9.4 The Origin of the Moon
3 min read•june 12, 2024
The Moon's origin has long puzzled scientists. The leading theory, the , suggests a Mars-sized object collided with early Earth, ejecting debris that formed our lunar companion. This idea explains the Moon's composition and its gradual drift away from Earth.
Earlier theories like fission, capture, and co-formation fell short in explaining the Moon's unique characteristics. The , however, aligns with the Moon's similarity to Earth's mantle, lack of iron core, and higher concentration of certain elements, making it the most plausible explanation.
The Origin of the Moon
Giant impact hypothesis for Moon
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- Proposes Moon formed from debris of collision between early Earth and Mars-sized object
- Collision occurred ~4.5 billion years ago
- Impact was glancing blow rather than direct hit
- Collision caused portion of Earth's mantle and impactor's material to be ejected into Earth's orbit
- Ejected material formed disk of debris around Earth
- Debris in disk eventually coalesced through to form Moon
- Newly formed Moon was much closer to Earth than today and has been gradually moving away over time
- This gradual separation is related to the conservation of in the Earth-Moon system
Early Moon origin theories
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- Suggests Moon was once part of Earth and broke off due to centrifugal force
- Not well-supported as Earth would need to have been spinning extremely fast to throw off Moon-sized object
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- Proposes Moon formed elsewhere in solar system and was later captured by Earth's gravity
- Unlikely as Moon's composition is too similar to Earth's to have formed independently (silicate rocks)
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- Suggests Moon formed together with Earth from same cloud of gas and dust
- Does not explain Moon's unique composition (lacks iron core) and problem in Earth-Moon system
Moon composition vs formation theories
- Moon's composition similar to Earth's mantle but lacks iron
- Supports giant impact hypothesis as Moon would have formed primarily from Earth's mantle material and impactor's material
- Lack of iron suggests Moon did not form from Earth's core material, challenging fission hypothesis
- Moon has lower density than Earth
- Supports giant impact hypothesis as Moon would have formed from mixture of Earth's mantle and impactor's material, which would have lower density than Earth's core
- Challenges co-formation hypothesis which would predict more similar density between Earth and Moon
- Moon has higher concentration of (calcium, aluminum, titanium) compared to Earth
- Supports giant impact hypothesis as these elements would have condensed at higher temperatures and would be more likely present in debris disk following impact
- Challenges capture hypothesis as a captured Moon would not necessarily have higher concentration of refractory elements compared to Earth
- of Moon rocks is very similar to Earth's, supporting the giant impact hypothesis
Moon's evolution and characteristics
- occurred after Moon's formation, resulting in a small iron core and large silicate mantle
- formed from ancient lava flows, filling large impact basins on the Moon's surface
- caused the Moon to always show the same face to Earth, synchronizing its rotation with its orbital period
Key Terms to Review (16)
Accretion: Accretion is the process by which particles in space stick together to form larger bodies, such as planets and stars. This occurs through collisions and gravitational attraction, leading to the growth of celestial objects.
Accretion: Accretion is the process by which matter, such as dust, gas, or smaller objects, accumulates over time to form larger bodies, like planets, stars, or galaxies. It is a fundamental mechanism underlying the formation and growth of many celestial objects in the universe.
Angular momentum: Angular momentum is the quantity of rotation an object has, which depends on its mass, shape, and rotational velocity. It is a conserved quantity in an isolated system, meaning it remains constant if no external torque acts on the system.
Angular Momentum: Angular momentum is a measure of the rotational motion of an object around a fixed point or axis. It describes the amount of momentum an object has when it is spinning or orbiting, and it is a conserved quantity in closed systems. This term is crucial in understanding the laws of planetary motion, Newton's synthesis, the dynamics of orbits in the solar system, and the formation of the Moon.
Capture Hypothesis: The capture hypothesis is a theory that proposes the Moon was formed when a large, Mars-sized object collided with the early Earth, and the debris from this impact was captured into orbit around the Earth, eventually coalescing to form the Moon. This hypothesis suggests the Moon was not formed alongside the Earth, but rather was captured and incorporated into the Earth's orbit at a later stage.
Co-formation Hypothesis: The co-formation hypothesis is a theory that proposes the Moon was formed at the same time as the Earth, rather than being a separate body that was later captured by the Earth's gravity. This hypothesis suggests that the Earth and Moon were formed from the same protoplanetary disk of gas and dust, and their formation was closely linked.
Fission Hypothesis: The fission hypothesis is a theory that proposes the Moon was formed from a portion of the Earth's crust that was separated due to a violent impact event in the early history of the solar system. This hypothesis suggests that the Moon was created as a result of a giant impact that caused a part of the Earth's outer layers to be ejected into orbit, eventually coalescing to form the Moon.
Giant impact hypothesis: The giant impact hypothesis suggests that the Moon formed from the debris ejected when a Mars-sized body collided with the early Earth. This collision is believed to have occurred around 4.5 billion years ago, shortly after the formation of the solar system.
Giant Impact Hypothesis: The giant impact hypothesis is a theory that explains the formation of the Moon as a result of a giant collision between the early Earth and a Mars-sized impactor. This collision is believed to have occurred during the final stages of the Earth's formation, leading to the ejection of material that eventually coalesced to form the Moon.
Isotopic Composition: Isotopic composition refers to the relative abundance of different isotopes of a given chemical element within a sample or substance. Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons, resulting in slight variations in their atomic masses. The isotopic composition of a material can provide valuable insights into its origin, formation, and chemical processes it has undergone.
Lunar Maria: Lunar maria are the large, dark, flat areas on the surface of the Moon. They are solidified basaltic lava flows that filled in large impact basins created by asteroid and comet impacts in the Moon's distant past.
Moon’s origin: The Moon's origin is the result of a massive collision between Earth and a Mars-sized body, leading to debris that coalesced into the Moon. This theory is known as the Giant Impact Hypothesis.
Planetary Differentiation: Planetary differentiation is the process by which a planet or other celestial body separates into distinct layers or zones with different chemical compositions and physical properties. This process occurs during the early stages of a planet's formation and is driven by the separation of denser materials, such as metals, towards the center, while less dense materials, like silicates, migrate towards the surface.
Refractory Elements: Refractory elements are a group of chemical elements that have extremely high melting and boiling points, making them resistant to high temperatures and corrosive environments. These elements are crucial in the context of the origin of the Moon, as they provide insights into the extreme conditions that existed during the formation of the Earth-Moon system.
The term 'refractory' refers to the ability of these elements to withstand intense heat and pressure without undergoing significant changes in their physical or chemical properties.
Theia: Theia is a hypothetical planet that is believed to have collided with the early Earth, leading to the formation of the Moon. This giant impact hypothesis is a widely accepted theory for the origin of the Earth's natural satellite.
Tidal Locking: Tidal locking is a phenomenon where a celestial body, such as a moon or a planet, rotates at the same rate as it orbits around another body, causing one side of the body to always face the object it orbits. This synchronization of rotation and orbital period is a result of the gravitational forces between the two bodies.