The mystery of Saturn's rings finally solved?

UPGRADE DAY

WASHINGTON | Of all the planets in our solar system, Saturn is certainly the one whose representation strikes the imagination the most, thanks to its immense rings. 

But even today the experts do not not all agree on the origin of their formation, or even their age.

To this burning question, a new study published Thursday in the prestigious journal Science intends to provide a convincing answer.

According to it, about 100 million years ago, an icy Moon broke up after s too close to Saturn, and the remains of this satellite then gradually orbited it.

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

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

Saturn, sixth planet in from the Sun, 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 observations of the Cassini probe, launched in 1997 and which bowed out in 2017.

“But since no one could find a process that resulted in these rings being only 100 million years old, some questioned the reasoning” that led to their dating, says Jack Wisdom.

He and his colleagues have thus built a complex model that not only explains their recent appearance, but also helps to understand another characteristic of this planet: its tilt.

Saturn's axis of rotation is in effect inclined at 26.7° with respect to the vertical (what is called its obliquity). However, Saturn being a gas giant, 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.

Gravitational forces

The researchers, who notably modeled the interior of the planet for their calculations, started from a recent discovery: Titan, the largest satellite of Saturn (the planet has more than 80 of them), is gradually moving away from 'she…and rather quickly.

According to their model, this motion gradually changed the rate at which Saturn's axis of rotation makes a complete turn around the vertical — somewhat like the axis of a spinning top forming an imaginary cone when it rotates slightly tilted (we speak of precession).

An important detail, because about a billion years ago this frequency entered into synchronization with the frequency of Neptune's orbit. A powerful mechanism, which to be maintained despite the continuous influence of the remoteness of Titan, caused the inclination of Saturn, up to 36°.

But the researchers found that this synchronization between Saturn and Neptune (called resonance) was no longer accurate today. Why?

Only a powerful event could have interrupted it.

They thus made the hypothesis of a Moon with a chaotic orbit, having gradually approached too close to Saturn, until that conflicting gravitational forces are causing its dislocation.

“It is torn down into multiple pieces, and these pieces are themselves still dislocated, and little by little form the rings”, although the majority fall towards Saturn, explains Jack Wisdom.

The influence of Titan, which continued to recede, then eventually reduced Saturn's tilt, down to that seen today.

Emerging from a Chrysalis

The missing Moon was christened Chrysalis by Jack Wisdom, an analogy to butterfly wings emerging from a cocoon — as here the unfolding of rings.

Scientists believe Chrysalis was a little smaller than our own Moon, and about the size of another Saturn satellite, Iapetus.

The latter is almost entirely made of ice water.

“So it is plausible to hypothesize that Chrysalis was also made of ice water, and that is what is needed to create the rings” , which are made up of 99% of them, notes the professor.

Does he have the impression of having finally solved the mystery of Saturn's rings?

“We have made a good contribution,” he replies soberly. Before adding: the system of Saturn and its satellites still conceals “a lot of mysteries”.