A ‘lost’ moon could explain several mysteries surrounding Saturn

New work reveals that Saturn’s rings may have formed during the time of the dinosaurs, around one hundred million years ago, when one of its moons was torn apart by the planet’s gravity. The disappearance of this moon could also explain the tilt of the planet. Details of the study are published in the journal Science.

Two questions

Saturn’s most striking feature is its rings. It was long thought that these were remnants of the formation of the planet itself, about 4.5 billion years ago. In the early 1980s, however, two MIT researchers estimated a relatively young age of 100 million years based on the speed of the icy particles contained within these rings, and the frequency with which they collide. More recently, data from the Cassini probe has confirmed the age of these rings, but how did they form?

We know that Saturn also “wobbles” on its axis. This phenomenon, called precession, is the same as that which causes the axis of rotation of a spinning top to turn in a circle. In the case of Saturn, this precession is mainly caused by Titan, its largest moon, which is “pulled” by the gravitational pull of the Sun.

At one time, as the frequency of Saturn’s precession increased, this is entering into resonance with the precession of the node of Neptune’s orbit (the place where Neptune’s orbit intersects the plane of the ecliptic). A resonance is an amplifying effect. Push a child on a swing at the right time and the range of motion will increase. In the solar system, resonances are gravitational and are tied to specific frequencies of occurrence, in this case the rate of Saturn’s precession and the precession of the node of Neptune’s orbit.

However, recent observations suggest that the frequency of Saturn’s precession and that of Neptune’s orbit are not in resonance. How to explain it?

Saturn photographed by Voyager 2 on August 4, 1981. Credits: NASA/JPL

A moon torn apart by Saturn

In a new study, a team of astronomers led by Jack Wisdom of MIT proposes a hypothesis to explain these two mysteries: Saturn once had another moon.

This hypothetical moon, which the team named Chrysalis, would have been roughly the size of the existing moon Iapetus, which is about 1,470 km wide. About 160 million years ago, gravitational interactions with other large moons, such as Titan and Iapetus, would have gradually made Chrysalis’ orbit more chaotic. Over time, the moon would then have come dangerously close before being torn apart by its intense gravity. Some debris would then have swirled around the planet, forming its rings.

Measurements of the planet’s gravitational field made by the Cassini probe also allowed the researchers to model the distribution of mass in its interior. From there, they were able to calculate that Saturn was just barely out of sync with Neptune. In other words, it still was until recently. The team claims that the gravitational influence of Chrysalis could have kept its tilt in resonance with Neptune for several billion years. The loss of the moon would then have allowed the planet to drift to its current angle of almost 27 degrees.

As neat and enticing as this explanation is, the team recognizes that it remains only a hypothesis. New evidence will have to confirm it.

Leave a Comment

Your email address will not be published.