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The Moon: Structure and Features
The Moon: Structure and Features
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The Moon is Earth's only natural satellite and the brightest object in our night sky after the Sun. Studying its structure and surface features helps scientists understand how rocky worlds form, cool, and change over time. Because the Moon has almost no atmosphere and very little active geology today, it preserves evidence from the early solar system. Its craters, maria, and internal layers act like a long-term record of planetary history.
The Moon has a layered interior made of crust, mantle, and a small core, much like Earth but on a smaller scale. Its surface shows dark basalt plains called maria, bright highlands, impact craters, mountains, and long cracks called rilles. Most of these features formed through impacts, volcanic activity, and slow cooling over billions of years. By mapping these structures and measuring the Moon's size, density, and motion, scientists can test ideas about its origin and evolution.
Key Facts
- Average distance from Earth to the Moon is about 384,400 km.
- Moon diameter is about 3,474 km, which is about 0.27 times Earth's diameter.
- Surface gravity on the Moon is g_moon about 1.62 m/s^2.
- Weight on the Moon = mass x 1.62 N/kg.
- The Moon rotates once and orbits Earth once in about 27.3 days, causing the same side to face Earth.
- Mean density of the Moon is about 3.34 g/cm^3, which is lower than Earth's mean density.
Vocabulary
- Crust
- The crust is the Moon's thin outer rocky layer above the mantle.
- Mantle
- The mantle is the thick middle layer of hot rock beneath the crust.
- Core
- The core is the dense central region of the Moon, made partly of metallic material.
- Maria
- Maria are large dark plains formed when ancient lava filled low areas on the Moon.
- Highlands
- Highlands are the older, brighter, heavily cratered regions that cover much of the Moon's surface.
Common Mistakes to Avoid
- Thinking the dark areas are oceans, which is wrong because maria are solid basalt plains left by ancient volcanic eruptions. They only looked like seas to early observers.
- Assuming the Moon does not rotate, which is wrong because it rotates once in the same time it orbits Earth. This synchronous rotation is why we keep seeing nearly the same face.
- Confusing craters with volcanoes, which is wrong because most lunar craters were made by impacts from meteoroids and asteroids. Only some surface features are volcanic in origin.
- Believing the Moon has no internal structure, which is wrong because measurements show it has a crust, mantle, and core. Its interior is simpler and smaller than Earth's, but it is still layered.
Practice Questions
- 1 An astronaut has a mass of 75 kg. What is the astronaut's weight on the Moon? Use weight = mass x 1.62 N/kg.
- 2 The Moon's diameter is about 3,474 km and Earth's diameter is about 12,742 km. Calculate the ratio Moon diameter divided by Earth diameter and round to two decimal places.
- 3 The Moon has many preserved impact craters, while Earth has fewer visible ancient craters. Explain how the lack of a thick atmosphere, liquid water, and active plate tectonics helps the Moon preserve old surface features.