Living on the Moon of a Gas Giant

[SamuraiChicken]

What would it be like if a campaign setting took place on a moon orbiting a gas giant planet?

When I first thought about this question, I didn't think much else other than, 'It would be just like any other setting, though someone on the moon would see the planet in the sky, just like people on Earth we see our moon (though the planet would look bigger).' When I thought about it more, I found that I wanted to explore all aspects of this lunar setting: Is it possible to have a moon that resembles Earth? What would an eclipse look like to someone on the moon? How would the planet's gravity effect the moon?

The following are my thoughts, theories, and ideas on how life would be like on a moon of a gas giant planet. The first few ideas are simply there to make the moon as Earth-like as possible, as well as to confirm that such an Earth-like moon is probable. If you want to get to the good stuff (the part that makes this setting far different than any other), skip ahead to the 'Total Darkness, Total Light' section.

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[note] I'd like to make this moon as close to Earth as possible, so that this format could fit easily into other campaign settings (especially for my next campaign setting, which for a while I have been assuming it would be on an Earth-like planet).[/note]
[spoiler=Size]Size
The size of the moon is very important. The smaller the moon, the less gravity it has. While a setting with a lighter gravity than earth is fine (and fun for jump checks), the gravity has to be strong enough to keep an atmosphere around the moon. To make things simple, let's make the moon's size equal to Earth, since that way the gravity is the same and we don't have to worry about the atmosphere problem.

We must also consider if a planet is capable of having a moon the size of earth. For this issue, let's take a look at Jupiter '" the largest gas giant in our solar system. Jupiter's largest moon (Ganymede) is slightly smaller than Mars, and Mars is about half the size of Earth. Considering that Earth is about 10 times smaller than Jupiter, I don't think that this is a problem. If it does pose as a problem later on, we can easily make the planet a little larger than Jupiter (we need not have this planet be an exact replica of Jupiter, but Jupiter is a good example to use in these calculations).[/spoiler]

[spoiler=Distance]Distance
Distance is important in two regards: How far away is the moon from the planet, and how far away the planet is from the sun.
If the moon is too close to the planet, then the gravitational effects that the planet has on the moon will be very strong (which can be a bad thing). Jupiter again provides a good example of this issue: Jupiter's closest moon (Io) actually bends and stretches from the gravitational forces. This constant bending and reshaping causes it to be very volcanic, and the moon's features change frequently (which is bad for life). Europa (the second moon closest to Jupiter) also feels these strong gravitational forces, but not to the extent of Io. The other two large moons of Jupiter are far enough away to be more stable than Io and Europa, so our campaign setting's moon should be around this distance (no closer than Europa).

How far the planet is from the sun is also a big issue. If we want this setting's moon to resemble Earth, then it has to have liquid water. Jupiter poses a bad example here, since it is too far away for water to exist in a liquid state (Europa is a special case). To me, I see no problem having the planet's distance from the sun be in the habitual zone (about the distance Earth is from the sun). My reasoning? Astronomers have discovered other solar systems in which a gas giant much bigger than Jupiter is orbiting it's star extremely close (closer than Mercury orbits our sun). Since this is possible, I have no problem imagining a gas giant forming in the habitual zone.[/spoiler]
[spoiler=Seasons][note]Contrary to popular belief, seasons are not caused by the distance a planet (or in our case, moon) is from the sun. To further prove this, Earth is the closest to the sun in January, and furthest from it in July. Also remember that when it's winter in the northern hemisphere, it is summer in the southern hemisphere (and the opposite is true as well).[/note]
 Seasons
If we want the moon to be more Earth-like, then we need to give it seasons. Seasons are caused by Earth's tilt, so if we give this setting's moon a similar tilt, everything should be ok.
[/spoiler]
[spoiler=Plate Tectonics]Plate Tectonics
Plate tectonics is important for life on Earth, as it is a great recycling system and temperature regulator. If our moon is the size of Earth, then plate tectonics are indeed possible, though there is an alternative: If the moon is closer to the planet (much like Europa is to Jupiter), then the planet's gravitational forces can cause something similar to plate tectonics as it bends and pulls the moon. I prefer giving the moon plate tectonics because I'm more familiar with how it works than with the other idea, but its good to remember there are always alternatives. (also if the moon is too close to the planet, the planet's gravity would steal the atmosphere from the moon).[/spoiler]

[spoiler=Locked or Unlocked]Locked or Unlocked
This is by far one of the most important issues for this campaign setting: is the moon spinning, or is it tidally locked to the planet? Tidal lock takes place when a moon (or any satellite) is relatively close to a planet, resulting in one side of the moon always facing the planet. Our moon is tidally locked to Earth, which you can confirm by noticing that the moon always has the same side facing Earth (it doesn't look like it is spinning from our perspective). While the answer to this question seems simple enough, it greatly changes what the skies look like from the perspective of those living on the moon.

Spinning: If the setting's moon spins (with no tidal lock), it acts much closer to Earth. The length of the day and night cycles all depend on the rotation speed of the moon, which we can easily match to be the similar to Earth.

Tidal Lock: If the setting's moon is tidally locked to the planet, the first noticeable thing is that the planet looks fairly large, probably taking up a large portion of the sky (probably about a fourth of the sky, but definitely less than half of the sky). This is due to the moon being fairly close to the planet. The second thing is that it has days and nights (since it is still spinning), though the days and nights would last longer. If we use Earth's moon as an example, the day and night would each last around 16 earth days in length. The longer it takes for the moon to orbit the planet, the longer each day and night would last. Last but not least, tidal lock causes the planet to appear as a fixed point in the sky. An observer on the far side of the moon would have night and day, but never see the planet. Those on the near side of the moon would be able to see the planet, and the planet's position in the sky depends on your location on the moon (there would even be a 'sweet spot' where the planet is directly above the observer).

I think I prefer the Spinning option because it would be easier to keep track of time within the campaign setting (days and nights would be just like on Earth). Having very long days and nights can get complicating. While the idea of a 'stationary planet' is indeed interesting, there is something even more fascinating:[/spoiler]

[spoiler=Total Darkness, Total Light]Total Darkness, Total Light
Whether the moon is tidally locked to the planet or not, two very strange phenomenon would occur every month: eclipses. If the moon has an equatorial orbit with the planet, then a solar and lunar eclipse happens ever month. The reason that Earth's moon doesn't have an eclipse every month is because it doesn't have an equatorial orbit. Rather, it orbits at an angle in which we can view it as a Full Moon or New Moon every month, and only occasionally does an eclipse happen. In this campaign setting, the planet is far greater in size than the moon so that even if the moon doesn't have a perfect equatorial orbit, the planet's size makes up for the odd angle.

Lunar Eclipse: A lunar eclipse is when a planet is between the sun and moon, with the planet's shadow being cast over the moon. To the perspective of someone on the moon, it would look like the planet covers the sun, blocking the sunlight reaching the planet (much like a solar eclipse on earth). However, because the planet appears much larger in the sky than the sun does, this eclipse would take longer than an eclipse on Earth. In fact, it would probably last for a few days. During this period no sunlight would fall upon the moon, resulting in a few days of total darkness.

Solar Eclipse: A solar eclipse is when a moon is between the sun and planet, with the moon's shadow being cast on the planet. To the perspective of someone on the moon, they would see the planet rise as the sun sets, looking exactly like a full moon on earth (the planet would be fully lit, and only out at night. The planet sets at dawn as the sun rises). While the moon would technically cast a shadow on the planet, it would only be noticeable if it were very close to the planet (the farther the moon is away, the fainter the shadow).

While this eclipse would seem rather uneventful, it is important to remember what happens when light reflects. Earth's moon reflects 16% of the sun's light to Earth, meaning that the moon can never get brighter than 16% of how bright the sun is. The Earth on the other hand reflects 40% of the suns light (thanks to it's atmosphere). Imagine if the moon had an atmosphere and was nine times larger than it is, and you would have a good idea as to how bright the campaign setting's planet would look at night during a solar eclipse. While it wouldn't be as bright as day, the reflected light would make the moon very well lit during a solar eclipse, somewhat resembling daylight. Just like with a lunar eclipse, this solar eclipse would last a few days.


After looking at eclipses, I like the idea of the moon spinning even better (as opposed to tidal lock). In the tidal lock situation, solar and lunar eclipses would only bee seen by observers on the near side of the moon, while those on the far side of the moon would never see such a phenomenon (day and night would look exactly the same year round, since they are not in view of the planet). With a spinning moon, everyone on the moon would be able to see these eclipses no matter where they are on the moon.[/spoiler]

[spoiler=Tides]Tides
Speaking of tides, it is important to note that tidal forces on the moon would be grater than those on Earth. The difference in high tide and low tide would be more extreme, which is important to note for seafaring civilizations. Port cities would either have to float on only be built upriver where tides are less extreme.[/spoiler]

[spoiler=Culture]Culture
On Earth, every culture has myths, beliefs, and stories about the sun and moon. These two stellar objects are found everywhere in folklore, so a civilization on this setting's moon would incorporate the sun and the gas giant planet into their folklore. With a planet appearing bigger than the sun in the sky, it would be interesting to see what mythologies evolve from such observations. I can imagine astrologers believing they can predict the future by reading the cloud formations on the gas giant, as they watch them slowly change over time.
Eclipses happening every month (days of total darkness and days of total light) would also be imbedded into every culture, and animals would also respond to these cycles as well. Perhaps there are some nocturnal animals that come out during total darkness, and hide underground at the return of the sun.[/spoiler]

[spoiler=Conclusion and Final Notes]Conclusion and Final Notes
In the next campaign setting I create, I plan on basing it on a gas giant's moon rather than having it on just another earth-like planet. While I don't know what kind of campaign setting this will become, I plan on making it more of a fantasy genre than sci-fi. Even if no culture on the moon can travel in space (like a medieval setting with humans, dwarves, and elves), just by changing the world from a planet to a moon makes the gaming experience far different than most standard settings. I think it's a nice little way of reminding players that this setting is on a completely different world (even if they never traverse into the final frontier).[/spoiler]

I hope you have enjoyed reading my ideas. If you have any additional suggestions, comments, constructive criticism, thoughts, or ideas on the subject, please let me know - especially if you see something I have overlooked. I'd like to get this setting as realistic as possible. Even if magic exists in the setting, I don't want to use the 'because its magic' explanation when it comes to the solar system.

Thank you for your attention.

[Llum]

There's one novel set on a moon orbiting a gas giant by Robert J Sawyer. Farseer/Fossil Hunter/Foreigner, however there are no humans, the people are evolved tyrannosaurs.

[O Senhor Leetz]

having this moon orbit like our moon (light side/dark side) would be neat for a setting. especially if you made this moon slightly inhospitable so only a twilit band would be inhabitable. Sun side too hot, dark side too cold.

[Xeviat]

Leetz, I think you might be mistaken about the "light side/dark side" of our moon thing. One side of the moon constantly faces the earth, but all of the moon gets light and darkness. As Samurai Chicken pointed out, the moon has a 16 day "day". That's why we see phases on the moon, since its angle is changing in relation to the sun.

I definitely like the idea of divination by watching the clouds on the gas giant. There would probably also be other moons, which would be watched with the same fervor that we watched the other planets as they meandered across the sky. One thing I have been looking at for my own setting (which is based on a future projection of Earth) is that most cultures have 7 day weeks and most cultures link these days to the 7 "planets" of antiquity: Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn is the order of the days of the week in most European cultures as well as Indian and Japanese cultures. I think you could have the number of days in the week be based on the number of observable moons around the planet.

As for the seasons, do you want the moon's axis to be different from the planets? The moon could have an equatorial orbit around the planet, but the planet could have a tilted axis like Earth. This would give the planet seasons like you want, and also give a slightly skewed angle from the sun, changing the frequency and intensity of eclipses. However long it takes the moon to orbit the planet, I'm pretty sure you're going to be getting a partial eclipse very frequently. These events should also be important calendar events. The year will track seasons, but now you also have a "month-like" orbit around the planet which will be measured from solar to lunar back to solar eclipse.

Another way you can handle the seasons is to have the moon's orbit be tilted in relation to the planet (could have happened from an asteroid impact early on). I think then you could have "summer" be when the moon is on the sun-side of the planet, and "winter" would be when the moon is mostly or partially in the shadow of the planet. The "year" would be the moon's orbit around the planet, and the people would probably not really recognize the solar year (except for astronomers who would recognize the rotation of the stars). You have a lot of options here.

On to tides. Having massive tides seems to mean you are going to have an explosion of amphibious oceanic life. Look at the creatures that live in tidal zones on Earth. Now you have a much larger tidal zone, so I think you're going to have many more tidal creatures. I would definitely advise having an amphibious oceanic race that lives in the tidal zones (if you wanted to have multiple races).

On to tectonics: You need tectonics, otherwise the moon will be dead like Mars. Our tectonics are indicative of our planet's molten interior. Since the core is largely a giant molten iron dynamo, it creates our magneto sphere. Without a magneto sphere, the moon would be irradiated by the sun. You could get around this if the planet has a massive magneto sphere (I believe Saturn's is unbelievably large), and the moon is within it. Just wanted to point out that there is another option to plate tectonics. The size of your moon means the interior can still be molten, and the presence of the planet's tidal forces might even keep the interior of the moon molten for longer (they think that Mars's interior cooled off faster since it was smaller).

I'm eager to see how this plays out. I have been toying with the idea of making a sci-fi setting with a bit of force-like magic, so I've been thinking about stuff like this too (also played too much Mass Effect).

[Ghostman]

Total Darkness, Total Light
Whether the moon is tidally locked to the planet or not, two very strange phenomenon would occur every month: eclipses. If the moon has an equatorial orbit with the planet, then a solar and lunar eclipse happens ever month. The reason that Earth's moon doesn't have an eclipse every month is because it doesn't have an equatorial orbit. Rather, it orbits at an angle in which we can view it as a Full Moon or New Moon every month, and only occasionally does an eclipse happen. In this campaign setting, the planet is far greater in size than the moon so that even if the moon doesn't have a perfect equatorial orbit, the planet's size makes up for the odd angle.

Lunar Eclipse:  A lunar eclipse is when a planet is between the sun and moon, with the planet's shadow being cast over the moon. To the perspective of someone on the moon, it would look like the planet covers the sun, blocking the sunlight reaching the planet (much like a solar eclipse on earth). However, because the planet appears much larger in the sky than the sun does, this eclipse would take longer than an eclipse on Earth. In fact, it would probably last for a few days. During this period no sunlight would fall upon the moon, resulting in a few days of total darkness.

If long lasting, it would not be simply darkness, but a world-wide cooling of temperature. Consider that the average difference between nighttime and daytime temperatures in Marseille, France is about 10 degrees (Celcius), and in Cairo, Egypt it can get close to 15 degrees in May/June, and that's with a 24-hour night/day cycle.

[Hibou]

If you need any help with the science, send me a message. Considering the drastically different setting you have, you may want to work out quite a lot of the planetary geometry when it comes to eclipses, day/night cycles, etc.

[Xeviat]

Are you an astronomical physicist FAH? I could use your help with some lunar orbit questions.

[Stargate525]

Even if the moon doesn't have en equatorial orbit, it'll still probably have an eclipse every orbit. Gas giants are HUGE (Jupiter's a small one, and the sun and it still orbit EACH OTHER), so your orbit would have to be pretty eccentric to avoid this.

[O Senhor Leetz]

this is somewhat off-topic, but "Living on the Moon of a Gas Giant" could be the name for the next Flaming Lips album.

[beejazz]

This is pretty interesting. Last scifi setting I wrote had moons orbiting a gas giant as the setting, but I can't say I thought about the physics of it in as much detail as you have. I may have to borrow some ideas.

[Ghostman]

For some extremely awesome scenery, you could make the gas giant be ringed like Saturn. I'm not sure if this would cause a major asteroid hazard though.

[Hibou]

Are you an astronomical physicist FAH? I could use your help with some lunar orbit questions.

Spent the last year in astronomy and physics courses. I can help you work out the basics, just send me the details over PM or contact me through AIM (nick is Mezerous).

[Stargate525]

For some extremely awesome scenery, you could make the gas giant be ringed like Saturn. I'm not sure if this would cause a major asteroid hazard though.

Depends on where the rings are in relation to your orbital plane.

[SamuraiChicken]

@Llum: Sounds interesting. I'll keep an eye out for it.

@Xeviat: Thanks for reminding me about other moons orbiting the planet. I'll be sure to add them into the folklore. I think the number of 'significant' moons (ones that formed with the planet) should be around 4 or 5, with the rest being captured satellites (and there will be plenty of those).

As for seasons and months, like the idea of having 1 year (in this system) equal to an orbit around the sun (just like on Earth), and 1 month is equal to an orbit around the planet (again, similar to our lunar calendar on Earth). I really want to go with the idea of two eclipses every month, as it would be a great way to keep track of time. I can imagine PCs in this setting keeping track of which day of the month it is so they can prepare for the eclipse.

I love your idea on amphibious life forms. I never thought about it when thinking about tides, and it makes perfect sense for this world.

As for plate tectonics, I completely forgot about the magneto sphere. That would be very important for life on the moon, as well as for compasses to be able to work. Currently, I prefer the moon to have its own magneto sphere, but the idea of the moon being within the planet's magneto sphere is equally an interesting idea (and good to keep in mind for an alternative).

@Ghostman: Thanks for reminding me about the cooling effect during the Total Darkness period. I don't know exactly how long an eclipse would last, but as soon as I do I can then figure out how cold it would actually get. I like the idea of making this period reflect a miniature winter, where the temperature decreases and stays down until the sunlight returns. In winter, this period would be especially harsh. Perhaps people go underground during this time (or underwater?) where it would be a little warmer.

@Freeeakin' Awesome Horse: Thanks for the offer! Yes, I think I will need some help on this project, so go check your inbox.

Thanks to everyone who replied to this thread - you guys have been awesome. As a reward, I'm giving anyone who posted on this thread (and whoever will post) the following badge:

[SamuraiChicken]

For some extremely awesome scenery, you could make the gas giant be ringed like Saturn. I'm not sure if this would cause a major asteroid hazard though.

One last Idea on multiple moons around the planet:[/b] Any moons that are farther away from the planet than the campaign setting's moon would retrograde occasionally, just like how Mars retrogrades from Earth's perspective (all other planets further from the sun than Earth is also retrograde, but Mars is the best example of this effect).
For those who don't know, retrograde in astronomy is when a planet is traveling in one direction (the same direction as the sun and moon), but then appears to stop and then move in the reverse direction for a period of time. Eventually, the planet comes to a stop again before it proceeds to go in the right direction again with everything else in the sky. This is purely an optical illusion cause by Earth orbiting the sun at a different speed than the observed planet, but only occurs with planets further from the Sun than Earth is (I've seen some 3d models of this effect, and recommend searching for them if you have no idea what I'm talking about).