Gas Giant Moons

A previous post talked about how Earth’s moon might not have been formed from a circumplanetary disk, but instead at a Lagrangian point in the planetary disk, specifically as another condensation of mass in the band of gas and particles at the orbital radius of the Earth. Then it migrated, impacted, and what remained did not have enough energy to escape the gravitational pull of the Earth, so it remained as a satellite. Having co-rotational velocity means it would impact with a relatively slow velocity, all the better to form a satellite rather than escaping into the distance.

What about other moons, using those in our solar system as examples? If, for example, Galilean moons initially formed in the band of gas at Jupiter's orbital radius, at the Lagrangian points or between two other moons, say Europa between Callisto and Ganymede, and one by one they migrated toward Jupiter, what would happen? How could they switch from being co-orbiting planetesimals to being planetary satellites? Crashing into the upper atmosphere of the planet would certainly tend to do that. What has to happen for something to be captured in orbit is for the velocity to be reduced, as by gas drag, and then the orbit circularized or for smaller objects, the perijove has to be raised so it doesn’t continue to enter the upper atmosphere.

Tides are often blamed for orbit circularization. A planet that rotates faster than the orbital period of a satellite will tend to circularize it, as the most velocity transfer occurs near the perijove. Adding velocity at the lowest part of the orbit will push that part outward, as more velocity means more distance. The semi-major axis of the elliptical orbit would increase, but this would also mean that the orbit would shift toward the perijove, lowering the apojove.

Having a deep atmosphere means that there is plenty of opportunity for tidal interactions. On Earth we are familiar with oceanic tides, but there is also one in the atmosphere. It doesn’t amount to much because Earth’s atmosphere is not massive compared to the planet or the ocean, much less than one percent of the ocean’s mass. But on a gas giant, a huge portion of the mass of the planet is atmosphere, just waiting for some nice satellite to come along and tug on it.

There is nothing preventing bodies from elsewhere in the solar system, not in the planet’s original disk nor in a disk around the planet, from being captured; it is simply a bit chancier. All of the non-Galilean satellites of Jupiter are small, and most of them have orbits which are not circular at all, even not prograde or in the equatorial plane around the planet. A large planet coming from a co-orbital location does not have to wind up in the equatorial plane of the planet, at first, but if the planet is rotating rapidly, it will be an oblate spheroid, and that means that there is more gravitational tugging near the equator, so the orbital plane may slowly tilt down toward the equatorial plane as it circularizes. Small intruding satellites might lose their solar orbital velocity, dropping it down to jovian orbital velocity, if they caught some gas drag in the upper reaches of the atmosphere, or perhaps, less likely, got a gravitational slingshot hit from passing near one of the large moons.

Tidal interactions do not affect small moons very much as the tidal mass that is moved in the atmosphere of Jupiter is proportional to the mass of the moon, and the gravitational attraction proportional to the square of the mass of the moon. With a small moon, there simply hasn’t been enough time to circularize.

With Saturn, there is one large moon, Titan, with 96% of the total satellite mass. Like the four Galilean satellites it is prograde, near Saturn’s equatorial plane and in a circular orbit. Its orbital period is longer than the day of Saturn. This does not necessarily mean it was captured as a co-orbiting planetesimal, but it does not have any characteristics that would indicate it could not be. The next four largest moons might also be.

For Uranus, the five largest moons are all orbiting in the equatorial plane of the planet, tilted extremely as it is. They have prograde, circular orbits, with orbital periods longer than Uranus’ day, meaning tidal pulls would have tended to circularize them. Neptune is the exception: it has one large moon, Triton, but Triton is neither in Neptune’s equatorial plane nor is it prograde. The orbit is nearly circular, but this does not explain the anomalous orbital plane and direction.

As a planetesimal approaches a co-orbiting planet from the rear, i.e. from the direction of the orbit opposite to the planet’s motion, it speeds up and this would tend to move it outward in orbital radius. This means that it would enter a prograde orbit around the planet, if gas drag and tidal forces were able to accomplish that. If the planetesimal approached from the front, meaning from the direction in which the planet was approaching, it would also increase speed and slide outward, but this would lead to a retrograde orbit. Tidal effects will destroy a satellite in a retrograde orbit, after sufficient time, and this may explain why there are few large moons in our solar system that have this. If the satellites that entered retrograde orbits around the other planets did so when there was more gas for drag, they could have disappeared already, leaving only Triton lingering here. This much detail calls for some simulation work, however.

Speculating about the origin and capture of the major satellites of planets in our solar system doesn’t immediately indicate any reason to think that capture of a co-orbiting planetesimal would be rare or difficult, and this indicates that other solar systems around other stars may have had the same phenomena at work. All this discussion points to one conclusion: having Earth-like worlds with large satellites following a mild impact event is not easy to rule out, and therefore it is not easy to rule out that there are alien civilizations arising through the early life origination method all over the galaxy.