Our moon, as you may know, is believed by many in the field to be a ‘chip off the old rock’. ie, once a piece of the Earth.
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“Origin of the Moon refers to any of the various explanations for the formation of the Moon, Earth’s natural satellite. The leading theory has been the giant impact hypothesis.[1][2]
However, research continues on this matter, and there are a number of variations and alternatives.[1]
Other proposed scenarios include captured body, fission, formed together (condensation theory), planetesimal collisions (formed from asteroid-like bodies), and collision theories.[3]
The standard GIH (Giant Impact Hypothesis) suggests a Mars-sized body called Theia impacted Earth, creating a large debris ring around the Earth which then formed the system.[1]
However, the Moon’s oxygen isotopic ratios seem to be essentially identical to Earth’s.[4] Oxygen isotopic ratios, which may be measured very precisely, yield a unique and distinct signature for each solar system body.[5]
If Theia had been a separate protoplanet, it probably would have had a different oxygen isotopic signature from Earth, as would the ejected mixed material.[6] Also, the Moon’s titanium isotope ratio (50Ti/47Ti) appears so close to the Earth’s (within 4 ppm), that little if any of the colliding body’s mass could likely have been part of the Moon.[7][8]”
https://en.wikipedia.org/wiki/Origin_of_the_Moon