I have a world in my novel that being investigated for terraformation. It's a large moon (slightly smaller than Earth) orbiting a gas giant. It's tidal-locked to its planet. Would the planet-facing hemisphere have a different climate than the non-planet-facing hemisphere? I currently have it written that the outward-facing side is more temperate, while the other is much warmer. But that just struck me as a fun idea and I don't actually want to use it if it's weird or implausible. Thanks!
Note: the following is for the moon to support life-as-we-know-it (carbon-based, some type of DNA or analogue, etc). Silicon-based life or other exotic life could live in very different environs..
A habitable moon orbiting a gas giant (which we will call the primary) would require both the gas giant primary and the habitable moon to have certain characteristics.
For analysis, lets assume the primary is a Jupiter-sized gas giant. We’d have to move it closer to it’s star than Jupiter is in our system, so there would be enough heat from the star to keep the planet habitable. The additional heat from the primary will help so we don’t have to move it as close to the star as we otherwise might.
This would make your primary a ‘hot Jupiter’ - a gas giant that orbits close to it’s star. We know of a lot of hot Jupiters in nearby systems.
Having an Earth-sized moon isn’t a stretch at all. The second largest moon in our Solar system is Titan, a moon of Saturn. It has a radius of 2575 km, about half the size of the Earth. It also has an atmosphere - one made of nitrogen, methane, and hydrogen. Ick, but this shows that a large moon with an atmosphere is possible.
The biggest danger to your habitable moon is the primary’s magnetosphere. This is the magnetic field generated by the primary. Although this magnetic field will help shield your moon from cosmic rays and radiation from its star, it also traps a lot of radiation particles and keeps them close to the primary - like Earth’s Van Allen Belts but much, much bigger and powerful. On moons close to the primary, that’s enough radiation to kill everything. Jupiter’s inner moons are orbiting inside a giant microwave oven.
To have a life-bearing moon, we need to do one of two things to it - preferably both to be on the safe side: move the moon outside of the radiation belts of the magnetosphere and/or give the moon it’s own magnetic field.
As your moon is about Earth-sized, it probably has a rotating iron core. It’ll have a magnetic field, but lets place it outside the primary’s radiation danger zone just to make sure.
For Jupiter, the safe distance is about 1.5 million km. So, lets put your moon at the orbit of Callisto at 1.8 million km. At this distance, what little radiation your moon gets from the primary’s belts is blocked by the moon’s magnetic field.
Good call on the moon being tidally-locked to its primary. Jupiter’s and Saturn’s largest, closest moons are tidally-locked to their primaries (including Callisto). What this situation will do is make your moon’s day-night cycle quite a bit more interesting than just sunlight and darkness.
As the above mages shows, your moon will have a varying amounts of illumination - a time where only the star is visible (true day), a time where both star and primary is visible (brighter day), a time where the primary only is visible (false day), and a dark time where neither primary nor star is visible (true night). The moon will have a day length equal to the time it takes to orbit it’s primary.
Putting your moon at the same, safe distance as Callisto, it will orbit it’s primary in about 16.5 Earth days, so the moon’s ‘day’ will be just under 400 hours. If your moon orbits in the same plane that the primary’s orbit is in (and it probably does, cause that’s how most orbits work), you’ll have a short eclipse of the star by the primary every day for a couple of hours. Indeed, it’ll happen at local noon.
The tidal-locking of your moon probably wont have much of an effect on the planet’s environment. The side pointing towards the star will get really hot, seeing 200 hours of daylight, but it’ll cool off at night. The heat from the star will have much more impact than the heat form the primary.
A good atmosphere and oceans
will do a lot to spread the heat across the planet more evenly. It’ll probably have more extreme days and nights than Earth, but probably not enough to make living there too difficult.
Of course, the view from your moon would be spectacular. The primary will hang in the same spot in the sky looking about nine times the diameter of Earth’s full moon. It’ll be dozens of times brighter than the full moon, as well.
Your setup is quite plausible and believable, and you’ve got the basics pretty good.. There is nothing known that would make such a setup implausible.
Hey, if it’s good enough for the Rebels…