### Origin of Lunar Material
Three hypotheses have been proposed to explain the origin of the Moon, particularly focusing on why the Moon's isotopic composition (such as the ratio of oxygen isotopes and ) is nearly identical to that of Earth's mantle, while other solar system bodies have distinct isotopic signatures.
*Hypothesis 1*
The Moon formed from a single, high-velocity, grazing collision between the proto-Earth and a Mars-sized planetesimal named Theia. The impact ejected a disk of molten debris into orbit. Because of the grazing angle, the debris disk was composed almost entirely (more than ) of mantle material from Theia. The Moon then accreted from this disk. The isotopic similarity between Earth and the Moon is an accidental consequence of Theia having formed in a similar region of the solar nebula as Earth, sharing the same isotopic reservoir.
*Hypothesis 2*
A high-energy, high-angular-momentum collision between proto-Earth and Theia completely vaporized both bodies, creating a giant, rapidly rotating, donut-shaped structure of silicate vapor called a *synestia*. The synestia was a single, fully homogenized system where turbulent mixing equalized all isotopic ratios. As the outer regions cooled below the condensation temperature of silicates, molten droplets condensed and accreted to form the Moon, while the inner region contracted to form the Earth.
*Hypothesis 3*
The Moon is the product of approximately 20 successive, smaller impacts by planetesimals (- of Earth's mass) rather than a single giant impact. Each collision ejected a mix of proto-Earth mantle and impactor material, forming a debris disk that accreted into a "moonlet." Tidal forces caused each new moonlet to migrate outward and merge with pre-existing moonlets, eventually forming the Moon. The isotopic similarity to Earth is due to the statistical averaging of the varied impactor compositions and the fact that a large fraction of the ejected material in each smaller impact came directly from Earth's mantle.
Based on Hypothesis 1 and Hypothesis 3, which of the following statements represents a core difference in how the two hypotheses explain the isotopic similarity between the Earth and the Moon?
- Hypothesis 1 attributes the similarity to the coincidental composition of a single giant impactor, whereas Hypothesis 3 attributes it to the combined effects of multiple smaller impactors' compositions averaging out and a high proportion of ejected Earth mantle.Answer
- BHypothesis 1 claims that the Moon's isotopic signature was determined by a series of small, localized mixing events, whereas Hypothesis 3 claims that a single grazing impactor contributed all the Earth-like isotopes.
- CHypothesis 1 assumes that the Moon is made primarily of Earth's mantle, whereas Hypothesis 3 assumes that the Moon is composed entirely of planetary material from outside the Earth-Moon system.
- DHypothesis 1 claims that the Moon's isotopic composition is identical to Earth's because of turbulent vapor mixing, whereas Hypothesis 3 claims that the isotopic similarity is due to a single impactor forming in a different region of the solar nebula.