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View Full Version : It's not a planet, it's a moon - worldbuilding questions.



Radar
2010-05-17, 06:01 AM
First of, i have a certain idea for a world and i seek all possible consequences (scientific and cultural alike) of initial choices, i will lay down below.

Imagine a star more or less similar to our Sun and a gas giant orbiting said star possibly in the same distance as Earth does the Sun. Now the gas giant has moons, one of which is earth-like in terms of mass, size and life on it. This moon is the world, i have in mind. Things I figured out on my own (enumerated are physical phenomena and pointed are their consequences):

1. Higher tectonic activity due to tidal forces (see Io for an extreme example) - dependant on mass of the giant and size of moons orbit.
- no high buildings and well made roads
- fewer places stable enough to build lasting settlements (cities)
- civilisation will not spread beyond patches of stable land (corrolary to the above)

2. If the moon orbit is anywhere near the ecliptic, then sun eclipse is a regular phenomenon (ones a revolution) lasting even a few days straight (depending on the revolution time). Even if not, it will come up once or twice a year anyway.
- low-light vision being more common?

3. ?

So, if anyone has an idea or sees a major problem with this setting draft, i will be glad to know. :smallsmile:

Greenish
2010-05-17, 06:18 AM
They might not even consider themselves the center of the universe (maybe they've even figured out who orbits who).

They're likely to have a host of explanations for the planet (home of gods/devils, etc).

Shorter daylight period (cut by the shadow of the plane) means colder climate, unless closer to sun, in which case the difference in temperature between day and night would be greater.

Different cycle. Day, night and sun eclipse would necessitate a different calendar.

hamishspence
2010-05-17, 06:18 AM
If the moon orbited further from the planet (more like Titan than Io)- it might solve some of the likely problems with life.

Less tectonics, radiation, and possibly able to not be tidally locked, and to have its own magnetic field.

Kingreaper
2010-05-17, 06:22 AM
How many other moons does this gas giant have?

If it doesn't have many, then low-light vision won't be that hugely useful, during the eclipses there won't be much light, while during the non-eclipse nights there'll be plenty (a gas giant behind you is going to reflect a lot of sunlight back at you)

It's possible that the more common distinction will be between eclipse and clearview, rather than day and night. Meaning; no timezones, the whole moon has the same values of time.

The moon will probably have a thick atmosphere for it's mass, which means flying should be slightly easier.

Altair_the_Vexed
2010-05-17, 06:26 AM
As I understand it - my geologist friend told me - Earth is alone of the rocky planets to have tectonic activity.
Io is very close to Jupiter. The tidal forces are enormous.
The moons just a little further out have much less tectonic activity, if any.

But if you want high levels of volcanic and tectonic action to be part of your game, then go for it.

What Io also experiences is a massive electrical flux. It moves through the magnetosphere of Jupiter, causing electrical discharge along a corridor between the two bodies. (http://en.wikipedia.org/wiki/Io_(moon)#Interaction_with_Jupiter.27s_magnetosphe re)
You might want to include the resultant electrical phenomena in your world, too. It would certainly make an interesting night sky.

Maybe electricity might be harnessed earlier by inhabitants of such a world?

Avilan the Grey
2010-05-17, 06:31 AM
They might not even consider themselves the center of the universe (maybe they've even figured out who orbits who).

They're likely to have a host of explanations for the planet (home of gods/devils, etc).

One explanation would be Gas Giant = Father, The actual moon they are on = Mother, and the Sun = the main deity of their pantheon.

Another one would be that the entire moon and them themselves are the Children, and they are the result of the joining of the Father (gas giant or sun) and the Mother (gas giant or sun).

Greenish
2010-05-17, 06:52 AM
I wonder if they could figure out which one is larger, their moon or the planet. Probably, if they use D&D: just teleport on the planet and have a looksee.

Kaun
2010-05-17, 07:03 AM
One explanation would be Gas Giant = Father, The actual moon they are on = Mother, and the Sun = the main deity of their pantheon.

Another one would be that the entire moon and them themselves are the Children, and they are the result of the joining of the Father (gas giant or sun) and the Mother (gas giant or sun).

It depends on how much and how often they are getting sun light but they are more likely to be a largly nocturnal world similar to a sort of surface underdark.

Eldan
2010-05-17, 07:39 AM
Hmm. Suggestion: give it synchronous rotation, similar to our moon, i.e. one side is always facing the giant.

Why?

Imagine orbits like this. (S: sun, 1:side one, 2: side two G: giant)

S-------- 12--GGG
Result: side one is bright, side 2 is dark to twilight, depending on reflection from the giant.

Then, half a, well, month later:
S-------------GGG--21

Side one is dark, side two is dark as well, with a dark giant taking up most of the sky obscuring the stars.


So you have two sides of the planet with different light systems.
Side one has half a month of light, half a month of dark, but the darkness has stars. In between, there is twilight.

Side two has half a month of giant-induced twilight, and one month of absolute darkness, with not even many stars in the sky. Only in the twilight days would you have any sunlight at all.

This means that you have a natural zone for darkness and cold loving critters, and gives you a natural starting point for a mythology.

hamishspence
2010-05-17, 07:48 AM
Most moons orbiting gas giants have synchronous rotation- so this would be plausible.

My guess is, to have even vaguely Earthlike conditions, the moon would need to be some way from the planet- more Callisto/Titan distance, than Io/Mimas.

Otherwise, too much radiation from the planet.

Main problem with synchronous rotation- may mean that the moon doesn't generate much of a magnetic field. Though maybe the magnetosphere of the planet will help keep off some of the cosmic radiation.

A Callisto/Titan type distance, combined with a moon size big enough to hold onto a reasonably dense oxygen/nitrogen atmosphere- might make for a moon that could support complex life.

It would also avoid it having a too-long day/night cycle. Just over a week of daylight followed by a week of night.

Note that at this distance, while the gas giant might eclipse the sun during the "day" for the planet-facing side for a while, it would be for a short time only.

Eldan
2010-05-17, 07:51 AM
Or it could be a water planet, like Europa is supposed to be, but not frozen... could be possible if it's closer to the sun, no?

Drakevarg
2010-05-17, 07:53 AM
I wonder if they could figure out which one is larger, their moon or the planet. Probably, if they use D&D: just teleport on the planet and have a looksee.

Teleporting onto a gas giant. Sounds like a family recipe for dead wizard.

hamishspence
2010-05-17, 07:54 AM
Could work. A Waterworld, with a sophisticated undersea civilization, might make for a change of pace. Maybe with small islands, and polar ice caps.

Yuki Akuma
2010-05-17, 07:55 AM
If Jupiter were in Earth's orbit, Europa would probably not be entirely frozen, no. Especially if it somehow had a thicker atmosphere.

Eldan
2010-05-17, 07:55 AM
Well, there's the Plate armour of the Deep (or some similar name) which protects from pressure. Or the Necklace of Adaption. If you include some gravity reducer (if there's reverse gravity, there could also be a Lessen Gravity), then a gas giant might be survivable.
Awesome place for a showdown, by the way. Floating stronghold in the upper atmosphere of a gas giant.

Greenish
2010-05-17, 08:00 AM
Teleporting onto a gas giant. Sounds like a family recipe for dead wizard.Meh, research an arcane version of Adapt Body (http://www.d20srd.org/srd/psionic/powers/adaptBody.htm) (unless one already exists) or just go Ethereal first.

Drakevarg
2010-05-17, 08:02 AM
Well, there's the Plate armour of the Deep (or some similar name) which protects from pressure. Or the Necklace of Adaption. If you include some gravity reducer (if there's reverse gravity, there could also be a Lessen Gravity), then a gas giant might be survivable.
Awesome place for a showdown, by the way. Floating stronghold in the upper atmosphere of a gas giant.

Which is of course assuming anyone had the slightest idea what a gas giant was (which without a god outright telling them, they probably wouldn't), as opposed to "big round thing in the sky that wizards never come back from."

Plus, there's the whole "planet-sized tornados" thing to worry about. Or the unbreathable atmosphere. Or the utter lack of a ground. Sure, magic could circumvent alot of these problems. The trick is suviving long enough to know that the problems exist at all.

hamishspence
2010-05-17, 08:05 AM
Maybe this civilization develops spelljammer-like vessels? And explores the immediate area, travelling to their gas giant (possibly one with a high-oxygen atmosphere) to set up a mining base?

Eldan
2010-05-17, 08:19 AM
Well, after a few mages die spectacularly after teleporting, they could invent various other ways of experimenting. Teleporting golems, maybe. Scrying. Teleporting slaves and testing the effects of various adaption devices.

hamishspence
2010-05-17, 08:22 AM
Apparently gas giants, even ones with large amounts of water vapor cloud, still have atmospheres of mostly hydrogen or methane.

So, some form of air-generation would be necessary- maybe mini-portals to the Plane of Air, on board flying ships.

Eldan
2010-05-17, 08:24 AM
That depends on the "tech" level, of course. A system with several inhabited moons would make a very cool setting for a kind of mini-Spelljammer.

Drakevarg
2010-05-17, 08:24 AM
So after sinking probably thousands of gold and dozens of lives into figuring out how to survive there, we've successfully turned the gas giant from a hellish deathworld to a hellish (but survivable) world utterly devoid of anything useful.

Hope it was worth it.

Eldan
2010-05-17, 08:26 AM
Curiosity. We sent men to the moon. They sent humanoids to the planet.

kestrel404
2010-05-17, 08:28 AM
As I understand it - my geologist friend told me - Earth is alone of the rocky planets to have tectonic activity.
Io is very close to Jupiter. The tidal forces are enormous.
The moons just a little further out have much less tectonic activity, if any.

Planets, yes. That's because no other solid planet has a moon of such huge proportion in comparison to the size of the planet. We have a molten core because we have a huge moon - otherwise, the core would have solidified a long time ago.

But for moons, it's different. There, you have two options:
First, you have tidally locked moons - that means the moon is so close to the planet, that tidal forces prevent it from rotating independently of the planet. This means that one side of that moon always faces that planet. Because of that, you don't have a molten core, so no tectonics, earthquakes, volcanoes, or other geographical oddities. This also means that, if the planet has any weather at all, it will be an ocean world - or at least a giant mudball, with the conscious inhabitants constantly trying to build livable land up above the continuously encroaching coastlines. This is to say nothing of the immensely long day/night cycle. A very alien world to live or game in.

Alternately, you move the planet a bit further out to mitigate the tidal forces, you can get a world with an independent rotation, giving you day/night cycles roughly equivalent to what we're used to. Now, you most likely will have earth-like tectonics, as well as high and low tide. However, the tidal height will be A LOT more than what you get on earth. Instead of a dozen feat at the extreme, you could have between dozens of feet and a half-mile of difference in ocean height between night and day (depending on how close you are to the gas giant).

You can partially mitigate this by having the moon farther away from the gas giant. However, the farther you move out, the more eccentric the moon's orbit becomes (meaning that the distance (and therefore gravitational pull) will vary greatly based on the time of the 'month'. Get an eccentric enough orbit and you'll find yourself with monthly 'tidal waves' eroding your land quicker than it can be built up by geological action, as well as 'volcano week', where geysers of molten rock become a regular, monthly activity for the whole family to enjoy.

Unfortunately, if you're going for realism, every moon capable of sustaining atmosphere in orbit around a gas giant in the solar system is tidally locked. Larger moons that are farther out have a tendency to either crash into other moons (generally slowing them down enough to fall quickly into the gas giant) or else break free and end up in their own orbit around the sun.

But hey, this is your game, do what you want.

Some things I do recommend. Every living creature with more than two cells to rub together gets the 'direction sense' ability for free. There' just this giant thing in the sky that lets everything know exactly which direction is north (or south or whatever). This sense is inborn, native to all creatures, and gets really confused by planar travel (generally leading to headaches, dizziness, and a -2 penalty to all actions for a full week after arrival in a new plane).

Also, sea craft will in general be much larger and more heavily constructed to survive the nastier oceanic forces caused by the constantly shifting strong gravity field of the moon. Between the massive tides and the enormous waves and the even heavier than normal winds, the behemoth ships will need to be able to survive getting stranded for a day on land, being carried away by a rogue tidal wave, or getting smashed between opposing giant waves without needing serious repairs.

Active volcanoes will be nearly as common as mountains, with any spot of geographical weakness offering a place for the planets molten and very active core to vent lava onto the surface. So islands will be plentiful, as will mountain ranges, foothills, and coastlines. Flat plains will be the uncommon thing.

That should account for most of the obvious differences. Of course, the disjointedness of the terrain and the difficulty of travel will lend itself to smaller and more local political systems, with alliances between minor lords and city-states being the norm rather than the exception. Continent-spanning empires will be nearly impossible until air travel is invented. And there will probably be more continents than on earth, with large islands (with a volcano or three in the middle) being the most common type of landmass.

Hope that helps.

Drakevarg
2010-05-17, 08:29 AM
Curiosity. We sent men to the moon. They sent humanoids to the planet.

True enough. Going to the moon was half ****-measuring contest anyway. But at least with the moon, we were fairly certain it was solid. And it probably had less deaths involved.

But either way, go ahead. FOR SCIENCE! (Or magic, as the case may be.)

Greenish
2010-05-17, 08:29 AM
So after sinking probably thousands of gold and dozens of lives into figuring out how to survive there, we've successfully turned the gas giant from a hellish deathworld to a hellish (but survivable) world utterly devoid of anything useful.

Hope it was worth it.Humans on Earth have visited our moon without even having magic.

Competing Universities of Magic would easily have the resources to mount a well prepared expedition.

hamishspence
2010-05-17, 08:29 AM
Maybe to extract hydrogen, or helium, from the atmosphere, to power the ships?

A gas giant might be worth visiting for resources, from the point of view of the moon's inhabitants.

Drakevarg
2010-05-17, 08:31 AM
Humans on Earth have visited our moon without even having magic.

Competing Universities of Magic would easily have the resources to mount a well prepared expedition.

Magic, no. But we had things like "physics", as opposed to the belief that gravity happened because God was pulling things down. Or the belief that the sky was a ceiling.

Of course, in DnD those very well may be valid beliefs.


Maybe to extract hydrogen, or helium, from the atmosphere, to power the ships?

A gas giant might be worth visiting for resources, from the point of view of the moon's inhabitants.

When did we go from "medieval fantasy" to "hydrogen-powered spaceships"?

DanReiv
2010-05-17, 08:33 AM
Funny you ask that, I'm currently in the middle of a moon adventure from a very old ad&d campaign. In this setting the moon is actually an obsidian shell covered with cosmic dust that serves as a antimagic prison for the BBEG cosmic being.

I can translate the part describing it if you want (not much, it's from an old french rpg magazine)

Greenish
2010-05-17, 08:35 AM
Magic, no. But we had things like "physics", as opposed to the belief that gravity happened because God was pulling things down.Who knows? Maybe if you put a lot of very bright people (with divination magic) together, they'll figure out some of the basics on how the world works.

Hell, it's not like real world never had superstitions.

Eldan
2010-05-17, 08:39 AM
That depends on whether or not D&D actually has the same physics. Remember: this is a world where the default assumption is that there actually is an aether, through which the four elemental forces move to create all matter; where dreams actually happen because your mind teleports to a different reality at night; where diseases are caused by the evil eye; where life happens because positive energy creates your soul...

So, yes, there might well be a gravity elemental or something like that sitting at the core of a planet, responsible for pulling thing down with it's magic, based on how much it likes them. (And boy, does it dislike helium and hydrogen.)

hamishspence
2010-05-17, 08:42 AM
And space might in fact be the Quasi-Elemental Plane of Vacuum, full of Vacuum Elementals (from the Tome of Horrors books).

D&D worlds can have some interesting things in place of physics.

Eldan
2010-05-17, 08:47 AM
Yup. See, as an example, the atomic theory of the elements (http://www.mimir.net/essays/planarphysics.html), which explains how four kinds of atoms can form so many quasi- and paraelements.

SlyGuyMcFly
2010-05-17, 09:18 AM
Yup. See, as an example, the atomic theory of the elements (http://www.mimir.net/essays/planarphysics.html), which explains how four kinds of atoms can form so many quasi- and paraelements.


Now that's awesome.

balistafreak
2010-05-17, 09:28 AM
Yup. See, as an example, the atomic theory of the elements (http://www.mimir.net/essays/planarphysics.html), which explains how four kinds of atoms can form so many quasi- and paraelements.

I have to show this to my chemistry teacher.

Cogidubnus
2010-05-17, 09:33 AM
Maybe the gas giant can function as one (or more) of the other planes. It would be easy for deities to fashion homes there just by defying the laws of phsyics, and air (well, hydrogen/methane) elementals would probably fit in quite well too.

Also, if you say that they have no way of discovering the gas giant is dangerous without sinking enormous resources into it, how did the Eberron lot discover that the Planes of Fire, Earth, Water and Positive/Negative energy could be instantly fatal. Cos unless you protected yourself beforehand, you can't breathe to cast spells on those elemental planes, and you might explode/disintegrate on arrival in the energy ones.

Drakevarg
2010-05-17, 09:37 AM
Maybe the gas giant can function as one (or more) of the other planes. It would be easy for deities to fashion homes there just by defying the laws of phsyics, and air (well, hydrogen/methane) elementals would probably fit in quite well too.

Also, if you say that they have no way of discovering the gas giant is dangerous without sinking enormous resources into it, how did the Eberron lot discover that the Planes of Fire, Earth, Water and Positive/Negative energy could be instantly fatal. Cos unless you protected yourself beforehand, you can't breathe to cast spells on those elemental planes, and you might explode/disintegrate on arrival in the energy ones.

Eh, I'd usually just say that they DON'T know. That way when the players plane shift and get themselves killed, I can just tell them they should have given more thought to the idea of traveling to a dimension made out of fire. (Exactly What It Says On The Tin (http://tvtropes.org/pmwiki/pmwiki.php/Main/ExactlyWhatItSaysOnTheTin), after all.)

Volthawk
2010-05-17, 09:49 AM
Magic, no. But we had things like "physics", as opposed to the belief that gravity happened because God was pulling things down. Or the belief that the sky was a ceiling.

Of course, in DnD those very well may be valid beliefs.


Actually, considering they would have Wizards with superhuman intelligence, maybe they could figure it out.

Cogidubnus
2010-05-17, 10:00 AM
I THINK he meant that in DnD these beliefs might actually be right?

Radar
2010-05-17, 07:03 PM
Sorry for the late reply, I was away for a long time. I'm so glad, that RPG gather creative minds. :smallsmile:

@Kingreaper: I haven't thought about other moons yet. I think i'll introduce some more to colour up the sky a bit.

@Altair_the_Vexed: yes, I am aiming for higher then ours yet livable tectonic activity. I would totally miss beautiful auroras! This would make eclipse not as dark as it would otherwise be. I like the effect a lot too. I think, i'll make moon's orbit highly inclined to gas giants orbit. For one think, I'll still have a more or less normal day/night cycle (with different illumination due to gas giant and moon's position). Second thing is, that then a sun eclipse and strong aurora will come together as a very significant event once or twice a year. :smallbiggrin:

@Eldan: I'll have to think the idea of tidally locked moon through much deeper. The bright side might experiance extreme temperature differences because of a long day/night cycle. Thicker atmosphere might mitigate it, but it would in turn darken the whole world and probably make the sky permanently clouded.
As for the water planet, it might cut all the other aspects.

@hamishspence: for the initial time period I was thinking about early Renaissance, where people just start to actively explore and reserch their world. A later time period would definatly include colonies on other moons and some space vessels.

@kestrel404: hmm... I'll have to wave with my hands, to make the orbit stable, but I think, I can live with that. Direction sense seems good - goes in my notebook. The funny thing is, that I got the whole idea, while thinking of a justification for a low population density with small, disconnected city-states. :smallsmile:

@DanReiv: talk about coincidences! Still, I will rather make my moon livable, so not quite the same. Thanks for the offer though.

As for replacing our physics with Handwavium or Mythologium, I find it easier to go as far as possible with physics (but not further), because it's easier to keep it consistant.

As for the mythology itself, I haven't thought about it much yet. I think I'll do the physical stuff first, to be sure, how the sky exactly looks like from the moon etc.

It also occured to me, that higher tectonic activity would make mining significantly more dangerous or almost impossible, so metals would probably be much more precious.


So now with a lot of new ideas to consider I will sit down and tweak all the numbers for this planetary system. Anyone knows an astronomic program in which I could visualise the thing and compute calendar?

Thank you all. I'll try to respond as soon as I have something done. :smallsmile:

Haarkla
2010-05-18, 08:03 AM
Greenish: I wonder if they could figure out which one is larger, their moon or the planet. Probably, if they use D&D: just teleport on the planet and have a looksee.

The ancient Greeks were able to figure out the distance to the Moon and therefore its diameter. (By timing eclipses and occultations IIRC.) They also (correctly) suspected that the Sun was larger than the Earth.


Hamishspence: Note that at this distance, while the gas giant might eclipse the sun during the "day" for the planet-facing side for a while, it would be for a short time only.

+1


Planets, yes. That's because no other solid planet has a moon of such huge proportion in comparison to the size of the planet. We have a molten core because we have a huge moon - otherwise, the core would have solidified a long time ago.

But for moons, it's different. There, you have two options:
First, you have tidally locked moons - that means the moon is so close to the planet, that tidal forces prevent it from rotating independently of the planet. This means that one side of that moon always faces that planet. Because of that, you don't have a molten core, so no tectonics, earthquakes, volcanoes, or other geographical oddities. (??? Whether a moon has tidally-locked rotation has almost nothing to do with whether its core is molten or not, indeed a moon closer to its primary would be more likely to retain a molten core do to tidal heating -for example Io. ) This also means that, if the planet has any weather at all, it will be an ocean world - or at least a giant mudball, with the conscious inhabitants constantly trying to build livable land up above the continuously encroaching coastlines. This is to say nothing of the immensely long day/night cycle. (Wrong again - the orbital period could be as short as a few days, not immensely long) A very alien world to live or game in.

Alternately, you move the planet a bit further out to mitigate the tidal forces, you can get a world with an independent rotation, giving you day/night cycles roughly equivalent to what we're used to. Now, you most likely will have earth-like tectonics, as well as high and low tide. However, the tidal height will be A LOT more than what you get on earth. Instead of a dozen feat at the extreme, you could have between dozens of feet and a half-mile of difference in ocean height between night and day (depending on how close you are to the gas giant). (half mile high tides would pretty much guarantee synchronous rotation)

You can partially mitigate this by having the moon farther away from the gas giant. However, the farther you move out, the more eccentric the moon's orbit becomes (not necessarily, and a large moon is likely to have a less eccentric orbit than a small one) (meaning that the distance (and therefore gravitational pull) will vary greatly based on the time of the 'month'. Get an eccentric enough (-as in far more eccentric than that of any known moon) orbit and you'll find yourself with monthly 'tidal waves' eroding your land quicker than it can be built up by geological action, as well as 'volcano week', where geysers of molten rock become a regular, monthly activity for the whole family to enjoy.

Unfortunately, if you're going for realism, every moon capable of sustaining atmosphere in orbit around a gas giant in the solar system is tidally locked. Larger moons that are farther out have a tendency to either crash into other moons (generally slowing them down enough to fall quickly into the gas giant) or else break free and end up in their own orbit around the sun.

But hey, this is your game, do what you want.

Some things I do recommend. Every living creature with more than two cells to rub together gets the 'direction sense' ability for free. There' just this giant thing in the sky that lets everything know exactly which direction is north (or south or whatever). (Except over half the planet) This sense is inborn, native to all creatures, and gets really confused by planar travel (generally leading to headaches, dizziness, and a -2 penalty to all actions for a full week after arrival in a new plane).

Also, sea craft will in general be much larger and more heavily constructed to survive the nastier oceanic forces caused by the constantly shifting strong gravity field of the moon. (I suspect not - tides are not a particularly great danger to ships) Between the massive tides and the enormous waves and the even heavier than normal winds, the behemoth ships will need to be able to survive getting stranded for a day on land, being carried away by a rogue tidal wave, or getting smashed between opposing giant waves without needing serious repairs.

Active volcanoes will be nearly as common as mountains, with any spot of geographical weakness offering a place for the planets molten and very active core to vent lava onto the surface. So islands will be plentiful, as will mountain ranges, foothills, and coastlines. Flat plains will be the uncommon thing. (Only if the distance between the gas giant and moon is very small)

That should account for most of the obvious differences. Of course, the disjointedness of the terrain and the difficulty of travel will lend itself to smaller and more local political systems, with alliances between minor lords and city-states being the norm rather than the exception. Continent-spanning empires will be nearly impossible until air travel is invented. And there will probably be more continents than on earth, with large islands (with a volcano or three in the middle) being the most common type of landmass.

Hope that helps.
Absolute rubbish.

hamishspence
2010-05-18, 08:16 AM
Periods of only a few days would have the problem of putting the moon in the radiation-heavy zone of the planet. And if it's not rotating fairly fast, it won't have much of a magnetic field, to keep the high-energy particles out.

If a moon is tidally locked, an intermediate distance from the planet (long enough to have a period of a week or more) and alone, its molten core might not rotate fast enough to generate a protective magnetic field.

Periods of more than a month or so, might put it outside the planet's magnetosphere- I'm not sure if this would be a problem though. Maybe such a moon would be able to not be tidally locked, thus it could rotate more rapidly, and generate its own magnetic field to keep the cosmic radiation out.

The high geological activity of Io is primarily due to tidal flexing- which comes from it's eccentric (slightly) orbit, and the other two moons (Europa and Ganymede) keeping it in that eccentric orbit.

A lone giant moon, might not experience such severe activity.

My guess is, a moon orbiting a gas giant which is itself in the habitable zone of the star, would need to be a bit further out than Io, to be safe from the radiation of its own planet.

Callisto and Titan, might be good examples of moons that orbit at a "safe distance".

Might be interesting to cover the different distances, and the problems involved.

A "super Io" might have radiation issues.
A "super Titan/Callisto" might be just right.

I wonder what conditions would be like on a giant moon orbiting a long way out? And if such a moon would be at all likely to form? Maybe large moons have to form fairly close to their giant planets.