Talk:Lunar mare

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[edit] Tidal locking

I have removed the following:

"This bias of distribution is thought to have assisted in the tidal locking of the Moon's rotation to its orbit (the phenomenon whereby the Moon spins on its axis in the same timespan as it takes to orbit the Earth). This results in only one side of the Moon being visible from the Earth (excepting some small leeway from libration). The reason the maria have assisted in tidal locking is that they are denser than much of the rest of the surface and are therefore more strongly attracted towards the Earth by gravity. Over many eons, the Moon's rotation has slowed so that the heaviest side of the Moon with the maria on it faces constantly towards the Earth."

This is wrong for several reasons. First, gravitationally, the lunar orientation is just as stable in its current configuration as in a state where it is rotated by 180 degrees. This means that it is only by chance that the mare basalts are predominantly on the hemisphere that faces the Earth. Second, tidal despinning occurs only because of the spherical harmonic degree two gravitational field (this is simply a triaxial ellipsoid in shape). It can be shown that the mare basalts make little to no contribution to the current degree-2 gravitational field of the Moon.

Mark Wieczorek Oct 29, 2006

I do not believe the maria are predominantly on the Earth-side of the moon by chance, but the explanation you (rightly) removed reverses cause and effect: the side of the Moon facing the Earth had a thinner crust due to tidal effects, and so lava could break through more easily and from the maria. This is my understanding of it, at least. --ScottAlanHill 07:41, 30 October 2006 (UTC)

As a geophysicist, I have searched in vain long and hard for a mechanism by which tidal forces could redistribute materials from one hemisphere of the Moon to another. Of course, the possibility exists that tidal effects somehow caused primordial crustal thickness variations during crystallization of the magma ocean, but the mechanism is at this point unknown.-- Lunokhod 10:50, 30 October 2006 (UTC)

This is discussed in the astronomy textbook I'm teaching out of, so if it's wrong I'd like to know. The theory is that the mantle is slightly offset with respect to the crust, due to the gravitational influence of the Earth. (I'm not sure it could be considered a tidal effect, as I said above.) The mantle is supposed to be slightly closer to the Earth, so that the crust on the "front" of the moon is thinner than on the back, which made it easier for lava to escape. (Is the thickness variation true?) I guess I don't understand exactly how that was supposed to work, either: if we consider the crust and the mantle as being two independent spheres orbiting at the same speed, the radius of their motion should be independent of their mass and density. Centrifugal force would actually result in heavier materials moving away from the center (naively considering the example of the centrifuge). --ScottAlanHill 22:40, 1 November 2006 (UTC)

[edit] Pronunciation

The article gives the pronunciation of mare but not maria; I tend to use MAH-ria, but I can also imagine saying it mah-REE-ah like the woman's name (and certainly this is acceptable Latin pronunciation, cf Ave Maria). Which is it?