Thought Experiment: Habitable Moon Around a Gas Giant

This year I hope to write a few thought experiments, and from these come up with plausible science fiction stories. I’ll start with one I’ve been thinking about for awhile: habitable moons.

First off, the orbit of the moon is absolutely critical. It cannot be too close to the gas giant, because of the gravity stresses from the gas giant will the moon to become more of an ellipsoid, the internal stresses from the planet’s gravity causing heavily volcanic regions.  The moon, Io, that orbits close to Jupiter is an excellent example of this. Because of its close proximity, the effects of gravity has squashed it into an ellipsoid, and the internal stresses from gravity has caused over four hundred volcanic regions to form across its surface. This renders it inhabitable. Another important point to consider is that the orbital period for the moon to remain in a stable orbit needs to be 40 to 65 days or less, according to simulations by scientists, if the gas giant or massive planet is within 1 astronomical unit from the host star. (1 AU equals the distance from our sun to our Earth.) Also important is that the moon needs to be well outside the Roche limit; the Roche limit is how close an object can be before tidal forces from the planet or star tears the object apart.

Another problem one may encounter is the albedo of the gas giant may reflect even more sunlight upon the moon. In addition to the planet’s thermal heating, which all planet’s release some heat to space over time, these two effects plus the effects from gravity provide additional thermal heating to the moon.  This may cause a runaway greenhouse effect, which will also make the moon relatively inhabitable.

What else can go wrong? Space radiation. Three sources of space radiation bombard the moon, and any planetary body without an adequate magnetosphere: stellar wind, cosmic radiation, and in the case of moons, particles trapped within the host planet’s magnetosphere.  If a moon (or a planet for that matter) is to be habitable, it needs protection from the massive amounts of space radiation that may barrage it. Over time, space radiation can strip away the atmosphere. This may be part of the reason why Mars has such a tiny atmosphere. It’s magnetic field is relatively weak, and thus it cannot provide enough protection against the solar radiation that barrages it every minute of every day of Mars’ existence.

Although moons may have their own intrinsic magnetosphere, it is unlikely to be strong enough to protect itself adequately from the intense barrage of the three sources of possible space radiation, particularly if the moon’s orbit puts it on the outer edge of the host planet’s magnetosphere. This means the third source — particles trapped within the host planet’s magnetosphere — may wreak havoc with the moon’s atmosphere and its ability to retain it over time. The moon needs to be close enough for the host planet’s magnetic field to actually protect it adequately, but it also needs an orbit far enough away from the planet to avoid the other negative effects I’ve already discussed. It’s very much a delicate balancing act to find that perfect habitable zone for moons around suitable gas giants. The good news is that for large gas giants, like Jupiter, the magnetosphere of the planet can extend up to fifty times the size of the planet itself. This provides some room for moons to orbit at a safe distance for habitability.

However, now we have a different issue; magnetospheres are not a constant size over the course of the planet’s existence. As the pressure of the stellar wind decreases over time, which it can as the star grows older, the magnetosphere will increase in size. In the following study, Jorge Zuluaga and Rene Heller determined that for a Jupiter sized gas giant, it would take 4.3 million years for the moon to be safely embedded within the planet’s magnetosphere. It would take even longer for a Saturn sized gas giant with the time span increasing the smaller in size the host planet is.

What the above study shows us is that it is indeed theoretically possible for a moon to be adequately shielded by the host planet’s magnetosphere. It may take some time for the magnetosphere to increase enough to enshroud the moon, depending on the mass of the host planet, but it is theoretically possible.

So now that we know a habitable moon can exist.  What would it be like? First off, the moon would be tidal locked, as in one side would face the host planet. The host planet would eclipse the host star on a regular basis.  Because of this, days could last for half of an orbital period, where the orbital period can range from 10 to 60 days depending on the above discussed factors.  In order for the moon to have seasons, it would need some tilt to its axis of rotation, but that’s wouldn’t be the only effect to its seasons: how elliptical the host planet’s orbit is around the host star can also influence the seasons.  Estimates from other studies, show that a tidal locked moon may end up with a fairly moderate climate, and have a fairly stable axial tilt due to its being tidally locked. Although the dark side may be somewhat colder than the side facing the planet, if there is suitable carbon dioxide amounts in the atmosphere, then this may cause enough of a warming effect for liquid water to exist on the dark side as well.

Another important aspect to the moon would be plate tectonics, which may be caused by heating within the moon itself — left over form its formation days — and tidal effects from the host planet. Again, there’s a delicate balance between too much tidal effects, which would cause the surface to be heavily chaotic and volcanic like Jupiter’s Io, and if the moon is too far away, and the effects aren’t enough to sustain geologically activity.

The moon’s atmosphere also has to be dense enough to support life, and it needs protection from space radiation in order to retain its atmosphere. geological activity within the moon can aid in replenishing the atmosphere as well. Also, the moon itself needs to be dense enough to hold onto important atmospheric gases like nitrogen, oxygen, and water vapor, so the moon’s own gravity needs to keep the escape velocity for gases high enough to avoid losing important gases necessary for life. In the above articles, scientists have determined that if a moon has the density of Mars, it needs to have at least 7% of Earth’s mass in order to retain an atmosphere for several billion years, necessary for life to evolve on the moon.

All these factors need to be examined and assessed when a writer is world-building a habitable moon around a gas giant. It takes a little bit of research and a small amount of math, and presto! You can create an possible habitable moon around a fairly large planet.

What is even more interesting is because of the above features, some interesting societies may develop. For instance, would the biorhythms of the alien life on this moon be linked to the orbital period of the moon? Similar to how there is some link with a lunar cycle on earth to biorhythms of some animals? What significance will the planet play in the developing societies myths and religions? What significance will the dark side and the light side of the moon play in such a society? Especially since the tidally locked side will experience more eclipses with the planet, which the dark side will rarely if ever see the planet at all. This may cause some interesting myths, religions, and practices to develop amongst a sentient society. There’s a lot to examine here, and I think, in time, I may come back to this idea through short stories or novellas. In the meantime, I hope the above helps other writers develop tools to help them build more diverse and interesting worlds in their fiction. The science may seem daunting, but if you break it into smaller chunks, you can tackle each problem one at a time, and build up your world from there. It’ll not only make your world fairly accurate science-wise, but may lead you to discover interesting effects that can influence the development of your societies, providing more inspiration to further stories.