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 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 her, about 100 million years ago, an icy Moon broke up after getting a little too close to Saturn, and the remains of this satellite then gradually placed themselves 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 observe through a small telescope in the solar system,” said 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, the sixth planet 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 leading to these rings being only 100 million years old, some have questioned the reasoning” that led to their dating, explains Jack Wisdom.
He and his colleagues have thus built a complex model allowing not only to explain their recent appearance, but also to understand another characteristic of this planet: its inclination.
Saturn’s axis of rotation is in fact 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 which 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 her… and rather quickly.
According to their model, this motion gradually changed the rate at which Saturn’s spin axis makes a full turn around the vertical — much like the axis of a spinning top forming an imaginary cone as it spins. 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) is no longer exact today. Why?
Only a powerful event could interrupt it.
They thus made the hypothesis of a Moon with a chaotic orbit, having gradually approached too close to Saturn, until the contradictory gravitational forces caused its dislocation.
“It is demolished 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 baptized Chrysalis (chrysalis in French) by Jack Wisdom, an analogy to the wings of butterflies emerging from a cocoon – as here the deployment of the rings.
Scientists think Chrysalis was a bit smaller than our own Moon, and about the size of another Saturn satellite, Iapetus.
However, the latter is almost entirely made up of ice water.
“It is therefore plausible to hypothesize that Chrysalis was also made of ice water, and that is what we need to create the rings”, which are 99% made up of it, notes the professor.
Does it feel like it’s finally solved the mystery of Saturn’s rings?
“We made a good contribution,” he replies soberly. Before adding: the system of Saturn and its satellites still conceals “many mysteries”.