Recently I had an idea for a world setting. A large part of this world focuses around a habitable satellite the size of Earth orbiting a Super-Jupiter. The fact that it's a moon is pretty integral to everything, so if it does't work out I'll probably scrap the idea as a whole.
My question is, could a satellite orbiting a Super-Jupiter be habitable?
If so, how different would it be from life on a planet? How would days be experienced? Would it be tidally locked? etc..?
If moons about our gas giants are an indication, the moons would likely be tide locked.
This makes for some interesting differences in cartography.
On earth's globe there's two non-arbitrary points: The north and south pole. And there's a non arbitrary great circle: the equator.
But for a tide-locked world, there are 4 non-arbitary great circles and 8 non-arbitrary points: Non-arbitary points: Near point Far point Leading point Trailing point North pole South pole
Great Circles: Equator Circle containing near point, far point, south pole and north pole Circle containing leading point, trailing point, north pole and south pole
(In this image I called the leading and trailing points the Left Ear and Right Ear.)
I would expect the tide locking to have a huge impact on evolution of the inhabitant's religion and culture.
For example I would expect the near point to become sort of a Mecca. They might build a temple on the the near point. Viewed from this temple, the gas giant would hover motionless at the zenith. Or the moon has some obliquity, the gas giant might trace an analemma with the analemma's center at zenith.
If this gas giant is like the giants in our solar system, it will likely have a whole family of moons, many of them near planetary size. It would be like a mini solar system. But travel between moons of a gas giant could take place at a much faster pace than travel between planets. Launch windows between destinations would occur on the order of weeks apart vs years apart (for example earth to Mars launch windows occur each 2.14 years). Trip times between moons would be on the order of weeks (Hohmann trip time between earth and Mars is about 8.5 months).
An elevator could be built from the moon's far point through the gas giant/moon L2 point and another elevator built from the moon's near point through the gas giant/moon L1 point. If such elevators were built on each moon, the civilization could travel between moons using virtually no reaction mass.
It could definitely be habitable. Beyond that, there's a huge range of possible environments such a planet could have.
In general, when we think of a 'moon,' we tend to think of smallish planetoids with hostile environments, but that's mainly because that's what we tend to see in our solar system. In another solar system, the moon orbiting your super-Jupiter could be the size of Earth.
The moon may or may not be tidally locked. If it is, it will experience one 'day' every time it revolves around its planet. If it has particularly long days, the day/night temperature swings will be a bit more extreme. If not, it will experience days like those we experience on Earth.
The biggest difference will be seasons, which will be based on the orbit of the moon around the planet, rather than the star. There will be seasonal effects both from any inclination of the rotational axis of the moon and from the position of the moon relative to the planet it orbits. If the moon is on the star side of the planet, it will receive more intense light from the star, due to its closer proximity, as well as additional light reflecting off of the planet it orbits.
As far as being tidally locked, the pieces that go into tidal locking are, the ratio of mass of both bodies, (planet and moon) the distance between them, how fast where they spinning at the beginning and how much time has passed in their little dance.
Our moon was not always tidally locked to earth, but there has been a lot of time for it to occur. The Earth used to have about a 6 hour day and it has been slowed by the moons tidal actions to our current ~24 hour day. eventually the Earth would be tidally locked with the moon (meaning that from the earth and the moon would always have the same view of each other. However this is estimated to take about twice as long as the remaining time of our sun's life.
So this means that an earth sized planet does not need to be tidally locked with it's super giant, but it would also likely have a much longer rotational period. Might have an orbital period of 60-100 days, farther away would reduce slowing affects of large planet and allow more reasonable seasons to measure. A 'year' of 100 days would give approx 25 days per 'season', 25 facing the sun, 25 behind the super planet, and then of course how long does the planet take to orbit the sun? Is every 4th earth year colder because it's at the planets farthermost reaches, and 2 years later warmest summer? Or is is farther out and a much slower solar year? To far away and the sun will have little affect, just a little more or less light.
Expanding a bit on HopDavid's answer, if the moon is totally locked, then the ecology also becomes different. The point closest to the Jovian would be the "hot pole", since it receives the heat and reflected sunlight from the primary, while the opposite point would be the "cold pole" (although that is a relative term; a moon with an atmosphere would have lots of ways to conduct heat between the poles.)
As well, the "Left Ear" (leading point) would be subjected to more radiation from the primary's radiation belts than the "Right Ear". We can assume that the moon has an active magnetic field of its own due to the molten core (hotter than the Earth's due to the constant "kneading" from the retinue of moons orbiting the primary, much like Io in our own Solar System), so the radiation is not instantly lethal to most life.
When plotting ecosystems, the moon will look a bit like a beach ball painted in a checkerboard pattern. One set of rings will run from the hot pole to the cold pole, and be areas of discreet temperature and illumination, while a second set of rings at 90 degrees to the first will run from the Left Ear to the Right Ear, indicating relative exposure to radiation.
Because of the active tectonic forces, the moon will have many active volcanos and subduction zones, so the various geological cycles will be quite rapid, with almost constant mountain building and oceans and seas opening and closing in relatively short (million year) timeframes. The carbon cycle and hydrological cycles will also be affected by the rapid "turnover" of the crust, so conditions will be rather different from Earth (although how different might depend on your starting assumptions.)
This will be a very interesting world to visit, but if we were able to get to the system, most people would probably prefer to remain in a colony in free space.
How would life be on a Jupiter moon? Nasty, brutish and very, very short.
Jupiter has these massive radiation belts generated by its magnetosphere, which are about a million times stronger than Earth's. They were powerful enough to damage or throw into safe mode all of our hardened interplanetary probes. It is estimated that the accelerated plasma is energetic enough to kill unshielded humans in minutes. This would scale up supra-linearly with the size of the super-Jupiter. Organic compounds (such as complex organic molecules, people or cats) on an exposed satellite surface would be destroyed in minutes or hours.
So colonies are unlikely, especially if you want them to live on the surface.
