400 years ago, astronomer Galileo Galilei announced the discovery of four moons orbiting Jupiter. For the past 40 years, scientists have been studying the satellites – Io, Europa, Ganymede and Callisto. However, how they formed remains a mystery.
Despite being all the same size – about a quarter of the Earth's radius -, Jupiter's four moons are very different: Io is violently volcanic, Europa is embedded in ice, Ganymede has a magnetic field and Callisto is full of ancient craters . In addition, Europa ice cream is considered a strong candidate to host life in the Solar System. But how did Jupiter's moons form?
Now, Konstantin Batygin, professor of planetary science at Caltech, and his collaborator Alessandro Morbidelli, from the Observatoire de la Côte d’Azur, in France, have proposed an answer to this long-standing question.
Using analytical calculations and large-scale computer simulations, the researchers propose a new theory for the origin of Jovian satellites.
During the first million years of life, our Sun was surrounded by a protoplanetary disk composed of gas and dust. Jupiter joined this disk and was surrounded by its own disk of satellite building material. The so-called Circum-Jovian disk was fed by material from the protoplanetary disk that rained on Jupiter at the planet's poles and returned from Jupiter's gravitational sphere of influence along the planet's equatorial plane.
How did the ever-changing disk accumulate enough material to form moons?
The new model by Batygin and Morbidelli incorporates the physics of interactions between dust and gas in the Circum-Jovian disk. The researchers demonstrate that, for grains of icy dust of a specific size range, the force that drags them towards Jupiter and the force that carries them in the external flow of the gas cancel each other out perfectly, allowing the disk to act as a trap. giant dust.
“I was going up a hill and I saw a bottle on the ground that was not going down the hill because the wind from behind pushed it up and kept it in balance with gravity. A simple analogy came to mind: if a bottle rolling on an inclined plane is similar to the orbital deterioration of solid grains due to hydrodynamic drag, particles of a certain size must find an equivalent balance in Jupiter's orbit, ”explained Batygin, in communication.
According to the study published in May in the scientific journal The Astrophysical Journal, the model proposes that, due to the balance between internal drag and external drag, the disk around Jupiter becomes rich in grains of icy dust, each about one millimeter.
The dust ring became so massive that it collapsed under its own weight into thousands of “satellitesimals” – asteroid-type objects, about 100 kilometers in diameter. Over thousands of years, satellitesimals coalesced into moons, one at a time.
When the first moon, Io, formed and its mass reached a certain threshold, its gravitational influence began to create waves in the gas disc of the material that surrounded Jupiter. When interacting with these waves, the moon migrated towards Jupiter until it reached the inner edge of the circum-Jovian disk, close to its current orbit. The process started again with the next moon.
This sequential process of formation and internal migration led Io, Europa and Ganymede to settle on an orbital resonance. Every four times that Io circulates Jupiter, Europa circulates two and Ganymede circulates one.
The model also suggests that the sun's radiation eventually expelled the remaining gas in the disk around Jupiter, leaving behind the residual satellites that formed the fourth and final main moon, Callisto. However, without gas to conduct the long-range migration, Callisto did not join the other moons and was trapped, rotating around Jupiter every two weeks.
There is still much to discover about Jupiter's moons. NASA's Europa Clipper mission, which will launch in 2024, will visit Europe with the aim of finding out whether or not it has favorable conditions for life.
The European Space Agency also plans to send a mission, called JUpiter ICy moons Explorer (JUICE), which will study Ganymede, the largest of the Jovian moons.