Possibility of a terrestrial moon orbiting a Jovian planet for the setting of a fantasy world
I am currently creating a world for a series of fantasy short stories (and hopefully movies one day), and I need some help with the science. Ideally, I would like the story to take place on a moon that is slightly smaller than the earth, with a similar atmosphere and temperature, that orbits a gas giant, which in turn orbits a star similar to the sun. The moon must be able to support human life. I have several questions that I have been unable to answer on my own:
- What sizes and masses of the star, planet, and moon would make this possible?
- What would the orbits of the planet and moon be (shape, distance, and speed), and how would I calculate the length of a day, month, year, and the seasons on the moon using this information?
- How would the gas giant affect the tides of oceans on the moon, as well as the seasons and how much would it light up the moon at night?
Since this is for a fantasy world, it doesn't need to be 100% realistic, but I would like to be as scientifically accurate as possible.
Thanks
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1 answer
Looking at current Jovian satellites, Europa and Io give you some idea of what you'd need to consider. Starting with the Solar system model:
Europa is (as Aaron commented) the most likely candidate, but is probably too small (little under the size of our moon) and too cold - but it probably has a water ice crust, which means it is not far off the necessary temperature.
Io maintains a slightly higher temperature, but is the most volcanic object in the solar system (these 2 facts are linked with the proximity to Jupiter - Io is the closest of the Galilean satellites)
So in an exact analogue of our solar system you probably would need this moon to be similarly close to Jupiter to allow for tidal heating, volcanism would be likely (and possibly help with warming if sufficient greenhouse gases were a part of this) but your real challenge in an exact Jovian copy is that you are so far from the sun, so solar heating will be tiny, photosynthesis will not provide plants as we know them, and hence oxygen will need to be sourced from somewhere.
The solution to the temperature/energy problem is to have the gas giant much closer to the star, or have a larger star. You do run up against a minimum orbital distance limit for gas giants, however if the gas giant was the only planetary body in the system, it could certainly be much closer than Jupiter is in our solar system.
So, a Jupiter-sized planet orbiting a little outside Mars' orbital distance, with a moon a fair bit larger than Europa could suit your needs. So you could use a year around 3 Earth years long for the gas giant, but need to look at how orbiting that gas giant affects things (an orbital period of 3 days or so is well within bounds for this model) in addition to the rotational period. For interest you could look at having the moon's orbit at a high inclination to the gas giant - this could simplify some of the concepts of days and years.
Rotational period may be very long - tidal locking is likely. If the moon isn't locked then the tidal effect on not just any liquid water, but the moon itself will be extreme.
Reflected light will make for impressively bright night skies on the side facing the gas giant (see how bright Jupiter is from Earth for an indication) - by default, gas giants have a large albedo so if you have tidal lock here the side facing the gas giant will only see darkness when eclipsed by the gas giant.
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