New theory about the formation and age of Saturn’s rings

To this burning question, a new study published on September 15 in the journal Science aims to provide a convincing answer.

According to her, about 100 million years ago, an icy moon broke up after getting too close to Saturn; Then the remains of this satellite were gradually placed in orbit around it.

“Saturn’s rings were discovered by Galileo about 400 years ago, and they are one of the most interesting objects to look at through a small telescope in the solar system,” said Jack Wisdom, lead author of the study.

“plausible explanation

“It’s satisfying to have found a plausible explanation” for their formation, modestly confesses this professor of planetary sciences at the Massachusetts Institute of Technology (MIT).

Saturn is the sixth planet from the Sun and was formed four and a half billion years ago, at the beginning of the Solar System. But a few decades ago, scientists suggested that Saturn’s rings appeared much later: only about 100 million years ago.

A hypothesis reinforced by the observations of the probe. casini, launched in 1997 and abandoned in 2017, after orbiting Saturn 294 times. He collected very important data, including measurements of the planet’s gravity, which were key to this study.

“Since no one could find a process that led to these rings being only 100 million years old, some have questioned the reasoning” that led to their dating, explains Jack Wisdom.

The tilt of each planet in our Solar System on its axis. [JPL-Caltech/Richard Barkus – NASA]He and his colleagues have thus built a complex model that allows not only to explain its recent appearance, but also to understand another characteristic of this planet: its inclination.

In fact, Saturn’s spin axis is tilted 26.7 degrees from vertical; this is called obliquity. By comparison, our Earth is tilted 23.4 degrees. However, Saturn being a gas giant, composed mainly of hydrogen and helium, it would have been expected that the process of accumulation of matter that led to its formation would have left it perpendicular to the plane of its orbit.

Conflicting gravitational forces

The research team, which notably modeled the interior of the planet for its calculations, started from a recent discovery: Titan, Saturn’s largest satellite –the planet has 83 known ones–, is moving away little by little and quite quickly: eleven centimeters per year.

It doesn’t sound like much, but it’s a long way back in time, especially for a moon that big. Titan is the second largest moon in the Solar System: it is larger than Mercury, the closest planet to the Sun.

According to the scientists’ model, this motion gradually changed the frequency at which Saturn’s axis of rotation makes a complete turn around the vertical, much like the axis of a spinning top that forms an imaginary cone when it rotates slightly tilted, a phenomenon called precession.

An important detail because about a billion years ago this frequency came into synchronization with the frequency of Neptune’s orbit. A powerful mechanism, which, if maintained despite the continued influence of Titan’s remoteness, caused Saturn to tilt up to 36°.

But researchers have discovered that this timing between Saturn and Neptune, called resonance, is no longer exact. Why?

Only a powerful event could interrupt it. Scientists thus formulated the hypothesis of a moon with a chaotic orbit, having come little by little too close to Saturn, until contradictory gravitational forces caused its dislocation: “It was demolished into multiple pieces, and these pieces fell apart again, and little by little. Little by little the rings formed,” although most fell toward Saturn, explains Jack Wisdom.

Titan’s influence, continuing to recede, eventually reduced Saturn’s tilt, down to what is seen today.

>> Read also: Liquid on Titan, a moon of Saturn

emerging from a chrysalis

The missing moon was named Chrysalis -or Chrysalide in French- by Jack Wisdom, an analogy with the wings of butterflies emerging from a cocoon, like here the deployment of the rings.

Scientists think that Chrysalis was slightly smaller than our own Moon and about the size of another satellite of Saturn, Iapetus, Saturn’s third largest moon, at about 1,470 kilometers in diameter. Prayed Iapetus It consists almost entirely of ice water.

>> A 3D model of Saturn’s icy moon Iapetus:

“Therefore, it is plausible to hypothesize that Chrysalis was also composed of frozen water, and that is what we need to create the rings”, which are 99% composed of it, says the professor.

Does it feel like the mystery of Saturn’s rings is finally being solved? “We made a good contribution,” she replies seriously. Before adding: the Saturn system and its satellites still hide “many mysteries”.

Stéphanie Jaquet and the agencies