Rotation rates of planets seem odd?

[JLT]

TL;DR Summary

rXmv, ice skater pulls their arms in, small r, fast rotation... debris in the universe pull into planets but.... smallest planets spin slowest? and the largest Jupiter spins the fastest?? What???

Ok, I know there are a lot of strange things in our solar system. Can anyone explain why the small planets spin so slowly? and why does Jupiter spin so quickly? It seems like a ball of debris, getting smaller and smaller, would increase its speed like an ice-skater pulling their arms in? rXmv+Mdt = rXmv. What moments are there? seems like gravitational forces are all acting on the center of mass, so there would be no "r", no reason for other planets to influence one another's rotational speeds as those are all center to center forces?

 

[russ_watters]

The inner and outer planets are vastly different compositions, so their rotation rates shouldn't be related to each other at all. And Mercury and Venus are close to the sun, which means they are being pulled toward tidal locking. So if you cluster the planets into three groups, the periods look a lot more similar in those groups and make a lot more sense.

 

[JLT]

I always thought of planets forming like whirlpools, or almost like hurricanes - perhaps something Coriolis like about them? Mercury does not have surface fluids, so how does tidal locking work there? Is it lopsided?

 

[russ_watters]

Mercury does not have surface fluids, so how does tidal locking work there? Is it lopsided?

The ground heaves.

 

[Nugatory]

Mercury does not have surface fluids, so how does tidal locking work there?

Our moon doesn't have surface fluids either, but it is well and thoroughly tidal locked to the Earth. At the time scales and with the forces that we're working with here, we can model solid rock as a viscous fluid - that's how gravity manages to pull the larger bodies into spheres in the first place.

 

[JLT]

Thanks for the replies, I guess I need to study tidal locking more. I can visualize the oceans on the Earth with high and low tide getting tugged around by the moon. It is much harder to visualize solid rock as a viscus fluid. Are there any good computer models of this? I am now googling which planets and moons are tidal locked, or at least the extent to which they are locked, trying to figure out why some objects are more locked than others. Very interesting, thanks!

 

[Nugatory]

I am now googling which planets and moons are tidal locked, or at least the extent to which they are locked, trying to figure out why some objects are more locked than others

Tidal lock is progressive; the things that aren't locked now will be more locked a few hundred million years from now. So the differences you see are the result of different rates at which the rotational energy is dissipated, and that depends on many factors: gravitational gradient across the orbiting body, its composition, how much rotational energy the orbiting body started with...

 

[A.T.]

I can visualize the oceans on the Earth with high and low tide getting tugged around by the moon. It is much harder to visualize solid rock as a viscus fluid.

This recent paper estimates the Earth's body tidal torque to be around 10% of its ocean tidal torque:
https://www.sciencedirect.com/science/article/pii/S1674984718301009

 

[davenn]

, I guess I need to study tidal locking more. I can visualize the oceans on the Earth with high and low tide getting tugged around by the moon. It is much harder to visualize solid rock as a viscus fluid.

You are completely misunderstanding the term "tidal locking" It has nothing to do with fluids, ie nothing to do with the tides
as we see in the Earths' oceans

https://en.wikipedia.org/wiki/Tidal_locking

https://www.universetoday.com/123391/what-is-tidal-locking/

https://phys.org/news/2015-11-tidal.htmlhave a read and see if that clears up things for you

 

[mfb]

Rock isn't that strong, on large scales gravity wins. That's why all the planets are fairly round, and that's why tidal forces can deform the rocks of planets.

 

[russ_watters]

You are completely misunderstanding the term "tidal locking" It has nothing to do with fluids, ie nothing to do with the tides
as we see in the Earths' oceans

I feel like this is a much too strong statement. The force that causes the ocean tides is exactly the same phenomena as the force that causes land tides. The ocean tides on Earth may not contribute much to slowing Earth's rotation, but that's only because the oceans are a miniscule fraction of Earth's mass. Even a gas giant or star can tidally lock.

The mechanisms of energy loss are the only difference.