Lunar mystery solved, near side Vs far side:

Originally posted by Paul Dirac II
My intuition suggests the moon didn't come into tidal-locking with earth until long after Earth cooled down from the collision.

Are there any relevant facts that guide that intuition?

I think the time it takes to obtain tidal locking depends on the initial rotation rate and how close the objects are. We know roughly how close they were.

Originally posted by twhitehead
Are there any relevant facts that guide that intuition?

I think the time it takes to obtain tidal locking depends on the initial rotation rate and how close the objects are. We know roughly how close they were.

What is really going on is Earth and Moon are tidally locking to each other, but it will never happen in the lifetime of the Solar system, assuming the Theia collision theory is true, the Earth had a 3 or 4 hour day when it started up 4 odd billion years ago and has been working its way down to the 28 day rotation period of the moon. Apparently about 600 million years ago the day was 21 hours long and now 24 and change and the interaction of the moon and Earth is speeding up the moon at the expense of Earth's rotational momentum so it is slowing down, day getting longer and the moon receding about an inch a year or so.

Originally posted by sonhouse
What is really going on is ....

Yes, I know most of that. What we do not know however is when the moon became tidally locked, because we have no way of knowing its initial rotation rate. Unless there is something I am missing.

Earths rotation is speeding up the moon, causing the two to drift furthur apart. Presumably the same or opposite was true when the moon was still spining. Would this put upper limits on its possible rotation? eg and argument like: if it took a long time to tidally lock then it should be much further out.

Originally posted by Paul Dirac II
My intuition suggests the moon didn't come into tidal-locking with earth until long after Earth cooled down from the collision.

Mercury is tidally locked to the Sun. The gas giants all have moons which are tidally locked to them. It does not require a solid earth for the moon to become tidally locked. The tidal locking requires that the smaller object becomes a prolate spheroid, otherwise the tidal locking won't happen. This will have happened while the moon was still molten. The moon will have solidified before the earth as it's smaller. So the earth probably hadn't reformed its crust when the locking happened, depending on how long it took the moon to form.

Originally posted by twhitehead
Are there any relevant facts that guide that intuition?

No, other than the rule of thumb that large things change slowly. But of course that argument works both for a very gradual slowing of the moon's rotation and a very gradual cooling of Earth.

For whatever reason, my intuition suggests millions of years for the former and mere decades for the latter. But my intuition shouldn't carry weight with anybody. 🙂

Originally posted by DeepThought
Mercury is tidally locked to the Sun. The gas giants all have moons which are tidally locked to them. It does not require a solid earth for the moon to become tidally locked. The tidal locking requires that the smaller object becomes a prolate spheroid, otherwise the tidal locking won't happen. This will have happened while the moon was still molten. ...[text shortened]... 't reformed its crust when the locking happened, depending on how long it took the moon to form.

Do you know if the gravitational tides put on Mercury are greater than the tides put on the moon by Earth?

It doesn't look like a slam dunk that Mercury would have greater tides since Earth is so much closer to the moon than Mercury is to the sun.

Originally posted by sonhouse
Do you know if the gravitational tides put on Mercury are greater than the tides put on the moon by Earth?

It doesn't look like a slam dunk that Mercury would have greater tides since Earth is so much closer to the moon than Mercury is to the sun.

Use a speadsheet to work it out. You can get the figures off Wikipedia. Assuming I haven't made a dappy mistake:

Earth's surface: 1 g
Sun's surface: 27.91 g (acceleration due to gravity on the Sun's surface in g's)
Mercury due to sun: 0.006 g
Earth due to sun: 0.0006 g (the same as moon due to sun)
Mars due to sun: 0.0002 g

And the shocker, Moon due to earth: 0.00024 g = 40% of acceleration due to the Sun.

Which means the force due to the sun's gravity on the moon is about 2 and a half times that of the force due to the earth's gravity.

The acceleration due to the sun on Mercury is 25 times that of the acceleration of the moon due to the earth.

The tidal locking had to occur when the moon was much closer to the earth, otherwise it wouldn't have happened. If the Moon started off 10 times closer the force would have been 100 times greater.

Originally posted by Paul Dirac II
No, other than the rule of thumb that large things change slowly. But of course that argument works both for a very gradual slowing of the moon's rotation and a very gradual cooling of Earth.

For whatever reason, my intuition suggests millions of years for the former and mere decades for the latter. But my intuition shouldn't carry weight with anybody. 🙂

You think the Earth's crust cooled and solidified in decades???

Try tens of millions of years at least.

The vacuum of space is a really good insulator.

After 4.5 BILLION years only the top few tens of miles has solidified.
Everything else is still molten.

If you assume an average thickness of 50km, then you get an average rate
of solidification of 0.01 mm per year.

Now it obviously hasn't been solidifying at a constant rate, as the crust cooled the
rate of heat loss and thus further solidifying would have reduced as well.
But it does rather suggest that solidifying the surface should have taken orders of
magnitude more than mere decades.

Originally posted by DeepThought
Use a speadsheet to work it out. You can get the figures off Wikipedia. Assuming I haven't made a dappy mistake:

Earth's surface: 1 g
Sun's surface: 27.91 g (acceleration due to gravity on the Sun's surface in g's)
Mercury due to sun: 0.006 g
Earth due to sun: 0.0006 g (the same as moon due to sun)
Mars due to sun: 0.0002 g

And the shoc ...[text shortened]... happened. If the Moon started off 10 times closer the force would have been 100 times greater.

The moon is supposed to have formed 10~20 times closer than it's current orbit.

Originally posted by googlefudge
The moon is supposed to have formed 10~20 times closer than it's current orbit.

Yes, the figure 10 was in my mind, but I couldn't be bothered to look it up so I just covered myself by putting an "if" in.

Originally posted by googlefudge
The vacuum of space is a really good insulator.

For conduction and convection, sure. For radiation, not so much. You'd have a surface temperature of thousands (?) of Kelvins radiating a huge flux of radiant energy toward 3 K space in all directions except toward the sun. So you've got a large amount of post-impact thermal energy, but also a large about of e.m. flux coming off Earth.

That said, I am completely at peace with the idea that my intuition about the time-response of planetary bodies post-collision could be way off-base.

Edited to add one more item: I imagine the current thermal state of the core and mantle of Earth have a whole lot more to do with their fissile element content than with the aftermath of the collision billions of years ago.

Edited to add another item: My intuition may be misled by the inverse-first-power scaling of the surface-area-to-volume ratio of a sphere. Heat up a 1 mm ball of clay to a red glow in a space probe's furnace, and release it into the vacuum of space. It might be dark within a few seconds. Release a 1 meter diameter red-hot ball of clay into space, and it might take minutes to stop glowing. Go up to the size of a planet, and that cooling time would be dramatically larger.