Exploring the Moon: Structure and Origin

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.

Lunar surface words

Four terms unlock the topic:

Match each Moon feature to how it formed

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

The Moon's gravity is about one sixth of Earth's. An object weighs 600 N on Earth. About how much does it weigh on the Moon, in newtons? (Divide by 6.)

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

Another object weighs 1200 N on Earth. About how much does it weigh on the Moon, in newtons? (The Moon's gravity is one sixth of Earth's.)

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

Put the stages of the giant impact hypothesis into the correct order.

• The early Earth has already formed
• A Mars-sized body collides with the Earth
• The collision throws debris into orbit
• The orbiting debris clumps together
• The debris forms the Moon

Think it through

Three questions about the Moon. Pick the best answer each time.

• Why is the giant impact hypothesis preferred over the older ideas?
• The Moon is covered in craters, but Earth shows far fewer. Why?
• An astronaut weighs 600 N on Earth. Roughly what will they weigh on the Moon?

Argue from the evidence

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.