How long ago did the Moon stop rotating...

I recently listened to a series of lectures by the geologist Robert Hazen (Robert Hazen - Wikipedia) - he suggested (if I recall correctly) that less than 2% of the original water of Earth survived the collision.

As Markn+ clarified:

A simplistic view would be that the Moon was at first rotating but as it slowed, it reached a point where it didn’t quite complete a rotation and instead behaved like a pendulum that was swinging +/- 179 degrees. After that the amplitude of the swings would decrease to the level of libration we see today. However, the Moon’s orbit is elliptical so I expect there would have been a brief period when it was a more complex mix of partial and complete rotations.

A separate interesting question would be how the eccentricity is affected by this process. My guess is that the tides are higher hence the Moon gets more of a ‘boost’ when closer which would tend to reduce the eccentricity. That would seem to be consistent with it forming from ejected material which is equivalent to launch from an Earth-grazing orbit.

That would be one of the arguments for asteroidal water. Yet the article makes it appear that in situ water as the source of the oceans has not been ruled out. That’s what I don’t understand.

If the original total water content was 10 times more than the present oceans, that would mean 20% is original while 80% arrived subsequent to the impact. A combination of the mechanisms does seem realistic though it’s not clear experimentally which dominates.

Where are you getting your claim that the water content on the original earth was 10 times more than today?

Not to nitpick, but after the sun swells up into a big red giant then blows off then collapses down into a little dwarf, wont the earth and the moon pretty much be melted slag?

Where would that much water vanish off to?
10 times more and i dont think anything would stick up above the water

Swept out of the solar system by the solar wind.

I’m not endorsing or questioning the “10x as much water” statement, but I have to say “so?” There is no reason that all ground couldn’t or shouldn’t be below water. And since the continental crust built up slowly from recycling of oceanic crust, the average elevation of ground on the early Earth was much lower than it is today, so even today’s amount of surface water may have been enough to completely cover the surface.

And the Earth really has a paucity of water anyway. There are moons that have more water than the vastly larger Earth. (Of course, lots of Earth’s water is in the mantle.)

The point is that we’re talking about the first 10 million years of the earth’s existence. There was no such thing as continental crust. Presumably, none of the water would be surface; instead all that water would have been embedded in the solids or liquids, depending how hot things happened to be.

If so, where did all that water come from? Was it part of the primordial gas clouds? Shouldn’t the other planets have had a similar percentage of water? Did they also all lose it? If so, how?

I’m not saying there are no answers. I’m saying that nobody has yet even tried to answer the questions this raises. Just pulling a number out of thin air doesn’t answer anything.

It was never intended as a “claim”, only pointing out that “2% of the original water of Earth survived the collision.” means nothing unless you know how much the original quantity was.

There have been several Straight Dope threads on this subject in the past, but these are still largely open questions.
http://boards.straightdope.com/sdmb/showthread.php?t=806537
Where does water come from? - Factual Questions - Straight Dope Message Board
Conservation of water molecules. Where and when do water molecules break down? - Factual Questions - Straight Dope Message Board

Certainly the four terrestrial planets (including the Earth) are very dry, but the outermost worlds contain plenty of water, mostly in the form of ice - Uranus and Neptune, and all the icy moons and Kuiper belt objects taken together contain fabulous amounts of water. This is supposed to be the result of formation beyond the ‘frost line’ where water is normally in the form of ice, so is much less volatile.

Jupiter, on the other hand, appears to have less water than expected in its upper atmosphere, so there is something else going on there, that may be answered by the results from the Juno probe.

After the collision that formed the Moon a lot of the Earth’s water would have been lost to space - but some of it would be deep underground, in hot hydrogenated minerals that would be largely unaffected by the impact. And after the impact some of the lost water would have fallen back down to Earth - probably only a small fraction. And some water came afterwards in the form of comets. Current theories are downplaying the role of cometary water, at least partly because of the results of the Rosetta mission. So it seems that outgassing may have been more important than subsequent delivery.

“solids or liquids, depending how hot things happened to be”

To be clear, water is still water if it is mixed in with other things in a fluid state.

But really it isn’t water that matters it is Oxygen and Hydrogen’s fairly common fairly low energy state. Free Hydrogen and Oxygen will tend to become water given enough energy to break up higher energy state compounds like, for a modern example trees.

Cellulose is C6-H10-O5, note how is is just the parts that make up water and some carbon? And when you “burn” wood with extra oxygen you get mostly CO2 and Water back along with charcoal (carbon and ash).

While there still unanswered questions it is quite clear that we had an atmoshpere, and in fact that is the differentiator between some other planet which either didn’t have the mass to have an atmosphere to contain hydrogen and oxygen or which lacked a magnetic field and thus the solar winds stripped them of that atmosphere over time.

This is pretty easily demonstrated with argon and helium, which are light enough that they escape our atmosphere and the earths supply is almost exclusively from radioactive decay. Helium is more than 20% more common in the universe but rare on Earth because of this.

The most common elements in the universe are:

Element Mass fraction PPM

Hydrogen 739,000
Helium 240,000
Oxygen 10,400
Carbon 4,600

This means that you will have Water, CO2 and hydrocarbons, they are the lowest energy state.

As we have lots of Iron and other heavy metals that result in the nice magnetic field which allowed us to maintain an atmosphere Water and Hydrocarbons would not be at the core of the planet or evenly mixed because those elements weigh less.
So while there may be questions to be answered it is highly unlikely that there was not water vapor or liquid water as the planet coalesced. As there is a tremendous amount of energy released as a planet forms which would be released as photons or heat it would be most likely that all available oxygen and carbon stored in more complex hydrocarbons would have been converted to CO/CO2 and H20.

This idea is far from being purely speculative.

What if it was counter-rotating?

Would it slow to zero, then speed up to 1 rev per month?

Tidal forces result in the transfer of angular momentum between bodies, the result of those transfers of energy depend on the conditions but in general the following is true.

If two bodies are in a stable orbit they will be tidally locked (ignoring the fact that in some cases the timeline will exceed the stars life etc)

If either body is rotating the tidal forces will typically transfer energy the orbiting system.

If the orbit is in retrograde the angular momentum will be transferred to one of the orbiting bodies, As an example Phobos is in a retrograde orbit with Mars and is increasing the rotational speed of Mars, causing the Martian day to decrease in duration.

If the orbit is in prograde the two bodies will tend towards tidal lock while increasing the orbital distance. This is what is happening with the Earth Moon system and if the Sun were not to go super nova the earth would become tidally locked with the Moon. This is why an Earth day is about 1.7 milliseconds longer than a century ago.

Except for external influences the angular momentum of our solar system has been preserved from the time that it was just an accretion disk. Some of this is in the 24.47 day solar rotational period, some is in the earths rotation but it really depends on the orientation of the orbiting bodies.

Our sun will never become a supernova. Its fate is to become a red giant, then a white dwarf. No booms.

Thanks for pointing out that error; Should have say expand to engulf the earth.

The point being that the time required to tidally lock to the moon is long enough that it will never happen.

Earlier in the thread I mentioned computer simulations suggesting that the moon could have formed at a distance of 25,000 km with a day on Earth lasting 6 hours. At that distance from the Earth, a lunar orbit is a little less than 11 hours. Those two numbers are pretty close–if the Earth had happened to spin a little slower, and the moon formed a little closer, the two figures could have matched, making the moon geosynchronous to the Earth. I’m trying to mentally model what would have evolved from those initial conditions. If Earth and the moon had become coincidentally mutually tidally locked very near the beginning, would they have remained that way (and that distance) from each other long term, or would some factor have broken the lock (and sent the moon spiraling outwards?)

OK I am going to have to dramatically simplify, so other posters this will ignore some realities.

But lets say you could some how magically tidally lock the earth with the moon right now over lets say Central America without effecting anything else. The earth would still have a day, but it would be roughly 29 1/2 days long.

The Earth and the Moon are a system, the moon really doesn’t orbit the earth they rotate around a common axis called the barycenter. When you are tidally locked with a body you just don’t rotate in relation to that other body but that doesn’t mean that the system as a whole doesn’t rotate in reference to something like a star.

By really rough calculations that are just a WAG the wobble of the Earth Moon system would be about 270 miles from the center of mass of the earth if both were tidally locked at current geosynchronous orbit.

Of course (ignoring the angular momentum that causes the earth to turn today) that 29 and 1/2 day would be much faster if it was that close just like an ice skater pulling in their arms. Currently the barycenter of the Earth/Moon system is about 2,902 miles from the Earth’s center so things would be spinning a lot faster.

Geostationary satellite don’t have enough mass to really effect the Earths rotation but the Moon would.

As a graphical representation here is a view of Pluto and Charon which are much closer in mass to each other.


To put this in perspective the Earth/Sun system wobbles by 449 km or 279 miles too assuming there was a distant observer.

But to maybe phrase this another way, if the Earth was still rotating in reference to the moon there would still be tidal forces, if they were both tidally locked that angular momentum still needs to be preserved so they would rotate together in relation to the Sun.
Here is another video that may show this although it is really about a binary planet system with a much closer mass ratio.

But this is where the comfy Newtonian model starts to break down but I will try. The tidal force and the Moon moving away from and slowing down the Earth is due to the fall off of gravity that is kind of like an inverse cube law.

The force applied would approximate the following. But those forces are due to the closer side being “accelerated” more than the farther side, and it is like grabbing onto a spinning shaft.


Due to various factors, and slightly modified by the liquid oceans but only a small portion the force applied to the moon lags behind a little bit.

It is similar to the red oval in the following image.

That force differential gives a continual “push” to the Moon moving it faster in it’s orbit from a heliocentric reference frame and because the Moon is moving faster it assumes a higher orbit in the Newtonian model.

TLDR; If two bodies are tidally locked, there isn’t a source of momentum that will result in a change in orbital distance. This is over simplified and ignores the solar tidal forces, frame dragging etc…

Over the weekend I was wondering about what a tidally-locked Earth would look like from the moon if it had it’s current axial tilt, so I put a camera around a rotating Earth, and got this. (The video played fine on the computer where I made it, but is choppy on the computer I’m posting from, so I’m not sure how it will do for anyone else.)

So the Bible was right! When all those pesky scientists say ‘Where did all the water go after Noah’s flood?’, they should have just asked their colleagues down the hall.