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10 Things: What We Learn About Earth by Studying the Moon

Earth rising above the lunar surface.
This view from the Apollo 11 spacecraft shows the Earth rising above the moon's horizon.

In the vast expanse of our solar system, there is one place that, in some ways, we know even better than parts of Earth. It’s a spinning rock that’s a constant throughout our lives: our natural satellite, the Moon.

NASA Science Live Ep. 1: To the Moon, and Beyond

Around the world, people have found ways to make the Moon their own. The Chinese tell the tale of Chang’e, a Moon goddess. The Ancient Egyptians had the Moon god Khonsu, protector of night-time travelers. The Ancient Greeks had the Moon goddess Selene, who was said to drive a Moon chariot across the dark sky.

Not only have our stories helped us make sense of the Moon, but the Moon has informed our understanding of Earth. So here are 10 things we’ve learned about Earth by studying our closest neighbor.

Animated GIF Showing how an impact may have formed the Moon.
Here a Mars-sized protoplanet strikes the proto-Earth at a 45 degree angle near the mutual escape velocity of both worlds. The “red” particles were found to escape the Earth-Moon system. Some of this debris may eventually go on to strike other solar system bodies like large main belt asteroids. “Yellow–green” particles go into the disk that makes the Moon. “Blue” particles were accreted by the proto-Earth.
Robin Canup/Southwest Research Institute.

1. The Makeup of a Newborn Earth

The Moon is not made of cheese; it’s made of remnants of a baby Earth!

Scientists believe that a Mars-sized object crashed into Earth 4.5 billion years ago. The force of this crash was so great it sent materials from Earth, and from the object that struck it, flying into space. Some of this debris stuck together to make the Moon.

Not only was the Moon constructed largely of Earth, but a lot of debris from Earth probably landed on the Moon in the period after it was formed. More clues about the composition of an early Earth could very well be hidden between layers of Moon dust.

Impact crater full of impact craters on the Moon.
The Moon's Mendeleev crater (named after Dimitri Mendeleev, the inventor of the periodic table of elements) is about 195 miles (313 kilometers) in diameter. It contains a chain of craters called Catena Mendeleev.
NASA/GSFC/Arizona State University

2. Earth's “Time Capsule” on the Moon

The face of the Moon preserves nearly every crater in its history. Earth, with its surface constantly churned and reshaped by plate tectonics, erosion and other elements, can hardly say the same. But because of how close the Moon is to Earth, we can look to the unchanging lunar surface to uncover Earth’s past. Most Moon craters are thought to have formed about four billion years ago, during a period called the Late Heavy Bombardment. During this time, we believe a huge number of asteroids and other objects pummeled the Moon, our planet and other planets. By studying lunar craters, along with rocks Apollo astronauts brought back decades ago, we have a better picture of what happened to Earth during that turbulent period of time and beyond.

Astronaut assembling equipment on the Moon.
A lunar sample returned by the Apollo 14 astronauts may contain a bit of Earth from about 4 billion years ago.

3. Earth Trades “Collectible” Meteorites with the Moon

Rather than playing cards, we think Earth and the Moon trade meteorites. Our studies of the Moon have taught us how meteorites ejected from its surface during asteroid impacts could have fallen to Earth, where they’ve been found by scientists.

These Moon samples come from all areas of the lunar surface, even the far side of the Moon, which we can’t see from Earth. While there is less evidence of stuff from Earth migrating to the Moon, scientists think it’s possible. One study even suggests, based on computer modeling, that there could be approximately 40,000 pounds (20,000 kilograms) of Earth rocks for every 100 square kilometers of the Moon.

Illustration of meteors crashing down on primordial Earth.
Artist'c concept of the heavy bombardment of early Earth.
NASA's Goddard Space Flight Center Conceptual Image Lab

4. Potential Clues to How Life Began on Earth

If Earth is, indeed, swapping meteorites with its satellite, these unique relics could tell us more about Earth’s conditions leading up to life.

Some scientists have suggested that microorganisms could have existed on the Moon, possibly carried there by meteorites from Earth. Others have also proposed combing through lunar soil for elements, including ancient nitrogen or oxygen, from Earth, which could fill in gaps in knowledge about things like the development of Earth’s atmosphere.

It’s even been suggested that the same materials that brought life to Earth could be preserved in lunar lava.

Lava glowing at night.
Night view of Hawaii's Kilauea Volcano, one of Earth's most active volcanoes. A NASA-led team is studying Hawaiian volcanoes from the air, ground and space to better understand volcanic processes and hazards.

5. Earth’s Volcanoes are a “Fountain of Youth”

Despite Earth and the Moon having formed around the same time, Earth’s surface looks younger. Our planet’s skin-deep secret? Volcanoes.

Plate tectonics and hot spots continuously help Earth spout rock, ash and gases from its interior. Some of this material settles, thereby renewing Earth’s surface and maintaining its youthful glow.

The Moon has maria, or plains of volcanic rock, that suggest past active volcanoes. Discoveries by NASA’s Lunar Renaissance Orbiter indicate that the Moon could have had volcanic flows up to just tens of millions of years ago, during Earth’s dinosaur age.

Because evidence of lunar volcanic activity has been so well preserved, we can study how it changed through time and under different conditions to better understand volcanic processes on Earth.

Visualization of Earth's magnetic field.
A simple visualization of Earth's magnetosphere.
NASA's Scientific Visualization Studio / JPL NAIF

6. The Moon May Help Enforce Earth’s Shield

Earth’s magnetic field is our shield, constantly protecting us from harmful solar wind or cosmic ray particles. This important buffer is generated by the fast-flowing movement of liquid iron and nickel in Earth’s outer core

One thing that makes this molten ocean of metal move is the Moon’s gravity. Recent research suggests that the Moon’s gravity tugs on Earth’s mantle layer (which sits on top of the outer core). This causes the liquid, outer core to slosh around, helping to generate the energy needed to maintain our magnetic field.

Four moonquakes compared (black) compared to an earthquake (red).
NASA/Marshall Space Flight Center/Renee Weber

7. Earthquakes are Nothing Like Moonquakes

Earthquakes generally last only half a minute. In contrast, shallow moonquakes, a type of Moon vibration that originates about 12 to 19 miles (20 to 30 kilometers) below the surface, can last at least 10 minutes. The cause of these rare quakes isn’t clear, but they do highlight one reason liquid water is critical on Earth: it helps spread energy out, muffling earthquake vibrations. Studying Moonquakes can help us understand what seismic activity on Earth could have been like during times with less liquid water on the surface, such as during major ice ages or during the Earth's early history, when the surface was much too hot to preserve liquid oceans.

The Moon above Earth from orbit.
The Moon above Earth as seen from the International Space Station. The orange-colored troposphere is the lowest and most dense portion of the Earth's atmosphere. The troposphere ends abruptly at the tropopause, which appears in the image as the sharp boundary between the orange- and blue- colored atmosphere. Silvery-blue noctilucent clouds extend far above the Earth's troposphere.

8. Mirror, Mirror on the Moon

Every planet or moon in our solar system can be measured by its albedo, or how much light it reflects. Think of it as a level of brightness: a brighter body will have higher albedo, while a dimmer body’s albedo will be lower.

On Earth, measuring albedo is especially important because it can help track changes in our climate based on the amount of sunlight Earth absorbs.

The Moon can actually help us measure this key property. Have you ever noticed that during a crescent Moon, you can sometimes faintly see the rest of the Moon's face? This fainter portion of the face is actually lit by sunlight bouncing off of Earth — something called earthshine.

By measuring this glow from the Moon, scientists can accurately estimate how much Earth itself shines, and even the composition of Earth’s atmosphere.

Starfish on rock with surfer in the background.
The ochre sea star helps specific diversity in low tide zones by eating mussels that might overrun other marine life.
National Science Foundation / Genny Anderson, Santa Barbara City College

9. The Moon Makes Our Existence Possible…

We have the Moon to thank for our way of life!

Earth’s signature 23.5-degree tilt on its axis is due to the Moon keeping it in check. The 23.5-degree angle ensures our planet is safe to live on, as a more exaggerated tilt would cause more extreme seasons.

Without the Moon’s gravity, Earth would wobble more violently on its axis, drastically altering the climate. Besides maintaining climate stability, the Moon also sets the rhythm of Earth — the highs and lows of our tides — which affects the variety of ways we use the ocean for food, travel and recreation. Precisely measuring the mass, size and orbital properties of the Moon is essential for predicting these rhythms of tides and seasons.

Moon over Earth with the Space Shuttle in the foreground.
Earth and its Moon are nicely framed in this image taken from the aft windows of the Space Shuttle Discovery in 1998.

10. ...Yet We’re Pushing It Away

Because Earth and the Moon interact through tides, our planet is actually pushing away its satellite about 1.5 inches (3.78 centimeters) each year — about the same rate fingernails grow. Here’s how: The side of Earth that faces the Moon gets pulled by the Moon’s gravity, creating what scientists call a “tidal bulge,” or a bulge of raised ocean water that’s drawn toward the Moon. Because Earth rotates on its axis faster than the Moon orbits it, the higher gravity from Earth’s bulge tries to speed up the Moon’s rotation. Meanwhile, the Moon is pulling on Earth and slowing the planet’s rotation. The friction that ensues from this tug-of-war forces the Moon into a wider orbit. Studying these tidal and orbital interactions is extremely important for understanding possible effects on Earth's climate.