Why is the Moon made of the same things as Earth

[Findthepin1]

I don't understand this. There are two facts I know that are relevant to this. A; the moon was formed in a glancing blow collision between Earth and Theia. B; the moon is made mostly of Earth.

If the collision was indeed a glancing blow, why is the moon's geology so similar to that of Earth? The material that went into Earth orbit should be mostly Theia. 

[Gaarst]

The impact of such a large body on Earth did have some effects on Earth itself. It's not just Theia crashing into Earth and sorta bounces off and make the Moon.

When Theia impacted the Earth, it was brutal: Earth was pretty much torn apart and large chunks of its mantle were sent into space. The parts of Theia that fused with Earth changed its composition and the parts that were sent in space eventually merged together and therefore gave the Moon a similar composition to that of Earth.

[Motokid600]

Both bodies were more or less completely liquified in the impact. All the material got mixed up real good.

[Scotius]

Not to mention, Theia probably formed from similiar stuff than Proto - Earth did. Moon probably contains some stuff that was part of Theia too - but it doesn't make much difference.

[Camacha]

Current theory is that it was less of a glancing blow and more of a full on collision. That would explain both bodies being of a very similar composition.

New research by

UCLA

based on rock analysis sourced from

Apollo

missions

12

,

15

and

17

, suggests Theia collided head-on with Earth,

^[2]

in contrast to the previous theory which suggested a glancing impact.[/]

Source.

[Temstar]

It's not that Moon was made from Earth material after Theia impact, it's more both Moon and Earth was made from material that came out of the proto-Earth / Theia impact.

Proto-Earth for example had a smaller iron core than our Earth today. Our big iron core today which produces the nice strong magnetic field that protect us is the combined product of both cores from proto-Earth and Theia.

[kerbiloid]

Because proto-Moon (or "Theia" if you want) hasn't crashed into the Earth.
It was captured, tidally locked on the Roche limit and melted.
The most part of it (including its iron core) has been destroyed, captured by the Earth and uttelry dissolved inside the Earth mantle during the following geological transformation.
While the outer tide bulge, consisting of the lightweight silicates, has been thrown away from the Roche limit by recoil and froze into a stoneball we know as "Moon".
As both planets have been formed at the same distance from the Sun (so from the same dust), the Moon is made of the same silicate material as the Earth's mantle.
But as the Moon, rather to the Earth, was totally melted, the Moon material differs from the Earth one. The Earth never had been totally melted, only its layer inside the mantle above the core.
(According to Sorokhtin O.G., Ushakov S.A., "The Earth Evolution").

Edited February 8, 2016 by kerbiloid

[sevenperforce]

Kerbiloid's explanation is not considered a likely one by the scientific community.

Earth has an oversized core and will remain geologically active for a long time thanks to subsuming Theia's core during the impact. The moon has an extremely small and inactive core because the collision ejecta which ultimately coalesced into the moon was mostly material from the crusts/mantles of Theia and proto-Earth rather than heavier, denser core debris.

As others have said, Theia likely formed from the planetary disc in a similar fashion to proto-Earth, which is why their compositions were already fairly similar. 

I wonder what the gravity of proto-Earth would have been.

Off-topic, but given the mechanics of impact formation, perhaps angular momentum could serve as a defining line between what constitutes a planet-moon system and what constitutes a binary planet system. If the combined mass and angular momentum of the system would make it impossible for a single body to remain stable, it's a binary system; if not, it's a planet+moon system.

[kerbiloid]

Earth remains geologically active for a long time due to the process of gravitational differentiation, when the original mixed silicates+FeO substance is separated into "heavy and sinking down" Fe+FeO material and "light and floating up" silicates. Due to this, appears the iron core of any solid planet and its silicate mantle.
While this process, the total potential energy decreases and the difference is being released as a heat which lifts up to the surface, not because of "Theia" impact heat lasts for billions years.

[sevenperforce]

4 hours ago, kerbiloid said:

Earth remains geologically active for a long time due to the process of gravitational differentiation, when the original mixed silicates+FeO substance is separated into "heavy and sinking down" Fe+FeO material and "light and floating up" silicates. Due to this, appears the iron core of any solid planet and its silicate mantle.
While this process, the total potential energy decreases and the difference is being released as a heat which lifts up to the surface, not because of "Theia" impact heat lasts for billions years.

Obviously.

The impact was what gave Earth a heavier core with more "mixed-up" heavy minerals, allowing it to sustain geologically activity for eons. The moon, coalescing from mostly-mantle/crust material, lacked the high mineral density and thus became geologically inert long ago.