Saturn’s rings may have formed 100 million years ago when one of its icy moons was torn apart by the planet’s gravity.
That’s according to a new study that links the creation of the magnificent rings to Titan’s outward migration and the resonance between Saturn’s rotation and Neptune’s orbit.
The debate about the age of Saturn’s rings has raged for decades. Some planetary scientists thought the rings could be as old as the planet itself, but in the early 1980s Peter Goldreich of the California Institute of Technology and Scott Tremaine of the Massachusetts Institute of Technology (MIT) estimated a relatively young age of 100 million years based on the velocity ice particles in the rings and how often they collide and wear each other.
Related: Saturn’s strange moon Titan looks a bit like Earth, and scientists may finally understand why
The end of the Cassini mission to Saturn in 2017 brought more evidence of a young age. The spacecraft’s space dust analyzer measured the fall of interplanetary dust onto the rings, and then, based on measurements of the mass of Cassini’s rings, found that only one percent of the rings are dust, meaning there could only be 100 million rings. years.
While Cassini’s findings were generally welcomed, some planetary scientists called for caution, pointing out that some of the dust could rain from the rings onto Saturn itself, thus keeping the rings clean and making them appear younger than they really are.
Now, a new study led by MIT’s Jack Wisdom has uncovered a physical mechanism that not only explains Saturn’s axial tilt and the orbital eccentricity of its largest moon, Titan, but also suggests the rings must be about 100 million years old. (will open in a new tab).
“Our scenario is the first explanation that predicts the age of the rings,” Wisdom told Space.com.
It’s a curious story about resonances, torques, deflections and precession. Saturn’s tilt—that is, how tilted the planet is to the plane of the solar system’s orbit—is 26.7 degrees. In 2021, scientists led by Melain Sailenfest of the Paris Observatory showed that the relatively recent outward migration of Titan’s orbit (will open in a new tab) could cause Saturn to tip over by that amount.
(Image credit: NASA/JPL) (will open in a new tab)
Saturn also wobbles on its axis in a phenomenon called precession. This is the same effect that causes the spinning pin of the top to pirouette in a circle. It is not unusual for a planet’s axis of rotation to precess—the Earth’s axis of rotation also precesses over thousands of years. Today, the north pole of the earth is more or less pointing at the North Star, Polaris, but in a few thousand years, precession will mean that the north pole will point at the star Vega instead.
In the case of Saturn, the precession is mostly initiated by Titan as the Sun’s gravity pulls on the Moon, causing a torque on Saturn. Torque is a twisting force, and in the case of Saturn, the torque acts on Saturn’s axis of rotation, causing it to precess.
At some point, when the frequency of Saturn’s precession increased, it entered into resonance with the precession of the node of Neptune’s orbit, that is, the place where Neptune’s orbit intersects the plane of the ecliptic. Resonance is a reinforcing effect, such as the classic example of a child swinging on a swing. Press at the right moment, and the amplitude of the swing may increase. Resonances in the solar system are gravitational and are associated with certain frequencies of occurrence, in this case the rate of precession of Saturn and the precession of the node of Neptune’s orbit.
Today, however, the frequencies of Saturn’s precession and the precession of Neptune’s orbit are not in resonance, but out of it, and their frequencies do not exactly match.
Could something have happened to bring Saturn and Neptune out of resonance? “We hypothesize that there was once an additional moon that was lost due to chaotic orbital instability and had a close collision with Saturn and formed rings,” said Wisdom, who dubbed the hypothetical moon “Chrysalis.”
In their hypothesis, Wisdom’s team suggests 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 million and 200 million years ago, Chrysalis itself would also have entered orbital resonance with Titan—for every three orbits of Saturn that Titan would have made, Chrysalis would have made one. This resonance would have given Chrysalis a boost like a baby on a swing and destabilized its orbit, eventually seeing it get too close to Saturn where gravitational tides tore it apart to form the famous rings. Without Chrysalis’ torque, Saturn’s precession frequency would decrease, causing it to go 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 moon was once present in the Saturn system, but by explaining four mysteries [Saturn’s obliquity, the existence of the rings, the age of the rings, and Titan’s eccentric orbit] with one hypothesis, a pretty good ROI.”
The study was published in the journal Science (will open in a new tab).
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