An image of Saturn taken in June 2019 by the Hubble Space Telescope, released by NASA (Hubble / A. SIMON)
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 are not all in agreement on the origin of their training, and not even on their age.
To this burning question, a new study published Thursday in the prestigious journal Science intends to provide a compelling answer.
According to her, about 100 million years ago, an icy moon broke after getting a little too close to Saturn, and the remnants of this satellite would then gradually go into orbit around it. .
“Saturn’s rings were discovered by Galileo about 400 years ago and 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 is satisfying to have found a plausible explanation” for their training, this professor of planetary science at the Massachusetts Institute of Technology (MIT) modestly confides to AFP.
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 the observations of the Cassini probe, launched in 1997 and released in 2017.
“But, since no one has been able to find a process that puts these rings 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 that allows not only to explain their recent appearance, but also to understand another characteristic of this planet: its inclination.
The axis of rotation of Saturn is in fact inclined by 26.7 ° with respect to the vertical (what is called its obliquity). However, being Saturn 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 specifically modeled the interior of the planet for their calculations, started with a recent discovery: Titan, the largest satellite of Saturn (the planet has more than 80), is gradually moving away from her. .. and pretty fast.
According to their model, this movement has gradually changed the rate at which Saturn’s axis of rotation makes one full revolution around the vertical, just like the axis of a top that forms an imaginary cone as it rotates, slightly inclined (we speak of precession).
An important detail because, about a billion years ago, this frequency went into sync with the frequency of Neptune’s orbit. A powerful mechanism, which to be maintained despite the continuous influence of the distance of Titan, caused the inclination of Saturn, up to 36 °.
But the researchers found that this synchronization between Saturn and Neptune (called a resonance) is no longer exact today. How come?
Only a powerful event could interrupt it.
They made the hypothesis of a Moon with a chaotic orbit, having gradually approached too close to Saturn, until the contradictory gravitational forces caused it to dislocate.
“It is being demolished into several pieces, and these same pieces are still displaced and gradually forming rings,” although most of them fall towards Saturn, explains Jack Wisdom.
The influence of Titan, which continued to diminish, eventually reduced Saturn’s inclination, to that seen today.
– Emerging from a chrysalis –
The missing Moon was named Chrysalis (chrysalis in French) by Jack Wisdom, an analogy to the wings of butterflies emerging from a cocoon – like here the unfolding of the rings.
Scientists think Chrysalis was a little smaller than our Moon and about the size of another satellite of Saturn, 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’s what we need to create the rings”, which are made up of 99%, observes the professor.
Do you feel like you’ve 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 hides “many mysteries”.