How can I make a planet covered in ice with an ocean beneath the surface?
In a story I have been working on, there is a planet called Ieuclite. I want to have its surface be entirely covered in ice and have the people live in cracks and fissures. But then I thought, what if there is an ocean beneath the surface? And what if these people evolved to be able to stay underwater for very extended periods of time (yet not fully aquatic). Some other information is that this planet is technically a moon that people from another planet colonized so they could collect the water (for other reasons that go way beyond the point of this question).
I want to keep my idea as realistic as I can, and I often tell myself, "it's just fiction, no one will really care," but at the same time I really want to have science backing up most, if not everything I write.
Also, any ideas regarding the actual species living on this planet would be greatly appreciated as well.
Edit: The aliens that colonized the planet for water come from a very dry and large planet with very heavy gravity. I read in a different Answer on this site that in heavy gravity, it would be favorable to sleep in water with breathing devices, because the water could still hold up your weight and keep you afloat. So they need to harvest more water that's readily available, and they decided to colonize the moon so they could just send little pods back with the water rather than travelling back and forth.
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1 answer
1. Orbit a high-mass planet
In the Solar System, two moons are covered with ice and have subsurface oceans: Europa and Enceladus. Europa orbits Jupiter; Enceladus orbits Saturn. In the case of the latter, jets of water vapor were observed, indicating an underground source. For both moons, the oceans survive through tidal heating by the giant planets they orbit. Energy is transferred to the moons, which heats up their interiors, keeping them warm enough for liquid water to exist below the surface.
Now, you've described the home planet as having "heavier gravity". If the planet's mass is anywhere near the level of that of, say, Saturn, then you should be able to come up with some pretty good tidal heating. You just need to have the right parameters for the moon's orbit, composition and structure.
2. Snowball Earth
RonJohn beat me to the idea of a snowball Earth, but I'll talk about it nonetheless, because it's a good option - and doesn't involve another body. Moreover, it may have happened, at multiple periods in our planet's history, and so it's quite feasible.
Essentially, you need a terrestrial planet with a good supply of water - oceans. You then need some sort of mechanism that will cool the planet and lead to more cooling - a feedback mechanism. (We see something similar in global warming, where increasing temperatures unlock more greenhouse gases, such as through the melting of ice.) This means that the cooling will continue, rather than being damped and eventually stopped.
This mechanism could be as simple as drastic snow buildup, which would lead to less heat being absorbed. Alternatively, some sort of mass reduction in greenhouse gases could reduce the heat trapped by the atmosphere. Both of these would be self-reinforcing, leading to a true snowball planet. They're better than one-time events, like an asteroid impact, because those effects will end on relatively short timescales (e.g. when dust dissipates).
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