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Astrophysicists suggest shaded outer disk regions formed Jupiter's largest moons

A pair of astrophysicists with Aix- Marseille Université, CNRS, and Institut Universitaire de France have developed a new theory about the formation of Jupiter's largest moons. In their paper in The Planetary Science Journal, Antoine Schneeberger and Olivier Mousis describe how they used a variety of factors to simulate the early years of Jupiter and the formation of some of its moons.

Prior research has suggested that the moon orbiting the Earth came about due to a collision with another large celestial body—but that was possible because Earth is rocky. Other planets, such as Jupiter, are gas giants, which means that their moons could not have originated in this way. Consequently, scientists have been finding it difficult to explain how Jupiter's moons came to exist.

Current theory suggests that during its early years, Jupiter was surrounded by a circumplanetary disk (CPD). Jupiter has 95 known moons and many moonlets, which are believed to have been created as the CPD coalesced and broke apart. But how that process came about and evolved is still mostly a mystery. For this new study, the researchers created a simulation to demonstrate how the four largest moons may have come to exist.

The team started with a generic CPD made of gas, dust and ice, and then gave it likely characteristics (such as proportions of ice, dust and gas) based on the makeup of Jupiter's moons. They also added other factors that would have impacted the formation of moons, such as the amount of time it took Jupiter to form, the heat it generated as it did so, and how much of that heat made its way to the CPD—and also the distance from the sun and the other planets.

When they ran their simulation and varied the element concentrations, they found the CPD split into two distinct regions, or rings—inner and outer. The ring closest to the sun, the researchers observed, was opaque, trapping heat, while the outer ring was nearly transparent, allowing heat to escape into space.

They also found that over time, the inner ring expanded, resulting in the formation of shadows on the outer ring. Such shadows, the researchers note, would have reduced the temperature of the parts of the CPD that were shaded. And it was those regions, the simulation showed, that eventually hardened and formed Jupiter's largest moons.