Term 2 Prep III
The City School
Handout: Science 2.6.3
Topic: Collision Theory and formation of Moon
Name: Sec:
Learning Objective(s):
2.6.3 Describe the evidence for the collision theory for the formation of the Moon.
The collision theory, also known as the giant impact hypothesis, is the most widely accepted explanation for the Moon's
formation. It proposes that a Mars-sized body, referred to as Theia, collided with the early Earth approximately 4.5 billion
years ago. The debris ejected from this massive impact eventually coalesced to form the Moon. This handout details the key
evidence supporting the collision theory.
Similarity in Composition Between Earth and the Moon
One of the strongest pieces of evidence for the collision theory is the similarity in the isotopic composition of rocks on
Earth and the Moon. Scientists have analysed lunar samples brought back by the Apollo missions and found that the
isotopes of oxygen, silicon, and titanium are nearly identical to those found on Earth. This suggests that the Moon
formed from material that originated on Earth, as would be expected if debris from an impact coalesced into the Moon.
Term 2 Prep III
Lack of Volatile Compounds on the Moon
The Moon has a significantly lower concentration of volatile compounds, such as water and certain gases, compared to
Earth. These compounds would have been vaporised and lost to space during the high-energy impact that created the
Moon. This scarcity of volatiles aligns well with the collision theory, as it predicts that the extreme heat from the impact
would drive off these materials.
High Angular Momentum of the Earth-Moon System
The Earth-Moon system has an unusually high angular momentum, meaning the total rotational and orbital energy is
greater than what could be explained by other formation theories, such as the Moon forming alongside Earth or being
captured by Earth's gravity. The collision theory accounts for this by suggesting that the impact imparted significant
rotational energy to both Earth and the forming Moon, resulting in the system we observe today.
The Moon's Lack of a Large Iron Core
Unlike Earth, the Moon has a relatively small iron core. This difference can be explained by the collision theory, as it
suggests that Theia's iron core merged with Earth's during the impact. The debris that formed the Moon was primarily
composed of lighter materials from the Earth's mantle and Theia’s outer layers, which would explain the Moon’s lower
density and smaller iron core.
Evidence from Computer Simulations
Modern computer simulations of giant impacts support the feasibility of the collision theory. These simulations show
that an impact between a Mars-sized body and the early Earth could eject enough material into orbit to form the Moon.
The simulations also replicate key features of the Earth-Moon system, including their size, orbital characteristics, and
angular momentum.
Lunar Surface Features and History
The Moon's surface provides additional evidence for the collision theory. The lunar highlands, composed of anorthosite,
are thought to have formed from a magma ocean that solidified over time. This supports the idea of a molten Moon,
which would have been a natural consequence of the intense heat generated by the impact.
Fossil Record of Tidal Locking
Studies of ancient tidal patterns preserved in Earth’s geological record suggest that the Moon has been gradually moving
away from Earth over billions of years. This movement aligns with the collision theory, which explains the Moon’s initial
proximity to Earth and its current position through gravitational interactions over time.
Key Terms to Remember
• Theia: The Mars-sized body that collided with Earth.
• Isotopic composition: The specific ratio of isotopes in a material, used to compare Earth and Moon rocks.
• Volatile compounds: Easily evaporated substances, such as water, that were lost during the impact.
• Angular momentum: The measure of rotational energy in the Earth-Moon system.
• Lunar highlands: Regions of the Moon’s surface formed from ancient magma.
