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This follows another question that brought me some curiousity about the general state of an earth-like moon tidally locked to a gas giant planet.

There's been questions about a world tidally locked to its star and how this affects their climate (the world being divided in an extremely hot side and an extremely cold side) and population distribution, but in this case since we have a moon revolving around a planet which in turn revolves around the sun, I'm making the assumption the difference in sunlight received won't be as radical.

Of course only one side will ever be fully exposed to the star while the other is blocked by the gas giant when it would be facing the light source, but it seems like to the people on that side of the moon it would just look like the planet "rises" from the side opposite to the sun, completely covers the sun towards midday, and then sets as the sun also starts to set? I assume for the moon's other side it would look like a normal day on Earth. This still results on a colder climate on the side that's facing the planet, right?

There's also the problem of all the water "bulging" towards the gas giant. I assume the extremity of this depends on the giant's pull (mass, distance), but there's also going to be rain, right? Can rivers and lakes still form on the opposite side of the moon in this planet I assume has one huge ocean? Can we still have green, lush, humid forests on the side away from the planet? Most I can think of is big basins creating oasis-like biomes... But what about the winds and erosion of the dry side?

In such conditions, is the limit between "cold" and "hot" sides still the best place to populate or could life be viable anywhere in this planet?

This post might be a bit vague as I'm trying to contribute with as much as what I have already thought of as possible, correct assumptions or not, as food for thought, so I'll re-state my question here in the end as being: what are the general climate conditions in such a world and how does it affect its population?

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  • $\begingroup$ Welcome to the site! When you get a chance, be sure to check out the tour and take a look at the help center if you have any concerns. As for this question, asking after the effects on population makes it far too broad (there are countless biomes and survival strategies to consider). I would edit your question to focus specifically on the climate effects of tidal locking. You're free to ask more questions at any time, but each one should be specific enough to answer. $\endgroup$ – Cadence Oct 12 '18 at 21:17
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First, it's worth considering that the angle of the plane of the moon's orbit around the gas giant will have a big impact on what effect that giant will have on the simpler "tidally locked to its star" story.

If the moon orbits the giant along a plane that is perpendicular to the giant's orbit around the star, you still wind up with an eternal dayside/eternal nightside situation, and the weather systems that flow from it.

If, on the other hand, all orbits occupy the same plane (which is what it seems you're imagining), then you dispense with the eternal stuff altogether. I think there would be no seasons; I can't see how a planet could become tidally locked with an axial tilt.

In that case, the giant would be an absolutely gargantuan presence in the sky, fixed above a single point. That point would likely be the middle of an ocean, a result of the combination of a permanent tidal bulge and the leveling effects on the moon of its own gravitational rounding.

Sailing that ocean would be an unimaginably harrowing experience. Each day cycle would be marred by an extremely long total solar eclipse. So, there's less daytime, and you have fewer stars to navigate by during night and day-night. The waters of this ocean would also be horrifically cold.

And always overhead is the giant, perhaps so close that its atmospheric features have visible depth during the scant hours when sunlight shines in from the sides. Sailors could very easily go mad staring up into the unblinking eye of that monster, night after night.

Or, it's so cold that the waters are frozen. Now you've got a merciless ice desert thousands of miles across, an endless dark mirror scoured clean by powerful winds that tear viciously at anyone foolish enough to attempt the crossing. Wind erosion will have a controlling impact on everything in this place.

This permanent cold zone is one of the important factors in the whole moon's weather systems. Even though the rest of the moon will have a normal day/night cycle, there will always be a permanent temperature gradient, which will probably play out in interesting ways.

The single most important factor in the weather systems -- to say nothing of life -- is that water could be pretty scarce everywhere else. Unless the moon has a lot of water, like Earth, it's likely that any water able to flow freely to the giant-side will have done so, where it will stay. Any moisture in the atmosphere that finds its way to the cold side will condense out and become trapped there. And I have a hunch that the temperature gradient will result in strong winds blowing from the warm side to the cool side, which will interact with the water cycle to slowly transfer all water to the dark side.


Finally, I guess humongous bodies like gas giants often (always?) generate massive, powerful radiation belts. I don't recall the distances at which that matters. But it's possible that anyone unfortunate enough to be on this planet will be killed by radiation very quickly. I honestly don't know if something like the Earth's magnetic field would be able to protect the surface of the moon, in part because I don't know how it would react in such proximity to the more-powerful field of the giant.

That said, the rest of this setting is maybe interesting enough that you might be forgiven for handwaving the radiation.

I like to imagine people living in trench villages dug into the solid ice of the frozen desert. Homes aren't built, they're carved out of the ice. Exile is a death-sentence. And there's always the danger -- perhaps unknown -- of digging too deep and tapping into a subsurface ocean of frigid black water.

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One of the biggest impacts to your seasons and overall environment is going to be how quickly the moon orbits the gas gaint. You are right in that a tidally locked moon will only receive sun on the non-planet face if it orbits in plane with the planet. However it will not have conventional "days" like a planet would. The time it takes Jupiter's moons to orbit it ranges from hours to thousands of years, with the vast bulk of them being in the hundreds of earth days. On a moon like that, with it being tidally locked to the planet, the sun facing side will see the sun rise and then it will not set again for many hundreds of days, and once it does set, it will not be back for an equally long time. A pattern like that is going to dictate a lot about how your environment works, and how life evolves to handle it. Maybe the whole moon freezes during the long night, then everything thaws out and rushes to complete a life cycle during the "day". Similar to brine shrimp that can complete their life cycle in days/weeks when it rains in the desert and their pond reforms temporarily. That said your moon is tidally locked so it likely orbits in closer, maybe you have a day/night cycle that is only a few tens of days, or a normal earth day. But until you know how quickly it orbits it is hard to come up with a solid climate model.

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Fortuitously, this is a relatively simple question because something very similar occurs in our own solar system. Saturn's moon Titan is tidally locked and has liquid lakes and seas on its surface. True, because of the temperature and atmospheric composition, they're lakes of ethane and other hydrocarbons rather than water, but fluid dynamics are fluid dynamics, and Titan's lakes show many of the same characteristics as Earth's.

So what does Titan's climate tell us? Well, for a start, it's surprisingly Earthlike, relative to being small and far out in the solar system (meaning extremely low temperatures). By that I mean that it has seasons, weather patterns, and an analog for a water cycle that are effectively similar to those on Earth.

Titan's seasons are extremely long because its orbit around the Sun (the same as Saturn's) is very long, almost 30 Earth years. However, if Saturn and Titan orbited the Sun more closely, the year would be correspondingly shorter. For a tidally locked moon in the system's habitable zone, this shouldn't be an issue.

Titan's weather is an interesting example, because most of the time it's fairly placid, except during the inversions caused by switching from one season to another (remembering that those seasons are up to a decade in length). Whereas on Earth there are about five distinct "cells" of circulating winds at different latitudes, on Titan there's just the one. Partially this is because Titan's lakes aren't as effective as Earth's larger oceans at breaking up atmospheric patterns. The slow pace of the seasons also plays a role.

One factor I had a hard time determining was whether eclipses of the Sun by Saturn played any role. (For what it's worth, the article on Titan's climate discusses how inclination and orbital eccentricity affect it, but not the difference - if any - between near and far sides.) My guess is that for most reasonable orbits, the effect would be pretty minimal. The only hard number I could find was that from Saturn's tiny inner moon Pan, Saturn is absolutely enormous, covering about 11% of the sky. Some back-of-the-envelope math suggests that from Titan, the Sun would be eclipsed by Saturn for about 3% of its transit across the sky; the difference in insolation should be similar. (Slightly less actually, since Saturn does reflect and re-radiate light and heat onto Titan's near side.)

Incidentally, the image you have of the planet rising and setting along with the sun is incorrect. From Titan, Saturn would appear to hang overhead in exactly the same spot (more or less; it would move slightly). The planet would only rise and set if the moon wasn't tidally locked.

Finally, Titan's hydrocarbon cycle. This is perhaps the most Earthlike part of all, aside from being comprised of different elements. It has lakes and seas, rivers, even tides (in this case driven by the in-and-out motion of Titan's orbital eccentricity). Right now, lakes are concentrated in the northern hemisphere, but current thinking is that this is seasonal (it's winter in the north right now) rather than driven by Saturn's gravity.

In summary: Tidal locking to a planet should have little effect on climate (particularly compared to tidal locking to the star, which has a drastic effect). Given the right conditions pretty much any Earthlike or Earth-unlike climate should be possible.

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