Define moon
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Defining the Moon: Composition, Origin, and Structure
Composition and Formation of the Moon
High-Temperature Accretion
The Moon's composition suggests it accreted at higher temperatures and lower pressures compared to terrestrial planets. This is inferred from models that simulate the condensation of planets from a solar composition cloud, indicating that the Moon's initial orbit had a high inclination1.
Age and Isotopic Composition
The Moon is estimated to have formed around 4.52 to 4.50 billion years ago. This age is derived from the tungsten isotopic compositions of lunar samples, which show a range from chondritic to slightly radiogenic. This isotopic heterogeneity is likely due to late radioactive decay within the Moon itself2. Additionally, uranium-lead dating of lunar zircons supports the formation of the Moon within the first 60 million years of the solar system's existence, around 4.51 billion years ago7.
Origin Theories
Giant Impact Hypothesis
The most widely accepted theory for the Moon's origin is the giant impact hypothesis. This theory posits that the Moon formed from debris ejected by a large off-center collision between the early Earth and a Mars-sized body. This impact would have produced an iron-poor Moon and is consistent with the current masses and angular momentum of the Earth-Moon system4 5. The chemical composition of the Moon, including a small nickel-rich metallic core, supports this model, suggesting that about 80% of the Moon's material came from the Earth's mantle, with the remaining 20% from the impactor and a late-stage veneer5.
Alternative Theories
Before the Apollo missions, alternative theories such as the resonance theory were considered. This theory suggested that the Earth and a proto-Moon initially formed a single rapidly rotating body that eventually split into two. However, this theory has been largely discounted due to the lack of supporting evidence6 8.
Structure of the Moon
Seismic Data and Crustal Composition
Seismic data from the Apollo missions have provided insights into the Moon's internal structure. The lunar crust is about 60 km thick in the Mare Cognitum region, with a high-velocity gradient in the upper 20 km layer and a nearly constant velocity in the lower 40 km layer. The mantle is believed to consist of an olivine-pyroxene composition, and there may be a thin high-velocity layer beneath the crust3.
Core and Mantle
The Moon's core is estimated to have a radius of about 380 km, with an average core mass density of 5200 kg/m³. The core is likely partially molten, as indicated by the detection of shear waves reflected from the core9. The mantle above the core is divided into a lithosphere and an asthenosphere, with the latter being partially molten and attenuating shear waves3.
Conclusion
The Moon's formation and structure are best explained by the giant impact hypothesis, supported by isotopic dating and seismic data. The Moon's composition, age, and internal structure provide critical insights into the early history of the Earth-Moon system and the processes that shaped our solar system.
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The Origin of the Moon
The Moon may have accreted at higher temperatures and lower pressures than terrestrial planets, possibly due to a high inclination in its orbit.
Highly CitedAge and Origin of the Moon
The moon formed 4.52 to 4.50 billion years ago, with its mantle remaining poorly mixed, likely due to late radioactive decay within the moon itself.
Highly CitedStructure of the Moon
The Moon's crust is 60 km thick and layered, with an olivine-rich mantle and no clear evidence of an iron-rich core.
Highly CitedOrigin of the Moon in a giant impact near the end of the Earth's formation
The Moon formed near the end of Earth's accumulation, with a smaller, more likely object than previously considered viable, and is iron-poor.
Highly CitedThe origin of the moon and the early history of the earth—A chemical model. Part 1: The moon
The Moon's chemical composition is similar to Earth's mantle, with 80% of its mass coming from the Earth's mantle and 20% from the Impactor and late stage veneer.
Highly CitedEarly formation of the Moon 4.51 billion years ago
The Moon formed within the first 60 million years after the birth of the solar system, using precise uranium-lead dating of Apollo 14 zircon fragments.
Highly CitedVery preliminary reference Moon model
The VPREMOON model suggests a core radius of 380 40 km and an average core mass density of 5200 1000 kg/m3, with a liquid state in the outermost part of the core.
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