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Origin of the Moon — Reading Comprehension

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MS-ESS1-2

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This comprehensive middle school science passage explores the widely accepted giant impact hypothesis for the Moon’s origin, aligned to NGSS standard MS-ESS1-2. Students will learn how a collision between early Earth and a Mars-sized object (Theia) led to the Moon’s formation from debris. The passage details the scientific evidence, including lunar rock composition and computer simulations, and discusses why rival theories are rejected. Real-world implications connect the Moon’s formation to understanding Earth's geological history. Key academic vocabulary is highlighted and defined. The resource includes a Spanish translation, a leveled version for struggling readers, and multiple activities—such as quizzes, writing prompts, and graphic organizers—to build analytical and scientific reasoning skills. This passage is ideal for science classrooms and is audio integrated for accessibility.

CONTENT PREVIEW

Artist's concept of collision at HD 172555 — "Giant Impact Hypothesis" by NASA / Wikimedia Commons (Public domain).

The Moon is the brightest object in our night sky, but for centuries, scientists wondered how it came to be. Today, the most widely accepted explanation is the Giant Impact Hypothesis. This theory describes a dramatic collision that happened about 4.5 billion years ago, soon after Earth formed. Understanding how the Moon formed is important because it gives us clues about the early history of our planet and the entire solar system.

How the Moon Formed: The Giant Impact Hypothesis

According to the Giant Impact Hypothesis, a Mars-sized object called Theia struck the early Earth with immense force. The impact was so powerful that it vaporized part of Earth’s outer layer and Theia itself, blasting huge amounts of rock and metal into orbit around Earth. This debris formed a swirling disk of hot, melted rock. Over time, gravity pulled the material together, causing it to cool and clump into a single large body—the Moon. Scientists estimate this entire process took only a few thousand years, a very short time in the history of the solar system.

Evidence for the Giant Impact Hypothesis

Several lines of evidence support this idea. First, rocks brought back from the Moon by astronauts have a chemical composition similar to Earth’s mantle, the layer beneath Earth’s crust. This suggests the Moon formed from material that once belonged to Earth. Second, the Moon has a much smaller iron core compared to Earth. If the Moon had formed elsewhere or split off from Earth, its core would likely be larger. Third, advanced computer simulations show that a giant collision could produce a Moon with the size and orbit we observe today. Finally, compared to other planets, Earth’s Moon is unusually large for a satellite, which matches what would be expected from a giant impact event.

Rejected Theories and Scientific Implications

Other theories have been proposed, but they do not fit the evidence as well. The capture theory suggests the Moon formed elsewhere and was pulled in by Earth’s gravity, but this cannot explain the Moon’s similar composition to Earth. The co-formation theory states that Earth and the Moon formed together from the same disk, but this theory does not account for the differences in their cores. The fission theory suggests the Moon split off from a rapidly spinning Earth, but the physics do not work out. By understanding the true origin of the Moon, scientists also learn about Earth’s early atmosphere, volcanic activity, and even the conditions needed for life.

In summary, the Giant Impact Hypothesis best explains the Moon’s origin using evidence from rocks, computer models, and comparisons with other planets. Studying the Moon’s formation helps scientists understand not just our own planet, but also how planets and moons form throughout the universe.

Interesting Fact: The Moon is slowly moving away from Earth at a rate of about 3.8 centimeters per year, changing the length of our days over millions of years.

What is the most widely accepted explanation for the Moon’s origin?

The Giant Impact HypothesisThe Capture TheoryThe Co-formation TheoryThe Fission Theory

About how long ago did the event that formed the Moon happen?

4.5 billion years ago500 million years ago2 million years ago10,000 years ago

Which object collided with early Earth to help form the Moon?

TheiaEuropaVenusTitan

What main evidence supports the Giant Impact Hypothesis?

Moon rocks are similar to Earth's mantleThe Moon is made of gasThe Moon has a giant iron coreThe Moon is smaller than most moons

In the passage, what does the word 'gravity' mean?

The force that pulls objects toward each otherA type of rockA spinning motionA metal found on Earth

What does 'mantle' refer to in the passage?

The layer of rock beneath Earth's crustEarth's outermost atmosphereA ring of debrisA type of planet

Why is the co-formation theory rejected?

It does not explain the different cores of Earth and MoonIt suggests the Moon formed from gasIt shows the Moon is older than EarthIt does not mention gravity

Which of the following is an effect of the Moon moving away from Earth?

The length of Earth's days changes over millions of yearsThe Moon becomes smallerEarth spins slower every dayThe Moon gets closer to the Sun

True or False: The Moon is unusually large compared to other planetary satellites.

TrueFalse

True or False: The capture theory is the best explanation for the Moon’s origin.

FalseTrue