that some of the dust could be raining out of the rings and onto Saturn itself, thereby keeping the rings clean and making them seem younger than they really are.
Now new research, led by Jack Wisdom of MIT, has found a physical mechanism that not only explains the tilt of Saturn's rotational axis and the eccentricity of the orbit of its largest moon Titan, but which also stipulates that the age of the rings must be close to 100 million years.
"Our scenario is the first explanation that predicts the age of the rings," Wisdom told Space.com.
It's a curious tale of resonances, torques, obliquities and precession. Saturn's obliquity — that is, how much the planet is tilted to the orbital plane of the Solar System — is 26.7 degrees. In 2021, scientists led by Melaine Saillenfest of the Observatoire de Paris showed that relatively recent outward migration of the orbit of Titan could have caused Saturn to tip over by this amount.
Saturn also wobbles on its axis in a phenomenon called precession. It is the same effect that causes the axis of rotation of a spinning top to pirouette in a circle. It's not unusual for a planet's rotational axis to precess — Earth's rotational axis also precesses over thousands of years. Today, Earth's north pole points more or less towards the Pole Star, Polaris, but in a few thousand years precession will mean that the north pole will instead point towards the star Vega.
In Saturn's case, the precession is instigated mostly by Titan as the sun's gravity pulls on the moon, incurring a torque on Saturn. A torque is a twisting force, and in the case of Saturn, the torque is acting on Saturn's rotational axis, prompting it to precess.
At some point, as the frequency of Saturn's precession increased, it entered into a resonance with the precession of the node of Neptune's orbit, i.e. the location where Neptune's orbit cuts across the ecliptic plane. A resonance is an amplifying effect, such as the classic example of pushing a child on a swing. Push at just the right moment, and the amplitude of the swing can increase. Resonances in the solar system are gravitational and are tied to specific frequencies of occurrence, in this case Saturn's rate of precession and the precession of the node of Neptune's orbit.
However, today the frequency of Saturn's precession and the precession of Neptune's orbit are not in resonance, but are just outside it, their frequencies not quite matching up.
Could something have happened to move Saturn and Neptune out of resonance? "We propose that there used to be an additional satellite that was lost through a chaotic orbital instability, and had a close encounter with Saturn and formed the rings," said Wisdom, who christens the hypothetical moon 'Chrysalis'.
In their hypothesis, Wisdom's team propose that Chrysalis also contributed to the torque on Saturn, bringing the ringed planet into resonance with Neptune. However, their computer simulations show that between 100–200 million years ago, Chrysalis itself would have also entered an orbital resonance with Titan — for every three orbits of Saturn that Titan would make, Chrysalis would make one. This resonance would have given Chrysalis a push, like the child on a swing, and destabilized its orbit, ultimately seeing it get too close to Saturn where gravitational tides tore it apart to form the famous rings. Without the torque from Chrysalis, Saturn's frequency of precession would have decreased, moving it just out of resonance with Neptune.
Scott Tremaine, who first hypothesized the young age, describes the new work as "remarkable", adding that, "Of course, we will never know for sure if an extra satellite was once present in the Saturn system, but explaining four puzzles [Saturn's obliquity, the existence of the rings, the age of the rings, and Titan's eccentric orbit] with one hypothesis is a pretty good return on investment."
The research was published online Thursday (Sept. 15) in the journal Science.