Moon from the Earth

[PhantomWolf]

Okay grabbed this from another thread where it was asked if the current idea of the Moon coming from a collision between Earth and a Mars sized planet was reasonble.

This brought a small question to mind. IIRC one of the reasons we are sure of the Moon missions is due to the unique features of the Moon rocks, especially that they are anhydrus, or were formed without the presence of water. Earth rocks all have evidence of having been formed in water whereas Moon rocks apparently don't have any evidence of such. If the moon was once part of Earth though, shouldn't it have some evidence in its rocks of water?

[kucharek]

Okay grabbed this from another thread where it was asked if the current idea of the Moon coming from a collision between Earth and a Mars sized planet was reasonble.

This brought a small question to mind. IIRC one of the reasons we are sure of the Moon missions is due to the unique features of the Moon rocks, especially that they are anhydrus, or were formed without the presence of water. Earth rocks all have evidence of having been formed in water whereas Moon rocks apparently don't have any evidence of such. If the moon was once part of Earth though, shouldn't it have some evidence in its rocks of water?

If I understand the theory corretly, the collision happened very early in Erath's history. The crust was still hot. Also, the energy of the collision heated the material very much, so all water evaporated from the materia that later formed the moon.

Harald

[Kullat Nunu]

If I understand the theory corretly, the collision happened very early in Erath's history. The crust was still hot. Also, the energy of the collision heated the material very much, so all water evaporated from the materia that later formed the moon.

Absolutely. One of the key evidence of the collision theory is that the Moon lacks volatiles, especially water. If the Moon formed directly by accretion it should have similar composition as the Earth (if it formed near Earth; on the other hand, there is no place in the Solar System where it could acquire similar composition). The Moon lacks heavy elements, too. Looks like it formed from material similar to the Earth's mantle, and the collision theory explains that nicely.

[John Kierein]

On the other hand, the moon is very lumpy, with lots of "masscons". This makes it look like it is a conglomeration of several smaller bodies.

[Jason Thompson]

On the other hand, the moon is very lumpy, with lots of "masscons". This makes it look like it is a conglomeration of several smaller bodies.

Like the accretion of bits of debris orbiting the Earth after a massive collision, perhaps? Could such debris have been only just molten when it accreted to form the moon, producing a more or less molten sphere with more solid lumps in it?

[Dgennero]

AFAIK, we know nothing about the composition of the intruder.
I suppose that during the collision, the core of the interloper settled down at the center of the earth and that, since the intruder was smaller than the earth (lower gravity), more of its mantle than of earth's mantle was ejected into orbit to form the moon. This accounts for the density differences as well as for any dissimilarities between the moon's and earth's mantle composition.

[umop ap!sdn]

But if there was such a Mars sized object, where did it go? Some of its material would have been incorporated into the Moon, but that still leaves a lot of mass unaccounted for.

[Dgennero]

A grazing blow supposed (a full hit might have destroyed both bodies), after losing part of the mantle some "chunks" of the intruder will have spun off into deep space (out of the solar system or into the sun), some reassembled to form the moon, and some were swallowed by the young earth.
I'll say more later, have to get offline for a sec.

[Dgennero]

The so-called "giant impact(or) theory" isn't all that new, but didn't become popular until 2001, when computer simulations showed that such an impact could indeed have formed the moon.
Of course there are countless sites describing or depicting this - just one example here: http://starchild.gsfc.nasa.gov/docs/...uestion38.html
I chose this for the well-known, stunning picture

[Kullat Nunu]

The so-called "giant impact(or) theory" isn't all that new, but didn't become popular until 2001, when computer simulations showed that such an impact could indeed have formed the moon.

Not earlier? I'm not so sure. Anyway, the theory was developed after the Apollo flights in 1975.

Of course there are countless sites describing or depicting this - just one example here: http://starchild.gsfc.nasa.gov/docs/...uestion38.html
I chose this for the well-known, stunning picture

Yeah, it's really cool. "Original" is here.

[The Shade]

But if there was such a Mars sized object, where did it go? Some of its material would have been incorporated into the Moon, but that still leaves a lot of mass unaccounted for.

The core of the impactor became incorporated into the core of the Earth. The moon formed from mantle material of both the young Earth and the impactor that was thrown into orbit following the collision. While some of the mass of the impactor was lost to space and some went into making the moon, the majority of it became part of the Earth.

The Earth-Moon combined angular momentum, the lack of volatiles in the Moon, the Earth 23.5 degree rotational tilt, and the Moon's 5.15 degree orbital tilt are the clues that led to what is now called the Giant Impactor Theory.

(Edited for grammar, and to correct moon's orbital tilt figure)

[Master258]

Yeah there are many places for the impacter to go. I say most went into the sun but I'm most-likely wrong.

[umop ap!sdn]

While some of the mass of the impactor was lost to space and some went into making the moon, the majority of it became part of the Earth.

Okay that makes sense, thanks!

...the Earth 23.5 degree rotational tilt, and the Moon's 18.5 degree orbital tilt are the clues that led to what is now called the Giant Impactor Theory.

Okay I'm confused. I know about the obliquity of the Earth's equator to its orbit, but as far as I know the Moon's orbit is inclined 5.16 degrees to the ecliptic. Or is it that the Moon originally orbited at an 18.5 degree angle and then got perturbed out by the Sun?

[The Shade]

Corrected. Thanks for the heads up.

[Kullat Nunu]

There were originally three different popular theories, everyone had their advantages and disadvantages. The Giant Impactor Theory nicely combines them all and gets rid of contradictions.

The Moon either
* formed around the Earth (The Condensation Theory; GIT: accreted from impact debris),
* got captured in an orbit around Earth (The Capture Theory; GIT: part of debris from the impactor), or
* split from the Earth (The Fission Theory; GIT: part of debris from the Earth debris).