# I Question about a magnetic dynamo on tidally locked moons

1. Apr 9, 2017

### Althistorybuff

All,

Non-scientist amateur looking for an answer.

I understand that planetary rotation aid is vital to maintain a magnetic dynamo. Venus' slow reverse rotation is often used as an explanation for its odd low magnetic field.

Assume there is a moon or a double planet that is tidally locked, therefore no rotation, but is in a very rapid orbit, say 20 to 40 hours. Would the orbital rotation do anything to stimulate the dynamo or would it likely die due to lack of spin?

For example, the Earth will eventually becoming tidally locked to the moon and the dual system will orbit one another. Would this mutual spinning orbit, but no individual rotation, do anything to maintain the dynamo?

Can you maintain a dynamo when mutally tidally locked, either the planet or the moon?

Thanks.

2. Apr 9, 2017

### Bandersnatch

Imagine you're on one of the tidally locked planets. Can you see the sky rotate around you or not?

3. Apr 9, 2017

### Althistorybuff

You would not have a planetary rotation but would have an mutual orbital rotation with the other body. Is that enough for a dynamo to keep going or does the planet itself have to rotate on its axis to keep that liquid metal flowing?

From the surface of the moon, you would see the stars and sun moving as it orbited the sun and the other body but the mutually tidally locked body would always stay in place for half the planet.

4. Apr 9, 2017

### Bandersnatch

How do you define planetary rotation and mutual orbital rotation? What do you think is the difference?

If you can see the sky rotate around you, then the body you're standing on is rotating. You can find out how fast by measuring the time for the background stars to make a full circle (i.e. the rotational angular velocity). On Earth it takes 24 hours. How long does it take on the Moon? How long does it take on a planet that is tidally locked to another body with mutual orbital period of e.g. 24 hours?

5. Apr 11, 2017

### Althistorybuff

I would have it as a very short "month" of perhaps 30 to 60 hours due to close co-orbit of the two mutually locked bodies.

6. Apr 11, 2017

### Bandersnatch

Then both planets will be rotating with the period of 30 to 60 hours. This is comparable to Earth rotation. Whatever causative effect on the internal dynamo that has, will be roughly the same.

Tidal lock means 1:1 spin-orbit resonance. For each one full rotation there is one full orbit.

Last edited: Apr 11, 2017
7. Apr 11, 2017

### snorkack

So suppose that Moon were lowered to the 24 hour geostationary orbit - leaving Earth´s rotation period unaltered.
Would there be any effects on geodynamo?

8. Apr 11, 2017

### Bandersnatch

I don't think tidal effects have any bearing on the dynamo. Maybe just the tidal heating keeping the interior a bit hotter than it'd be otherwise.