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I am trying to make a moon orbiting a large ice giant that has a liquid ocean via this thought process: The moon has high eccentricity (though it is still in spherical shape), so raw tidal forces already warm it up, but this also gives it volcanoes which put greenhouse gases into an atmosphere, which collect and preserve the scarce sunlight and eventually melt some of the icy crust. This moon has an ice surface, a silicate mantle, and and an iron core, similar to Europa.

Does this thought process correctly align with science, and if so, could the moon I'm thinking of exist? If not, what would be needed to make it exist?

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  • $\begingroup$ Yes it can happen. In our solar system it did not happen: there are some moons (Europe, Ganymede, Titan) with quite interesting properties, but unfortunately they all are still very, very cold. $\endgroup$
    – Gray Sheep
    Mar 27 at 20:47
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    $\begingroup$ Your title says Ice Giant, but the question says Gas Giant - which case are you interested in? $\endgroup$
    – codeMonkey
    Mar 27 at 21:24

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This is definitely possible -- largely because we believe similar mechanisms affect planets!

Simulations have shown that some planets may become "tidal Venuses" (Barnes et al. 2013) thanks to tidal forces from their parent stars. Planets orbiting close to low-mass stars are particularly susceptible to tidally-induced runaway greenhouse effects, even in the absence of extreme volcanism. The results are catastrophic, but it's possible that milder versions could yield oceans of liquid water. All it needs is the right balance of planetary, stellar and orbital parameters.

A version of that milder scenario may have taken place in the early years of Earth, with an increase in volcanism and a temperature rise of several degrees (Heller et al. 2021). In this scenario, energy was actually provided by the Moon, shortly after it formed, on a timescale of ~100 million years. This may have enabled liquid water to arise on Earth earlier than expected.

Putting these two bits of evidence together: Since stars can induce greenhouse effects on planets orbiting them, and since the Moon may have induced a greenhouse effect on Earth, it stands to reason that a body of intermediate mass, like a giant planet, could have generated enough tidal heating to warm one or more of its moons enough to allow liquid oceans to form. Your scenario seems pretty darn plausible.

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  • $\begingroup$ An answer to both my question AND the faint young sun paradox?! This I was not expecting. $\endgroup$
    – Pointy Orb
    Mar 27 at 22:58
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Yes, that works.

As comments will point out, moons similar to what you're looking for - Europa, Ganymede, Titan, Io, etc. - all have very interesting properties similar to what you described but are still quite cold.

There's no reason that this couldn't've happened - you'd just need more greenhouse gases and a hotter Sun to add a ton more heat. Titan is already kind of screwed over because Saturn is so far out, but if your gas/ice giant is closer to its hot sun and has a lot of methane, carbon dioxide, or even some fluorides or chlorides, then you will get a good greenhouse effect and might be able to have an ocean moon.

Of course, this largely depends on the parameters of the atmospheric composition of the moon, its exact eccentricity, the star brightness, the albedo of the moon, etc. etc. etc. but if you're looking at answers, it's reasonable that such an ocean moon could exist. Worst case scenario, you can have your moon be extremely tectonically and volcanically active like Io and just have the volcanoes pour hot greenhouse gases out until the temperature is right.

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The "habitable Edge" of a giant planet is the distance at a which moon orbiting it that is otherwise suitable for life will have too much tidal heating and will suffer a runaway greenhouse.

I discussed this in an answer to a recent question.

If too much tidal heating of a large moon of a giant planet can cause a runaway greenhouse or even turn it into a volcan hell like Io, a lesser amount of tidal heading might melt some of the ice covering the surface of a moon.

I note that many of the larger moons in the outer solar system are believed to have internal oceans of water below miles thick ice crusts. Inreasing the degree of tidal heating would increase the chances that the ice crusts would melt.

I found the most recent question where I discussed the "habitable edge" in an answer. The scientific articles mentioned in it may help you to design a system with just the right amount of tidal heating.

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