Exploring the Moon: Structure and Origin
How did the Moon form, and why does its surface look the way it does? Explore the Moon's structure, its craters and seas, the giant impact hypothesis, and its low gravity.
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A world next door 🌙
The Moon is the only other world humans have walked on. Its cratered surface and its very existence both tell a story - of violent impacts and one enormous collision. This module covers the Moon's structure, its surface features, the leading theory of how it formed, and why its gravity is so much weaker than Earth's.
The words you need 🔑
Four terms unlock the topic:
Match each Moon feature to how it formed 🔗
- A crater
- The maria (dark plains)
- The highlands
- No atmosphere
- A meteorite struck the surface
- Ancient lava flooded and hardened
- Old crust left heavily cratered
- Gravity is too weak to hold on to gases
The lunar seas 🌑
The dark, flat plains on the Moon (the "seas") are actually made of what?
- Hardened ancient lava
- Liquid water
- Thick sheets of ice
- Loose blown sand
The giant impact 💥
The accepted theory for the Moon's origin is the **giant impact hypothesis**: A **Mars-sized body** (sometimes called Theia) collided with the **early Earth**. The collision threw a huge amount of debris into orbit, and that debris slowly clumped together to form the **Moon**. This is preferred over older ideas - capture, co-formation and fission - because it fits the evidence best.
Evidence for the impact ✅
Select the TWO pieces of evidence that support the giant impact hypothesis.
- Moon rocks are similar in composition to Earth's mantle
- The Moon has only a small iron core
- The Moon has a thick atmosphere
- The Moon is made entirely of ice
Why this theory wins 🆚
Scientists once had several ideas. Only one fits the evidence well.
Weight on the Moon 🪶
An interactive activity.
Always the same face 🔄
Why do we always see the same side of the Moon from Earth?
- It rotates exactly once for each orbit of the Earth
- The Moon does not rotate at all
- The Moon has no far side
- The Earth physically blocks the other side
A heavier object ⚖️
An interactive activity.
Put it together 🧱
The Moon has almost no _____, so its surface is covered in impact _____. The dark plains are called _____. Most scientists accept the _____ impact hypothesis for its origin. The Moon's gravity is about one _____ of Earth's.
How the Moon formed 🪜
An interactive activity.
Think it through 🧭
An interactive activity.
The grade-9 habit 🌟
Top answers **argue from evidence**: the giant impact hypothesis wins because the Moon's rocks match Earth's mantle and its iron core is small - facts the older theories struggle to explain. Link every **feature to its cause**: craters to impacts, maria to ancient lava, the missing atmosphere to weak gravity. And remember the Moon's gravity is about a **sixth** of Earth's. Evidence, cause and effect, and the numbers - top band.