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Bonus: How much would the increase in lunar mass increase average tides? & which factors would cause the Earth to become uninhabitable?

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    $\begingroup$ In that case, Moon can be as big as the Earth - that will make them tidally locked, which might affect the development of life on Earth, but won't make it uninhabitable. $\endgroup$ – Alexander Feb 22 at 1:42
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    $\begingroup$ can you elaborate why do you think that a massive moon would cause Earth to be sterile? $\endgroup$ – user6760 Feb 22 at 2:30
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    $\begingroup$ Hi, 007, do you mean completely uninhabitable? Or for humans and existing complex Earth life? When do you want this to happen, before life existed or tonight or...? $\endgroup$ – Mikey Feb 22 at 11:53
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    $\begingroup$ This question definitely needs clarification. Key information that is missing includes, but may not be limited to: 1. how this increase in mass is manifest (does the moon just inexplicably increase mass from the inside out? or is it bombarded by bazillions of tiny meteors? or several large ones? or does the method not matter, and only the end result, after the increase is important? etc) 2. how the mass affects the radius of the moon itself (and thus its density) 3. how the mass affects the orbital distances of the Earth and Moon. $\endgroup$ – Dalila Feb 22 at 16:23
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    $\begingroup$ You may also have luck, possibly better luck, asking this on astronomy.SE $\endgroup$ – Thesaurus Rex Feb 23 at 15:15
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It'd have to be pretty big. And by 'big', I mean instead of the moon orbiting the earth, the earth would become a satellite of the Moon.

And the Moon would be around the size of Jupiter.

I'm taking a guess from the context that the mechanism of uninhabitability that you're thinking about is tidal forces ripping the earth apart, or at least causing sufficient levels of vulcanism to make the formation of life impossible.

Basically, you're thinking about Io.

Io orbits Jupiter only about 10% further out than the Moon orbits the Earth (420,000km vs 385,000km), and because of that it's the most tectonically active thing in the solar system. Certainly no life is likely to ever form there.

HOWEVER, Europa orbits Jupiter at around 660,000km and even though the amount of radiant energy available is miniscule at that distance from the Sun, there's still a lot of hope that life could naturally evolve there. The earth would be more tectonically active at that distance than it is now, but not catastrophically so.

At a more reasonable 1millionkm orbit (where Ganymede sits relative to Jupiter), things would be just fine.

In fact, if you had the Earth orbiting a Jupiter-like gas giant at the same distance from the sun we are currently, then it would be just about as habitable as it is now. You'd have a much more complicated seasonal variation of course because instead of just the axial tilt you'd also have to take into account the effects of passing in front of and behind the gas giant, but other than that, humans would be just fine.

We'd just have MUCH more spectacular sunsets and eclipses.

EDIT: I realized there's another important factor that I didn't address. The other issue to consider is atmosphere retention.

Jupiter's moons all have very thin atmospheres for the same reasons that Mars does. First, they're not massive enough for gravity to do all the work and Second, they don't generate enough of a magnetic field to prevent an atmosphere from being ionized and stripped away by solar wind or (in the case of Jupiter's moons), by the massive magnetosphere of Jupiter itself.

Now, this would be a problem for earth too if it had a really massive neighbor because now you've got the electromagnetic effects on the atmosphere to consider. Earth generates a MUCH stronger magnetic field of its own than any of Jupiter's satellites, which would mitigate the effects somewhat, but not entirely.

Here again proximity is the key factor. If Earth were orbiting Jupiter at the same distance as Io does, not only would you have the tidally created vulcanism to worry about, you'd also have massive magnetic storms. I'm not enough of a physicist to know exactly where the safe limit would be, but there's a range where Earth might be far enough away to avoid the catastrophic volcano problem, but still close enough that the magnetic storms would render the atmosphere unsuitable for life.

On the other hand, you also still have a nice sweet spot the Earth could sit in where its own magnetosphere is strong enough to protect the atmosphere, and everything is copacetic.

You'd probably have some pretty spectactular Aurora Borealis EVERYWHERE, EVERY NIGHT, but that's sort of a win/win in my book.

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    $\begingroup$ The moon's core is dead right? Does that change anything when it gets larger in regards to magnetic fields? $\endgroup$ – Trevor Feb 27 at 15:23
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    $\begingroup$ Well, given OP's response to @Alexander above, I'm reading this as "What would happen if Earth had a MUCH more massive companion" rather than literally "What would happen if the moon we had suddenly got MUCH larger". Thus my response hypothizes naturally occuring bodies of the appropriate mass. Trying to make something the mass of Jupiter out of the same materials as the moon would be... funky... it certainly couldn't form NATURALLY that way. $\endgroup$ – Morris The Cat Feb 27 at 15:30
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    $\begingroup$ Io is already tide-locked. Earth would not be(it takes time) so tidal effects would be orders of magnitude bigger. For Jupiter-sized "moon" just ocean tides would be on order of 10 km. $\endgroup$ – Vashu Feb 28 at 3:32
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    $\begingroup$ It doesn't have to be tidal forces ripping Earth apart, it could just be tidal forces creating, well, tides. If tides are sufficiently powerful that they erode away any potential landmass, that's generally going to be a big red flag for habitability. (Notice the OP tagged the question with tides.) So, don't think how big before it destroys Earth, think, how massive before the tides and other effects destroy its potential biosphere. $\endgroup$ – Cadrac Feb 28 at 5:30
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    $\begingroup$ @Cadrac by the time you had a biosphere to worry about, the Earth would be tidally locked by its partner and the ocean tides wouldn't be destructive. $\endgroup$ – Morris The Cat Feb 28 at 14:48

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