Inspired by this article about the Kepler-enabled search for life-supporting moons, I came over here to learn more about the concept. I found this very complicated answer to a very complicated question on the subject, part of which says:

Tidal locking will occur at some point in time. You can't get around it.

I want to understand this statement more without all the equations.

  • For one, is it true?
  • What causes the situation?
  • Over what period of time?
  • Does it last forever?
  • Is it periodic (i.e. the moon becomes tidally locked, stays that way for a while, then "breaks free" and rotates again)?
  • Intuitively, if the difference in masses is large enough and/or if the moons are far enough out, then the quoted statement must be false. Case in point: Earth isn't tidally locked to the Sun. However, I don't have any real evidence with which to back this up, so I can't tell you if the quoted statement is true for some cases, or even possibly all realistic cases of planets and moons. – a CVn Apr 13 '16 at 18:22
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    @MichaelKjörling Isn't the Earth's rotation slowing down, though? It may not have stopped yet, but it's getting there. – DaaaahWhoosh Apr 13 '16 at 18:24
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    @DaaaahWhoosh Earth's rotation is slowing down; see for example Change's in Earth's rotation and ΔT. However, at least Wikipedia claims that this is due to interaction with the Moon, not with the Sun. The present net rate is claimed to be +1.7 milliseconds/day/century. I'm getting lost in all the exponents, but it'll be a while before we at this rate hit a sidereal day the same length as the sidereal year, if indeed it ever can happen for the reasons quoted. – a CVn Apr 13 '16 at 18:30
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    While Wikipedia has issues, they are usually a good place to start looking. Try here: en.wikipedia.org/wiki/Tidal_locking – Thucydides Apr 13 '16 at 20:35
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    "Always be" is not "always becomes". I think the distinction is important. – a CVn Apr 14 '16 at 15:29
up vote 6 down vote accepted

Tidally locking happens to all orbiting bodies. It is only a matter of time. Once tidally locked, they stay that way, unless acted upon by an outside source.

Differences in mass and distance affect the time tidally locking takes to occur. Our moon is tidally locked with the Earth. that took some time, and it also slowed the earth's rotation down by a factor of 4. Earth's 'days' used to be about 6 hours long.

But the distance between the earth and the moon has also increased and continues to do so.

Moons around Gas giants have a much smaller mass ratio and tend to be 'closer' so are going to generally tidally lock much 'faster' than it took for the earth and our moon.

However, the one main outlier are liquid moons. Moons that are primarily liquid in the center (or have a very large liquid layer) tend to NOT (won't?) tidally lock such as Europa. And that was one of the main clues that it had a lot of liquid, since it SHOULD have been tidally locked to Jupiter.

It appears there might be some dispute on Europa's tidally locked status. But I think a more scientific paper needs to be found to support the 'not tidally locked'. This was from Wiki.

...Like its fellow Galilean satellites, Europa is tidally locked to Jupiter, with one hemisphere of Europa constantly facing Jupiter... ...Research suggests the tidal locking may not be full, as a non-synchronous rotation has been proposed: Europa spins faster than it orbits, or at least did so in the past. This suggests an asymmetry in internal mass distribution and that a layer of subsurface liquid separates the icy crust from the rocky interior.

They used to think that Mercury was tidally locked with the sun, but it has been discovered that it rotates at a ratio of 2/3 it has a ~58.5 day long day (Earth Days) and 88 day (Earth Days) long year. And according to JDługosz this is a stable type orbit so it is possible that some moons in an eccentric orbit might stabilize in this pattern, as a 3rd option.

  • I've pointed out on other Qs about planets of red dwarfs that the situation with Mercury is commonly expected: for high eccentricity, an odd half multiple is more stable than 1:1 locking. And meanwhile other bodies in the system can promote high eccentricity. So, the same might happen to moons of a gas giant: expect 3/2 etc. for the same reasons. – JDługosz Apr 14 '16 at 6:33
  • @JDługosz interesting! Until I looked up Mercury for that answer, I was still under the impression that it was tidally locked! I'll update my answer. – bowlturner Apr 14 '16 at 12:51
  • Huh? Europa is tide locked with Jupiter. – HopDavid Apr 21 '16 at 23:51
  • @HopDavid interesting, I'm sure I had read that one of the reasons they thought Europa had a large water mass was because it wasn't tidally locked. – bowlturner Apr 22 '16 at 13:14
  • @bowlturner Please show a cite. I believe it is a false memory. Am voting this answer down because of the 5th paragraph. – HopDavid Apr 22 '16 at 14:49

I think they've already answered your question. It's buried in the math, yes, but it's there.

While true that tidal locking is 'inevitable.' This period can be extremely long.

The are three factors that determine this period:

  • Mass of the moon. The greater the mass, the smaller the tidal forces *edit - the less significance the tidal forces will have on the... * influence the moon. Since you'll need a moon at least as big as mars... and more likely the size of the Earth, this shouldn't be a problem. I base this assumption on the fact you'll need a magnetosphere. Even if the gas giant is far from the primary star(s), gas giants, especially those that are brown dwarfs like Jupiter, emit a hellish amount of radiation onto their poor moons. You'll need a decent magnetosphere to protect you from that.

  • Distance of the moon from the Gas giant. The farther the orbit, the less tidal influence you'll get. This is the average orbit (at least from what I understand in the equations), so a highly eccentric orbit would also qualify.

  • Orbital Period. A fast orbital period will take longer to influence.

So, if the gas giant captured a planet sized moon with a fast and eccentric orbit, it should take a good long while before it even started to become tidally locked. Use poetic license here would be my advice.

  • Not quite. It may lock at a resonance faster than 1:1 and then not slow any more. E.g. 3:2. You can also have chaotic tumbling, and chaotic non-periodic orbital periods (like Pluto's 3 outer moons). – JDługosz Apr 14 '16 at 6:36
  • What if the moon had it's own moon captured at or fairly near the same time as the original capture? It would seem to me that this would keep the 'primary' moon from becoming locked. At least if the 'secondary' moon was of sufficient mass compared to the 'primary.' Much like Luna and Terra. – Hirahito Apr 18 '16 at 20:38
  • "it's" always means "it is"; "its" is the pronoun. – JDługosz Apr 18 '16 at 22:04
  • There are other WB questions on moon of a moon, with analysis of our Earth Moon subMoon in particular. It would not last for very long. IAC, our Moon was not captured, but formed from an extended debris ring (actually an atmosphere!) after a Earth-Shattering impact. – JDługosz Apr 18 '16 at 22:06
  • @JDlugosz - Honestly? Private comments are for this. This was an honest comment and request for information. It's comments like this, reactions like this, that make people turn away from open forums. – Hirahito Apr 19 '16 at 16:18

The exception that proves the rule: Hyperion is not tidally locked and as far as we can compute will never be tidally locked longer than some "time".

Hyperion is highly irregularly shaped which results in a chaotic rotation. It's like trying to balance an irregular potato on one point. Which is possible. And then open the window to simulate the gravitational influence of the other moons with some wind.

This was not mathematically understood until recently, see the links on Wikipedia like The Chaotic Rotation of Hyperion.

So in very specific circumstances more than temporary tidal locking is not likely and might even be impossible.

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