<p>The vulcanism on Io is one solid piece of evidence. Io is very close to tidally locked to Jupiter. It is also very close to being in a 1:2:4 orbital resonance with Ganymede and Europa. The forces from the orbital resonance tend to make Io's orbit more eccentric, while the forces from the tidal interactions tend to make Io's orbit less eccentric. This is not a balancing act. It instead forms a nice hysteresis loop.</p>
<p>Suppose Io is in a state of having a near circular orbit. This causes Io to cool off as the tidal stresses are minimal. This in turn causes Io to become more solid. The forces and torques that circularize Io's orbit and synchronize its rotation with the orbit diminish. The resonant forces that make Io's orbit more eccentric start to dominate. An increasing eccentricity results in increasing tidal stresses as the tidal locking time is very long compared to Io's 42.5 hour orbital period. Those increasing tidal stresses cause Io to heat up internally (which in turn results in the vulcanism seen on Io). This increased warmth and increased eccentricity eventually enable the tidal forces and torques to dominate over the resonance forces and torques. Io's orbit circularizes, and then the cycle begins anew.</p>

CC BY-SA 4.0
2024-10-17T20:07:11.913Z
The interacting orbits of Io, Jupiter, Ganymede, and Europa are not relevant to the question, which asks essentially whether moons were formed asynchronous, and then got tidal locked or formed already in synchrony, and why. There are several published theories about the formation of moons including ours. Some require that the moons were formed in synchronous rotation naturally. Therefore, there is no need for a tidal lock (none involves magic as you question in one of your comments). This is consistent with Garcia’s answer because it implies that the Moon was formed in synchrony. See his ref.
Tony Anderson
CC BY-SA 4.0
2024-10-05T09:31:12.707Z
2024-10-05T09:31:12.707Z
David Hammen
<p>NOTES FOR CLARIFICATION: I am asking for actual evidence, not theories. E.g. If a theory says that they tidal lock takes x million years, what evidence shows they were not locked before that time? This is not a textbook question, I know what textbooks say. This is a scientific question challenging the common textbook explanation. References to scientific papers will be appreciated.
I am reposting the question because it seems the original question was deleted by someone who thought that tidal lock MUST be the one and only answer WITHOUT CITING ANY EVIDENCE. For those who think like that, there are alternative theories. A complete answer would be very informative to elucidate what is the real cause.</p>

CC BY-SA 4.0
2024-10-05T05:10:31.313Z
What does the textbook say? What are your "alternative theories" which you want to hear about? Please be verbose and cite any evidence
planetmaker
CC BY-SA 4.0
2024-10-05T06:28:55.397Z
If you have any clarification, please edit your previous question instead of asking a new question.
Nilay Ghosh
CC BY-SA 4.0
2024-10-05T13:18:10.503Z
I voted to close and then I answered the question. My answer is aimed at other readers of this StackExchange; the OP apparently doesn't understand science. As another user commented, "We don't post crazy stuff here."
David Hammen
CC BY-SA 4.0
2024-10-05T15:08:01.187Z
This is similar to the occasional question about whether there's any non-rotating objects. There is exactly one rotation rate that would result in synchronization, and the probability of all the matter comprising a body coming together with the exact amount of angular momentum needed to result in that exact rotation rate is essentially zero. When you look at a system that has evolved over billions of years with 20 large moons rotating synchronously after all the impacts and other forces they've experienced, chance is simply not a credible explanation.
Christopher James Huff
CC BY-SA 4.0
2024-10-08T07:03:48.340Z
@DavidHammen Since this question is the exact duplicate of other question, wouldn't it be better if you had edited your previous answer? If the question end up getting merged that you would have two answers for same question. Is this answer different than the other?
Nilay Ghosh
CC BY-SA 4.0
2024-10-17T19:35:57.223Z
I found a partial answer to this question in by Connor Garcia: Is there any evidence that the rotation period of the Moon was different to its orbital period in the past? His answer is: “No, there is no evidence of a period when the Moon wasn't tidally locked.” It is a partial answer because it refers only to our Moon, while the question refers to all Regular moons. It includes a supporting publication. His answer highlights the confusion created in other answers about “tidal lock” and “synchronous orbit.” They are different things but some answers treat it as the same.
Tony Anderson
CC BY-SA 4.0
2024-10-26T02:27:55.120Z
For clarification: When I refered to alternative theories I was not referring to a theory of mine, but to well-known theories of moon formation such as Darwin's fission or the Moon impact theory. They naturally convey the synchronous rotation since the moon was once part of the planet and separated so they were already facing each other at the time of formation. Textbooks, as some answers here, assume that the synchronous motion is caused by tidal lock.
Tony Anderson
<planetary-formation><planetary-science>
Why is the rotation period of regular moons in sync with the orbit period? What is the evidentiary support for tidal lock?
Tony Anderson
# Why is the rotation period of regular moons in sync with the orbit period? What is the evidentiary support for tidal lock?

(Asked by: username_0 on 2024-10-05)

NOTES FOR CLARIFICATION: I am asking for actual evidence, not theories. E.g. If a theory says that they tidal lock takes x million years, what evidence shows they were not locked before that time? This is not a textbook question, I know what textbooks say. This is a scientific question challenging the common textbook explanation. References to scientific papers will be appreciated.
I am reposting the question because it seems the original question was deleted by someone who thought that tidal lock MUST be the one and only answer WITHOUT CITING ANY EVIDENCE. For those who think like that, there are alternative theories. A complete answer would be very informative to elucidate what is the real cause.

**Tags:** planetary-formation, planetary-science

> **Comments:**
> **username_1**: What does the textbook say? What are your "alternative theories" which you want to hear about? Please be verbose and cite any evidence  
> **username_2**: If you have any clarification, please edit your previous question instead of asking a new question.  
> **username_3**: I voted to close and then I answered the question. My answer is aimed at other readers of this StackExchange; the OP apparently doesn't understand science. As another user commented, "We don't post crazy stuff here."  
> **username_4**: This is similar to the occasional question about whether there's any non-rotating objects. There is exactly one rotation rate that would result in synchronization, and the probability of all the matter comprising a body coming together with the exact amount of angular momentum needed to result in that exact rotation rate is essentially zero. When you look at a system that has evolved over billions of years with 20 large moons rotating synchronously after all the impacts and other forces they've experienced, chance is simply not a credible explanation.  
> **username_2**: @username_3 Since this question is the exact duplicate of other question, wouldn't it be better if you had edited your previous answer? If the question end up getting merged that you would have two answers for same question. Is this answer different than the other?  
> **username_0**: I found a partial answer to this question in by Connor Garcia: Is there any evidence that the rotation period of the Moon was different to its orbital period in the past? His answer is: “No, there is no evidence of a period when the Moon wasn't tidally locked.” It is a partial answer because it refers only to our Moon, while the question refers to all Regular moons. It includes a supporting publication. His answer highlights the confusion created in other answers about “tidal lock” and “synchronous orbit.” They are different things but some answers treat it as the same.  
> **username_0**: For clarification: When I refered to alternative theories I was not referring to a theory of mine, but to well-known theories of moon formation such as Darwin's fission or the Moon impact theory. They naturally convey the synchronous rotation since the moon was once part of the planet and separated so they were already facing each other at the time of formation. Textbooks, as some answers here, assume that the synchronous motion is caused by tidal lock.  

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## 1 answers

### Answer by username_3 on 2024-10-05. Score: 6

The vulcanism on Io is one solid piece of evidence. Io is very close to tidally locked to Jupiter. It is also very close to being in a 1:2:4 orbital resonance with Ganymede and Europa. The forces from the orbital resonance tend to make Io's orbit more eccentric, while the forces from the tidal interactions tend to make Io's orbit less eccentric. This is not a balancing act. It instead forms a nice hysteresis loop.

Suppose Io is in a state of having a near circular orbit. This causes Io to cool off as the tidal stresses are minimal. This in turn causes Io to become more solid. The forces and torques that circularize Io's orbit and synchronize its rotation with the orbit diminish. The resonant forces that make Io's orbit more eccentric start to dominate. An increasing eccentricity results in increasing tidal stresses as the tidal locking time is very long compared to Io's 42.5 hour orbital period. Those increasing tidal stresses cause Io to heat up internally (which in turn results in the vulcanism seen on Io). This increased warmth and increased eccentricity eventually enable the tidal forces and torques to dominate over the resonance forces and torques. Io's orbit circularizes, and then the cycle begins anew.

> **Comments:**
> **username_0**: The interacting orbits of Io, Jupiter, Ganymede, and Europa are not relevant to the question, which asks essentially whether moons were formed asynchronous, and then got tidal locked or formed already in synchrony, and why. There are several published theories about the formation of moons including ours. Some require that the moons were formed in synchronous rotation naturally. Therefore, there is no need for a tidal lock (none involves magic as you question in one of your comments). This is consistent with Garcia’s answer because it implies that the Moon was formed in synchrony. See his ref.  

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