Do all / most / any other moons orbiting a planet synchronize their rotation so as to become tidally locked with it, or is the earth a special case?
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$\begingroup$ I believe tidal locking between moons / planets is an eventuality in almost every case. Many of the Jovian moons are tidally locked. Charon and Pluto are tidally locked. The Moon / Earth system is not special. $\endgroup$– Ezra BaileyApr 6, 2016 at 21:13
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1$\begingroup$ @SarahBourt Some examples of exceptions from Wikipedia which actually strengthen your argument: "Notable exceptions are the irregular outer satellites of the gas giants, which orbit much farther away than the large well-known moons." "Pluto's other moons are not tidally locked; Styx, Nix, Kerberos, and Hydra all rotate chaotically due to the influence of Charon." $\endgroup$– called2voyage ♦Apr 6, 2016 at 21:42
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1$\begingroup$ @called2voyage Having researched it a bit more now, there's a lot to tidal locking and how it affects satellites! Objects want to face each other, but if the planet is faster than the satellite, the satellite will increase orbit height until they either it's orbit is outside the planet SOI, or the planet is tidally locked with it. On the other side, if a planet rotates slower than the orbit, tidal deceleration will make its orbit smaller until it eventually falls into the planet! Neat! $\endgroup$– Ezra BaileyApr 11, 2016 at 13:14
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2 Answers
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Moons are not always tidally locked.
They are formed with a certain spin angular momentum that will be dragged by asymmetries on the hosting planetary body. Those asymmetries then backreact onto the moon and drag it's spin to become synchronized with the orbit angular momentum.
How rapidly this process happens is roughly a function of the tidal forces
$$F_{Tid} = G \frac{m_{Moon} \cdot m_{Planet}}{r^3} d$$
exerted by those asymmetries. Here $d$ is the moon size, $r$ the distance between planet and moon and the other quantities are masses and the gravitational constant.
The time $t_{lock}$ this locking takes can be estimated by comparing the excess angular momentum $L_{ex} = r^2m_{Moon}(\omega_{actual} - \omega_{orbit})$ with those acting forces: $$t_{lock} \approx L_{ex} / \left(r F_{Tid} \right) \approx \frac{r^5}{ d \cdot m_{Planet}}$$
The latter equation makes it fairly obvious that this is not a process that happens at a certain magical orbit or at a certain time. It is an asymptotic process, and the only coincidal thing is that we observe any given moon at a time where it may not have lost enough spin angular momentum yet.
This should hopefully also clarify some confusion from the comments:
- Yes, the tidal locking time is also dependent on the mass of the host-planet, but much more sensitive to the planet-moon-distance. Note that this derivation is independent of the moon's mass, and only dependent on it's size. The literature knows much more sophisticated variants of this, that also incorporate a (realistic) weak dependency on the moon's mass.
- For Hot Jupiter exoplanets it is thought they'd be in tidal lock when the asteroseismologic age of the system is bigger than the derived tidal lock time.
Concluding
1.) Any moon would be tidally locked if we could wait long enough.
2.) There are much more effects / torques to consider for a detailed understanding than I did here.
answered Apr 7, 2016 at 9:58
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Many, many moons throughout our solar system are tidally locked. https://en.wikipedia.org/wiki/Tidal_locking This wikipedia page lists 34 moons throughout our solar system that are known to be tidally locked, and another 26 that are suspected.
It is very common for moons within a certain distance of their planets to be tidally locked. If they are too far away, they won't be tidally locked. Astronomers have even deduced that some exoplanets are tidally locked, but I have no idea how they know that. This leads to some interesting questions:
- What is the distance at which moons are likely to be tidally locked?
- At what distance are moons less likely to be tidally locked?
- How can we know an exoplanet is tidally locked to its star?
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1$\begingroup$ I think it's more about the mass of the two orbiting objects than distance, and also the time elapsed since the capture or the formation of the moon. At every distance, the oldest a moon is, the highest the probabilty of it being tidally locked. Also, the bigger the difference in mass is, the higher the probability. Distance matters at some point but I don't think it is the most important criteria. $\endgroup$– NicoApr 7, 2016 at 7:59