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15.3Chapter 15: The Earth in Space (Universe and Stars)

Collisions and the Moon — Alhussan Physics - Stage 9 Revision Notes & Topical Questions

5 min read 8 parts Lesson 3 of 6 2 quizzes

📚 Alhussan Physics - Stage 9 Hub / Topical Revision
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Lesson summary

Physics 15.3 Collisions and the Moon — The Giant Impact Hypothesis (Theia) — Alhussan International Academy — Physics (Stages 7–9) notes: Describe the giant-impact hypothesis for the formation of the Moon, the evidence for and against it and the other theories.Earth's origin — Th…

  • Later impacts
  • Molten surface
  • Cores merged
  • Moon formed
  • Iron core evidence
  • Not certain

Section 1 of 8Formation of the Earth

Earth's origin
  • The Earth formed about 4.54 billion years ago from gas and dust left over from the formation of the Sun.
Later impacts
  • Various objects hit the Earth later and were absorbed into it.
Iron core
  • Gravity pulled dust and gas together, and iron made in other stars formed the central core.
Molten surface
  • Debris raining on the young Earth probably kept its surface molten for a long time.
The Moon
  • It was not there when the Earth formed.

Section 2 of 8The giant-impact hypothesis

Theia
  • An object the size of Mars collided with the young Earth, at some time before 4 billion years ago.
Heating
  • The energy of the collision heated both bodies.
Cores merged
  • The molten iron cores of the two objects merged to form the Earth's present core.
Moon formed
  • The lighter rock was thrown out into orbit and formed the Moon.
Cooling
  • The surfaces of both the Earth and the Moon then cooled to form a crust.

Section 3 of 8Predictions and evidence

PredictionEvidence
The Earth should have an iron coreNot directly seen, but an iron core accounts for the Earth's density and its magnetic field
The Moon should orbit at a distance predicted by modelsIt does orbit at that distance
The Moon's orbit should align with the Earth'sThe orbits do align
Some Moon material should match the Earth and some should differApollo samples show very little difference in composition, which is evidence against
  • Iron core evidence
    • A rock core would make the Earth's density too low, and a magnetic field suggests a moving heavy metal in the core.

Section 4 of 8Co-formation theory

Idea
  • The Moon formed alongside the Earth as gravity pulled dust and gas together.
  • In another version two objects the size of Mars collided and the Earth and Moon formed from them.
For
  • The Moon has a similar composition to the Earth, and it would be in orbit at its present location.
Against
  • The Moon is less dense than the Earth and does not seem to have a heavy core.

Section 5 of 8Capture theory

Idea
  • The Moon formed elsewhere in the Solar System and the Earth's gravity captured it as it passed by.
For
  • The Moon is less dense than the Earth.
Against
  • Captured objects are not usually spherical, and their orbits do not align with the planet's.
Mars
  • Some astronomers think its two small moons, Phobos and Deimos, are captured asteroids.

Section 6 of 8A hypothesis, not a certainty

Not certain
  • We are not completely sure the giant-impact hypothesis is correct.
Testing
  • Scientists compare predictions with evidence from Moon rocks collected by Apollo astronauts and from computer models.
Accepted
  • A hypothesis is accepted when it explains most of the evidence and is kept open to change.
Old theories
  • The co-formation and capture theories have been around for a long time.

Section 7 of 8Key terms

TermMeaning
Giant-impact hypothesisThe idea that the Moon formed from debris after a Mars-sized object (Theia) hit the Earth.
TheiaThe Mars-sized object thought to have hit the young Earth.
Co-formation theoryThe idea that the Moon formed alongside the Earth.
Capture theoryThe idea that Earth's gravity captured a passing Moon.
HypothesisScientific knowledge used to explain a prediction.

Section 8 of 8Exam technique

1
Thinking the Moon was always there — it formed after the Earth.
2
Thinking the giant-impact hypothesis is proven — there is evidence both for and against.
3
Thinking Theia was a small asteroid — it was about the size of Mars.
4
Thinking the Earth's iron core has been directly observed — it is inferred from density and magnetism.

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