Currently, the Giant-Impact hypothesis is the most convincing argument for how our moon was formed. According to it, a proto-planet – a planet in its embryonic stages – called Theia and Earth participated in a colossal collision, causing the maimed part of Earth to melt and a chunk of it to be flung into space. The splintered debris gradually cooled off, coalesced into a stable solid, and became every werewolf’s muse.
However, the hypothesis is only feasible when the participants are planetary or proto-planetary bodies with a solid core. Only the first four planets exhibit such a core and are aptly known as terrestrial planets. The other four are known as gas giants, as they are literally massive balls of accreted gas.
Saturn’s density is so paltry that it would float in a tub of water, if there was a tub so large. Strangely, the number of moons orbiting these gas giants is more than a hundred, and all of them are made up of solid, cratered rocks. Why aren’t the moons of these gas giants also gas moons?
The formation of gas giants
While the Solar System’s oldest dust and gas attracted more dust and gas to form terrestrial planets, the gas giants, owing to their distance, had a head start. The gas proto-giants started out as ice, which aided in them accreting mass with more ease. Eventually, their gravitational pull grew so strong that they were able to clutch even the lightest fumes of hydrogen and helium dispersed in the solar flares.
Furthermore, these clouds of particles weren’t simply strolling by like soap bubbles, they were whizzing by swiftly, such that they did not seamlessly fall inward and settle down. Instead, by the virtue of their kinetic energy, they briskly rammed into the planet, thereby raising the planet’s overall angular momentum. Subsequently, as the mass progressively increased, the planet, like an ice skater bending her hands inwards, rotated faster and faster, obeying the law of conservation of angular momentum.
Jupiter spins so fast that a single day on the planet only lasts for about 10 hours. The friction between its gases caused by such immense rotational velocity, coupled with its gravitational compression, generates so much pressure and heat that the gases are unable to cool. Like the rotation of a tightly-strung bucket filled with water, if the gas giants weren’t large enough or spinning at such an exorbitant velocity, they wouldn’t be tenacious enough to sustain the bolus of gas. This is exactly why gas giants don’t have gas moons.
Gravity, of course
Other than undergoing a devastating collision, there is another, much easier way for a lonely planet to befriend a rock. Like Mars, a planet can cajole another extraterrestrial body by its pull of attraction. The moons of Mars are probably asteroids that it gravitationally lured from the asteroid belt between it and Jupiter while they were passing by. At least 33 of Jupiter’s moons spin in the opposite direction from the planet, which implies that they are also asteroids it might have captured.
The formation of the rest of Jupiter’s moons mimics the formation of planets around the Sun. The variation in the size and composition of these moons roughly emulates the variation in size and composition of the planets. The moons are believed to be formed by the accretion of gases that constituted a circumplanetary disk, a ring of debris similar to the debris that concentrated to form the planets. This was the mist of gases that floated around Jupiter following its formation.
However, the moons did not grow massive enough to hold onto those gases. The gases which accreted to form the moons cooled off and the remnants coalesced into a hard solid. Moons of a planet are so small that they cannot even sustain a thin peel of atmosphere, an entire body of gas is out of the question. Every planetary body in the universe is subject to such an escape of gases, but the rate of this escape varies inversely with size. However, there is a threshold beyond which planets are massive enough to maintain an atmosphere.
How small bodies cool off in space is evident from the thick crust of ice on Jupiter’s moon Europa. Because Jupiter’s moons are predicted to have formed simultaneously during the planet’s formation, the ice is probably formed by the same ice that constituted its core. However, astronomers predict that beneath Europa’s ice is an ocean of seawater twice the amount as what is found on Earth. The water is a result of its gravitational exercising with Jupiter. Even if a layer of gas were to form around Europa, Jupiter’s inescapable gravity would simply strip it away.