Habitable tidally locked moon

Hello, im new here and found this forum while searching the web for some information that might help me.

Im hoping to start a project to develop a game for handheld devices and wondering if someone here can give me some ideas for a scenario that is plausible enough.

Im thinking of having the game take place on a world (moon) colonized by our race (human ), so it should be earth-like enough, with a few differences to make it more original.

I thought of a moon tidally locked to a gas giant in the system's habitable zone, should have an interesting sky and the "dark" side shouldnt be too hostile since the planet is in the habitable zone. Another option I thought of was make it a planet with a close orbit around a brown dwarf that in turn orbits another star, but dont know if that would be more or less plausible.

I suppose the side facing the planet should be more volcanic, so higher elevation and more rich terrain features, while the other side could be mostly ocean, wouldnt the planet's gravity interference push the water more to the opposite side?

What would the atmosphere be like? no seasons? would a scenario be possible where there's no rainfall, but instead have a variable level of humidity, forming mists near sea level in some time of the day perhaps (day being the two times the sun is visible when its not behind the planet or facing the opposite side)?

And finally for now there's another feature in my mind that I need some advice on its plausability: could the parent planet's gravity have a an effect on the moon's in a way as to have near zero gravity at some altitude, while still inside the moon's atmosphere, with enough pressure, oxygen and heat so as to allow a human to survice with just the help of a breating mask? Imagine a scenario where a zeppelin-like large ship takes off from ground-level with the help of jet engines, and is able to stay airborne almost indefinitly at these altitudes. Would gravity at these altitudes increase when approaching the "dark" side?

Thanks in advance for any comments.

Madeira wrote:

Hello, im new here and found this forum while searching the web for some information that might help me.

Im hoping to start a project to develop a game for handheld devices and wondering if someone here can give me some ideas for a scenario that is plausible enough.

Im thinking of having the game take place on a world (moon) colonized by our race (human ), so it should be earth-like enough, with a few differences to make it more original.

I thought of a moon tidally locked to a gas giant in the system's habitable zone, should have an interesting sky and the "dark" side shouldnt be too hostile since the planet is in the habitable zone. Another option I thought of was make it a planet with a close orbit around a brown dwarf that in turn orbits another star, but dont know if that would be more or less plausible.

I suppose the side facing the planet should be more volcanic, so higher elevation and more rich terrain features, while the other side could be mostly ocean, wouldnt the planet's gravity interference push the water more to the opposite side?

What would the atmosphere be like? no seasons? would a scenario be possible where there's no rainfall, but instead have a variable level of humidity, forming mists near sea level in some time of the day perhaps (day being the two times the sun is visible when its not behind the planet or facing the opposite side)?

And finally for now there's another feature in my mind that I need some advice on its plausability: could the parent planet's gravity have a an effect on the moon's in a way as to have near zero gravity at some altitude, while still inside the moon's atmosphere, with enough pressure, oxygen and heat so as to allow a human to survice with just the help of a breating mask? Imagine a scenario where a zeppelin-like large ship takes off from ground-level with the help of jet engines, and is able to stay airborne almost indefinitly at these altitudes. Would gravity at these altitudes increase when approaching the "dark" side?

Thanks in advance for any comments.

Not sure what you need help on.
Pretty much you can make the world as you see fit.

As for the zero gravity at higher altitude, that is unlikely.
For something of that nature, you need a very little mass world, but then it'll probably not hold an atmosphere.
Let alone hold an atmosphere before the frost line.
Artificial atmosphere is possible, but we are talking about Sci-Fi technology here.

Ok thank you!
I am looking for some opinions and fishing for ideas

So the low gravity idea below the frost line like you say is not a good one, I was just wondering if there's something that could make that happen naturally, like the influence of the parent planet's much higher gravity.

About the atmosphere, do you think a tidally locked moon in the system's habitable zone would have strong winds? The opposite? Or would that depend on different factors?

Madeira wrote:

So the low gravity idea below the frost line like you say is not a good one, I was just wondering if there's something that could make that happen naturally, like the influence of the parent planet's much higher gravity.

The reason the atmosphere would stay on the body is because of the body's gravity. If there were a tidal influence that was strong enough to negate the surface gravity of the body, then the atmosphere would get up and leave.

Madeira wrote:

could the parent planet's gravity have a an effect on the moon's in a way as to have near zero gravity at some altitude, while still inside the moon's atmosphere

These altitudes are what we call the Hill radius. It's the radius from the body that it's gravitational field dominantes the local space. Usually, orbits around the body aren't very stable until within 1/3 of the Hill radius. If a body overflows its Hill radius, as in the case of the hypothetical body you speak of, then it will lose matter rather rapidly.

In summary, we can rule out a zero-g environment within a planetary atmosphere, especially at a pressure remotely approaching anything habitable.

Madeira wrote:

About the atmosphere, do you think a tidally locked moon in the system's habitable zone would have strong winds? The opposite? Or would that depend on different factors?

It's dependent on several factors, a lot of them being the orbit of the moon around the planet, and the planet around the star.

Madeira wrote:

I suppose the side facing the planet should be more volcanic, so higher elevation and more rich terrain features, while the other side could be mostly ocean, wouldnt the planet's gravity interference push the water more to the opposite side?

Not quite. The tides will affect a body as a whole. Consider our oceans, there's a tidal bulge of the ocean on both sides of Earth (the sub-lunar side and the anti-lunar side). Similarly, note that Jupiter's moon Io does not have a hemisphere preference for its volcanism, and Europa does not have a hemisphere preference for the cracks in its crust.

Madeira wrote:

What would the atmosphere be like? no seasons?

One needs only to look at Titan to see that such a body as you describe would indeed have seasons, if the orbit and inclination of the moon is supportive of it. Titan has slow surface winds but more rapid, upper altitude winds. But that's a single case study, and unfortunately, all the Solar system has given us.

Thank you Sirius_Alpha, very usefull information you gave me.

I suppose I'll have to look for other features for my moon.

I dont actually need zero-g environment inside atmosphere, I could go with low-g and low pressure, but at a lower altitude than on Earth (I suppose that might be possible if the world was smaller? dont need it to be Earth or even Mars sized either).

In your opinion what could be the conditions if the parent planet was outside the habitable zone? Could the moon support life if the planet gave enough heat to warm the side facing it? what would happen to the other side? What about the brown dwarf idea instead of a gas giant?

Titan has low surface gravity and fairly high surface pressure, it would appear to someone there to be a very low-g environment. I'm not sure if that would be possible for a habitable-zone Titan-like world, with the extra insolation heating the atmosphere and all.

Madeira wrote:

In your opinion what could be the conditions if the parent planet was outside the habitable zone? Could the moon support life if the planet gave enough heat to warm the side facing it? what would happen to the other side? What about the brown dwarf idea instead of a gas giant?

I wouldn't know the exact conditions, and again, it will depend on several factors, like the orbits of the two bodies. But I think what you describe would be possible. What would happen to the other side? Depends on the atmosphere of the moon and the orbit of its planet.

Ok, one last question concerning gravity, can you think of any factor that might make the moon's surface gravity close to 1g, but loose it more quickly than normal as altitude increases?

One idea is make the moon's terrain be mostly plateaus, and have some very deep and wide valleys, which are the only habitable regions. So im wondering if gravity is low on the surface of the plateaus (0.2 to 0.4 perhaps), could it increase to around 0.8 in the deep valleys (difference in altitude could be perhaps 10 to 20km, or miles)?

Madeira wrote:

can you think of any factor that might make the moon's surface gravity close to 1g, but loose it more quickly than normal as altitude increases?

Remember Newton's law of gravitational attraction:

f(g) = m₁ m₂ / r².

The gravitational attraction between you and another body will decrease at a rate of r^-2. Thus, if you move something twice as far away, the gravitational attraction between it and you will be 1/4th of what it was. Similarly, increase your separation by four times as much, and the gravitational attraction will be 1/16th as much.

Madeira wrote:

So im wondering if gravity is low on the surface of the plateaus (0.2 to 0.4 perhaps), could it increase to around 0.8 in the deep valleys (difference in altitude could be perhaps 10 to 20km, or miles)?

If a 20 km difference in altitude results in doubling the gravitational attraction between you and the planet, then the planet is probably extremely small.
Early in the morning point source math... if a 20 km increase in altitude (with the surface being zero altitude) results in half the gravitational pull, then the radius of the planet is 20√2 = ~28 km.
This is simply not planetary (or even a decent moon), and for the gravities you mention, the "planet" probably has the density of a compact star.

Madeira wrote:

Ok, one last question...

You can ask as many as you want

Ok thats not an option then Lol

thanks for your simple math, its good enough for me
seems I'll have to come up with some Avatar-like sci fi if I want something like what I described to be possible

Quote :

You can ask as many as you want

thats good to know
I might get back to this when I have a new one

Madeira wrote:

...seems I'll have to come up with some Avatar-like sci fi if I want something like what I described to be possible ...

That's the difference between Sci-Fi and Hard Sci-Fi.

Up to you on how much real science you want to add.

Sirius_Alpha wrote:

One needs only to look at Titan to see that such a body as you describe would indeed have seasons, if the orbit and inclination of the moon is supportive of it. Titan has slow surface winds but more rapid, upper altitude winds. But that's a single case study, and unfortunately, all the Solar system has given us.

Ok im droping the low gravity with altitude idea, and have some other questions

Could you give me an idea why this happens in Titan, slow winds on the surface but faster ones higher up?

I would like to keep the idea that my Moon's terrain has either a large number of high mountain chains with huge plateaus, or is mostly a plateau with some deep and wide valleys, where the plateaus are uninhabitable due to lower pressure. So im thinking it could have very slow winds at low altitude, and wonder if there's any interesting factor that might make it have stronger winds at plateau altitude? And when I say strong, I mean dangerous for any incomming or outgoing spacecraft, and near impossible for atmospheric aircraft.

I think it's because the top of the atmosphere is what is exposed to the stellar insolation, but I don't know particularly much about atmospheres.

If so, winds like you're looking for, though, are probably not achievable by a world that would be considered "habitable."

But I'd rather differ to someone who knows more about atmospheric science than I.

Yes, I suppose Titan's lower winds are slow because its too cold at those altitudes. But that is not the case with my Moon, maybe the different altitudes would make no big difference.

What could make a difference is if the habitable lower altitude regions are deep valleys which are protected from the strong winds. But how to make that feature plausible? I thought of some sort of continental rifts, but dont know if 8 mile high rifts are realistic, how's your geology?

8 km rifts are indeed realistic, low-gravity bodies may have them, like Miranda for example.



Generally, the lower the surface gravity of a body, the higher the terrain formations can be. A major difference between Miranda and Earth, aside from mass of course, is Earth has surface processes like plate tectonics and erosion.

If your habitable moon has large amounts of water (oceans, rainfall, etc), then I suspect this deep valley would be underwater. Water tends to accumulate in lower elevations of course.

Ok but Miranda is too small compared to what I want, I need my moon to be slightly similar to Earth, I think a gravity of 0.7 or 0,8g is close enough.

Another idea I had is a geological active moon with one or two large continents in the process of breaking up like it happened with Pangea and other super continents before, but with a much lower percentage of wather perhaps.

Sirius_Alpha wrote:

Not quite. The tides will affect a body as a whole. Consider our oceans, there's a tidal bulge of the ocean on both sides of Earth (the sub-lunar side and the anti-lunar side). Similarly, note that Jupiter's moon Io does not have a hemisphere preference for its volcanism, and Europa does not have a hemisphere preference for the cracks in its crust.

New question: I was thinking of the solar system set up when I remembered what you said above.

So if the tidal stresses affect the whole planet, the moon's hemisphere facing out will not necessarily be much colder than the hemisphere facing the gas giant, right? Unless perhaps if the gas giant is outside the habitable zone, and it radiates enough heat to warm up the side facing it to a confortable level, while the opposite side gets a bit chilly (even with tidal heating, but not chilly enough to freeze the oceans for instances, maybe Antartica chilly). Is this a good assumption?

From what I understand of winds, this might cause some strong winds constantly flowing from the warm hemisphere to the cold one, so it might also make the characteristic I was looking for possible.

Madeira wrote:

From what I understand of winds, this might cause some strong winds constantly flowing from the warm hemisphere to the cold one, so it might also make the characteristic I was looking for possible.

Sure, but consider the kinds of winds you're trying to achieve. Even Venus's winds aren't even deadly to aircraft or spacecraft (recall a hand-full of spacecraft landed on Venus back in the 70s, and some balloons were used in the atmosphere).

Ok, not so strong then
I'll come up with a different scenario to account for the near disaster of the colony ship.

So if you agree with the rest I wrote above, do you think a Gas giant should be good enough? Or might a Brown dwarf be a better idea?

I honestly don't know much about the luminosities of brown dwarfs and super-Jupiters. It will be strongly dependent on age, though, with older planets or brown dwarfs providing less energy than younger ones.

Ok thanks. If someone else has an opinion I'll welcome it when it comes. That decision can wait.

Next I have to think of the geology, the last ideia that crossed my mind is a super continent that goes all the way around the moon, with one ocean above and another below. And possibly use the rifts idea in areas where the continent is beggining to break up, some rifts will be narrower, others might be so large as to no longer being considered rifts, or possibly make them wider near the coast, and get narrower as distance from the coast increases, along with altitude.

Saw this available on Steam for less than 10$, seems interesting, just want to share it with you:

http://universesandbox.com/screenshots/