So as I understand it the moon was formed when a chunk of earth was blown into orbit when a mars like planet collided with earth, if this is the case why is the composition of moon rock not similar to the composition of rock found on earth, when the moon if just a piece of the earth its self?
In: Planetary Science
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Based on your question, it sounds like you think the Moon came from something like a wedge or chunk *broken off* from the Earth. Scientists currently think what actually happened is, a planetary body had a *glancing* collision with the Earth, liquefying the proto-Earth’s surface and most of the impactor’s material and tearing the impactor apart. This created a huge spray of material around the Earth, coming from *both* the impactor and the proto-Earth. The Earth reformed and cooled; *most* of the sprayed-out material fell back to it, but a portion of it was able to form a stable orbit as a satellite.
Because of this glancing blow, and the liquefaction of (part of) both bodies, the material of the two planets intermingled. The Moon’s development changed some of its composition (as others noted, the heavy stuff sank to the core, for example), but it’s got a mix of both the impactor’s material and Earth’s material, just like the Earth today does.
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The composition of the moon IS similar to the composition of the Earth. That’s literally the what clued us in on the Moon being formed from Earth material, as opposed to forming separately from and independent source.
But on the other hand, it formed from a massive collision between the Earth and another planet right at the dawn of the solar system ahd hasn’t changed much since, while the Earth itself has been recycling it’s surface continuously since then, so it’s a convenient window to the past.
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Most of the surface rock on Earth wasn’t formed that long ago, geologically speaking, and certainly long long after both the Earth and moon cooled and formed their own relatively stable surface layers. The rocks we have on the surface of the Earth are formed from a lot of geological processes that involve not only the pressure from the earth itself (Granite) but also the presence of life (Limestone) and the actions of active volcanoes (Basalt). It’s also greatly affected by weathering from wind and water.
What makes the rocks from the moon so different is that they didn’t really have the same situation as there is on Earth. So the dust on the moon is sharp and jagged like fiberglass because there’s no weathering. Some of the rocks were formed from impacts with meteors or comets and, because there’s not much turnover, they’ve essentially stayed there the whole time unless moved by something else.
Regardless of the source, as others have addressed that, Moon rocks are very different from Earth rocks because of weathering and cycling. Water and weather have had large effects on the elements in Earth rocks, and their shapes. Everything on the Moos in razor sharp as a result. The Moon is also dead, there is no tectonic activity over a molten core. The Earth has plate tectonics, which produce lava and other metamorphic rocks; it essentially recycles rock into and out of the molten core.
These are the differences that made Moon rocks special, they are closer to their original state.
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