Our Moon

xMonty

Is our moon slowly receding? is it getting further from us? if so why?

HallsofIvy

Yes, at about 3.8 cm per year.

See http://eclipse.gsfc.nasa.gov/SEhelp/ApolloLaser.html.

S.Vasojevic

Ok, I read the article, and they suggest that Moon, they believe, may harbor liquid core. But they never say what it could be. So, what it could be, magma? I thought that moon is geologically dead.

qraal

They thought it would probably be dead, but all the evidence suggests there's significant residual heat closer to the core. There's probably some magma around a small iron core.

As for why the Moon is receding, that's due to conservation of angular momentum - as orbital energy is lost due to tidal interactions with the Earth, the Moon has to move outwards to conserve angular momentum. Energy is inversely proportional to the radius separating the two, while angular momentum is directly proportional to the square root of the radius - thus as one decreases, the other increases. Eventually the two will lock into a synchronous orbit - BUT solar tides will cause energy loss to continue, so the Moon will have to spiral back inwards to compensate. Eventually it will approach the Earth and break-up... or it will if the red-giant Sun doesn't fry them both. Total time to Moon break-up is roughly ~ 15 billion years or so.

Tongue-in-cheek J.B.S.Haldane predicted the accelerated break-up of the Moon because the people on Earth enhanced the tidal coupling between the two to increase the power output (for central heating.) Thus the Moon will spiral in and break up c. 3,000,000 AD, causing the inhabitants to emigrate to Venus (which was considered more clement back in the 1920s when Haldane wrote that little piece.)

Ophiolite

Since there isn't any magma around the Earth's large iron core why would you expect around the Moon's?

Sorry!

Woah what? There's no magma around Earths solid core? References please.

diogenesNY

Just guessing here, but I _suspect_ that the disconnect may be a semantic one:

Earth's core is a solid metalic surrounded by a liquified molten iron (and some other heavy metalic crud) molten core.... thus the dynamo effect giving us that lovely magnetic field that keeps us from getting fried....

...as distinct from _basaltic magma_ (and a few other kinds) which is what is often what is meant in casual conversation when speaking of 'magma', notwithstanding more precise definitions and terminology.

If I am off base on what was meant, however, please call it.

diogenesNY

Ophiolite

Consult any basic textbook on geology. (I presume the routine on this forum is the same as on other forums i.e. the person making the extraordinary claim is the one required to provide the references, not the person stating the facts, so it's really up to you to provide references for the idea of magma around the core.)

The mantle is solid. In the upper mantle partial melting occurs to a sufficient degree to enable the generation of magma that will then, through buoyancy, migrate towards the surface. Even in the low velocity zone, at the base of the lithosphere, the melt-solidus proportion is not high and the small concentrations of partial melt are distributed through the crystalline matrix in the form of intergranular films and small droplets. (1).

The primary evidence for the solid mantle is of course its ability to pass S-waves, which cannot be transmitted by a fluid. The general character of the mantle, solid but denser than crustal rocks, was identified in 1909 by Andrija Mohorovic (2). Earlier, in 1906, Richard Oldham had recognised the liquid nature of the outer core because its inability to transmit S-waves.

For a fuller treatment of these matters you may wish to consult

Gerald Schubert et al, Mantle Convection in the Earth and Planets, Cambridge University Press 2001 ISBN 0-521-35367-X

Now perhaps you are thinking of the promising research (4) that has identified ultra-low velocity zones at the base of the mantle at the CMB (Core Mantle Boundary). These are certainly consistent with partial melts at the CMB that could initiate the mantle super plumes responsible for the convection driving the plate tectonic system. However, saying there are localised partial melts is not the same thing as saying the core is surrounded by magma. So, if you have some research I am unaware of that claims this I would be delighted to see it.

References:
1. Trucotte,D.L. Magma migration. Ann.Rev.Earth Planet.Sci. 10, 397–408. 1982
2. Andrija Mohorovicic, Yearbook of the Meteorological Observatory, Zagreb, 1909
3. Richard Dixon Oldham, "The Constitution of the Interior of the Earth, as Revealed by Earthquakes" Quarterly Journal of the Geological Society; 1906; v. 62; issue.1-4; p. 456-475
4.Garnero, E.J et al, Fine-scale Ultra-low velocity zone layering at the core-mantle boundary, in Yuen, D. et al, Superplumes - Beyond Plate Tectonics Springer 2007 p. 137-157

Edit for Diogenese, who posted while I was still typing:
You almost seem to imply that the mantle is basaltic in composition (I don't think you meant that - it might just be read that way). It isn't. You can derive a basaltic magma from it by partial melting (as you can on Mars and Venus too), but that is not at all the same thing.

Sorry!

yeah seems just a semantics issue on my part. sorry.

qraal

Luna "molten rock" aka "magma" is typically described as being deep, near the *solid* iron core.

Bob S

The angular momentum of the Earth-Moon system (Earth rotation plus Moon orbital angular momentum) is a conserved quantity. Tides on Earth caused by the Moon are slowing down the Earth's rotation. The Earth's rotational angular momentum is being transferred to the Moon's orbital angular momentum.
Bob S

MikeyW

I remember reading about the theorem (Tisserand's criterion rings a bell) that the moon will never actually leave the Earth's orbit, and is confined to within a certain orbital radius as a result of the findings of the circular-restricted three body problem. It was shown that the moon is in a stable orbit that will never reach a certain critical "zero-velocity surface", I think this is summarised in Moulton's book on celestial bodies. I don't know if the assumptions made are invalid in our system,

qraal

Sounds about right. Of course a key assumption is that no other celestial body enters the system and adds angular momentum...