Scientists have a new theory on how Jupiter's moons were formed

Science has known that Jupiter has a quartet of moons since the days of Galileo. Those moons include Io, Europa, Ganymede, and Callisto. Each of the four moons is significantly different than the other. Interestingly, all of the satellites are of similar size at about one-fourth of the Earth's radius.

Despite their similar size, each moon is wildly different with Io having significant volcanic activity, Europa is encrusted in ice, Ganymede has a magnetic field, and Callisto is covered in ancient craters. Perhaps the most interesting moon of Jupiter is Europa, considered one of the most likely locations for life outside of Earth in the solar system. Scientists have long wanted to know how the Jovian moons formed, and Caltech researchers have a new theory.

The scientists used analytical calculations and large-scale computer simulations. They believe that during the first few million years of the sun's life, it was surrounded by a protoplanetary disc of gas and dust. Jupiter coalesced from the disk and was encircled by its disk of satellite-building material called a circum-Jovian disc. That disc was fed by the protoplanetary disc that delivered material to the poles of Jupiter and flowed out to the planet's sphere of gravitational influence along its equatorial plane.

The Caltech researchers' new model incorporates the physics of interactions between dust and gas in the circum-Jovian disk. The researchers were able to demonstrate that for icy dust grains of a specific size range and the entrainment force carrying them and the outward flow of gas canceled each other perfectly. That allowed the disk to act like a giant dust-trap enabling the disc to become rich in icy dust grains, each about 1 mm in size. Eventually, the ring of dust became so massive that it collapsed under its weight, and thousands of "satellitesimals," which are icy asteroid-like objects about 100 km across, were formed.

Over thousands of years, those objects coalesced in the moons one at a time. The model predicts that Io was the first to form, and its gravitational influence raising waves in the gaseous disk of material surrounding the planet. The moon migrated towards Jupiter until it reached the inner edge of the disk near its current orbit. The process began again, ultimately leading to the creation of Europa and Ganymede. The three moons are locked in a so-called Laplace resonance, which is one of the best-known features of the moons' orbits. The sun's radiation eventually blew away the remaining gas in the disc. The leftover satellitesimals formed Callisto, but it had no gas to drive the migration towards the planet, leaving it out of resonance with the other moons.