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Artifexian in his Worldsmith and other videos of his recommends a moon of a habitable planet must be a tidally locked moon. But why is that? I fail to understand how rotation of a moon about its axis would prevent life.

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    $\begingroup$ I believe his statement is a simplification, which is common in worldbuilding. Unsimplifying a little bit, a habitable world requires a minimum mass to ensure the atmosphere stays in place. That mass exceeds the minimum amount needed to tidally lock the world's moons (it's more complex than this, but...). For example, all of Jupiter's moons are tidally locked (big mass, that Jupiter). A planet of such little mass that the moons wouldn't tidally lock has too little mass to support life. Note, however, that just because we haven't seen a contrary example yet doesn't mean it's not out there. $\endgroup$
    – JBH
    Commented Jan 18, 2023 at 18:22
  • $\begingroup$ Note that I didn't post an answer because this kinda isn't the kind of question we post here. It borders on an open-ended (prohibited, see help center) discussion (prohibited, see tour). The help center states that if you're looking for a general process, this might not be the right place to ask - and that's what this is. In the future, this is a great kind of question to ask on our Worldbuilding Chat, The Factory Floor. You can ask anything at all there and it's haunted by HDE226868, who's an astrophysicist. $\endgroup$
    – JBH
    Commented Jan 18, 2023 at 18:25
  • $\begingroup$ @jbh I agree. The statement should be reported verbatim for a complete answer. And probably Astronomy SE would provide more insight in the matter. One thing I wonder: would it be possible for a planet like Earth to have a much bigger moon than ours but at a greater distance? Probably not given the current theory of moon formation but maybe with a different generation? $\endgroup$ Commented Jan 18, 2023 at 18:28
  • $\begingroup$ @DuncanDrake Sure, it's called a binary or double planet system. In a sense, it's all the same thing. There's really no such thing as one object orbiting around another object. What you have are two objects orbiting around a barycenter. In the case of big planet, small moon, the barycenter is inside the radius of the planet. In the case of two objects the same size, the barycenter is between them. So, sure, no problem. $\endgroup$
    – JBH
    Commented Jan 18, 2023 at 18:38
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    $\begingroup$ @JBH Thanks for letting me know. I'm still pretty new to SE. $\endgroup$
    – user100304
    Commented Jan 18, 2023 at 19:57

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It isn't the rotation (or lack thereof) that supports life, but rather the situations in which we expect life to be able to exist, given a sample size of 1, lead to a tidally locked moon.

Firstly, Earth is Quite Large. If it weren't, it wouldn't have held on to its atmosphere and water so well, and in the absense of those things it is difficult for life to get started or survive and branch out into a variety of forms. If it were much larger though, there's a risk of it holding on to too much atmosphere and ending up as a gas dwarf or hycean world... an interesting body to be sure, but not the sort of thing that has life much like our Earth.

There is a volume of space around any gravitating body called the Hill Sphere. The size of the Hill Sphere is related to the mass of the body (eg, a planet), the mass of the thing it orbits (eg. a star) and the distance between the two. Any orbit of a planet outside of 1/3 to 1/2 of the radius of its Hill Sphere is not stable, so any moons out there will eventually fall out of orbit around the planet and into orbit around the star. Earth's Hill radius is about 1.5 million kilometers, and the Moon orbits at about 400000 km, comfortably within the 1/3rd limit. The Hill radius can't be bigger without Earth being larger (which could be a problem, see above) or the Sun being smaller or further away, which has implications for the planet being warm enough for liquid water.

The time taken for an orbiting body to become tidally locked to its parent is described by a slightly unwieldy formula. This locking timescale is increased when the moon is a long way from its planet, spinning very quickly and has a high rotational inertia. Tidal locking timescales are reduced when the planet is very heavy, or when the moon has a large radius. The Moon's tidal locking timescale is about 180000 years, a blink of an eye in astronomical terms (working in another answer of mine, timescale probably correct to an order of magnitude).

To maximise the tidal locking timescale, therefore, you need a small dense moon a long way away. The Hill radius limits how far away it can be, so you're left with something like a modestly sized metallic asteroid. Lets call this sort of a thing a "boring, small moon".

Things a boring, small moon moon probably won't be doing:

  • Stabilising Earth's axial tilt. It isn't entirely clear how unstable Earth's axial tilt is, but Mars' tilt has varied by as much as 60° of its history and that sort of variation could cause catastrophic climatic disruption. The influence of the Moon on Earth's rotation limits how quickly or dramatically the axial tilt can change, because tidal forces will counteract the movement.
  • Have any tidal effects on the geodynamo producing Earth's magnetic field. Again, it isn't clear how much effect the Moon has here, but the effect is nonzero and Earth has a reasonably strong magnetic field which has various benefits like reducing the rate of atmospheric loss. Contrast with the sorry state of Mars' magnetosphere and atmosphere.
  • Having a catastrophic origin story that might have kickstarted Earth's magnetic field. How the geodynamo got to its current state is also a bit of a mystery, but there's a good chance it is linked with the hypothesized Theia impact that created the moon.
  • Provide nice big tides of the conventional, wet kind. The Moon has a much more substantial influence on tides on Earth than the Sun (the princple lunar semidiurnal constituent is substantially larger than all other contributors to tide range), which in turn drives all sorts of interesting depositional and erosional effects and produces complex intertidal habitats with all sorts of interesting biology. Even below the tidal range, tidal currents distribute nutrients across wide areas facilitating diverse submarine biomes.
  • Providing lots of lovely moonlight. Good for nocturnal hunters, also used (for whatever reason) as a signal for all sorts of other biological processes. No big moon means no werewolves, which sounds pretty boring.
  • Pushing astronomical discoveries back by centuries, because not having an interesting nearby astronomical body to look at is going to delay things until the science of optics has marched on enough to start visualizing planets, which are rather further away than the Moon.

It is also possible that the Moon provides some slight protection against asteroid impact, but it isn't clear how much of a factor that is. For worlds other than Earth, it may be possible for a moon of a gas giant which had its own moon to have a sufficiently high rotational inertia that it could itself avoid being tidally locked to its parent, even if its own moon was tidally locked.

So, there isn't a single definitive reason why you have to have a big, interesting moon, but it seems like it is quite a beneficial thing and if you do have a big, interesting moon then it is inevitably going to be tidally locked.

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