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 astronomers do not all agree on the origin of their formation, or even their age.
To this burning question, a new study published on September 15 in the journal Science intends to provide a convincing answer.
According to her, about 100 million years ago, an icy moon broke apart after getting a little too close to Saturn; the remains of this satellite were then 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 observe through a small telescope in the solar system,” said Jack Wisdom, lead author of.
“It’s satisfying to have found a plausible explanation” for their formation, modestly confides this professor of planetary sciences at the Massachusetts Institute of Technology ().
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 observations of the probe, launched in 1997 and bowed out in 2017, after orbiting Saturn 294 times. She collected very important data including measurements of the gravity of the planet which were a key for this study.
“As 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.
The tilt of each planet in our Solar System on their axis. [JPL-Caltech/Richard Barkus – NASA]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 indeed tilted at 26.7 degrees from the vertical – this is called the obliquity. By comparison, our Earth is leaning 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 which 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, the largest satellite of Saturn – the planet has 83 known ones –, is moving away little to little of it and rather quickly: eleven centimeters per year.
It doesn’t seem like a lot, but that’s a lot of distance over time, especially for such a large moon. Titan is the second largest moon in the Solar System: it is larger than Mercury, the planet closest to the Sun.
According to the scientists’ model, this movement gradually changed the frequency at which Saturn’s axis of rotation makes a full turn around the vertical – much like the axis of a spinning top forming an imaginary cone when it spins. spins slightly tilted – a phenomenon called 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 researchers have found that this synchronization between Saturn and Neptune – called resonance – is no longer exact. Why?
Only a powerful event could interrupt it. The scientists 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 was demolished in multiple pieces, and these pieces again fell apart themselves, and little by little formed the rings”, although the majority fell towards Saturn, explains Jack Wisdom.
The influence of Titan, which continued to recede, then eventually reduced Saturn’s tilt, down to that seen today.
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Emerging from a chrysalis
The missing moon was baptized Chrysalis – or Chrysalide 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, Saturn’s third-largest moon, about 1470 kilometers in diameter. Goldconsists almost entirely of ice water.
>> A 3D model of Saturn’s icy moon Iapetus:
“It is therefore plausible to hypothesize that Chrysalis was also composed 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 conceal “a lot of mysteries”.
Stéphanie Jaquet and the agencies