WHERE DID THE
MOON COME FROM?

We know it’s not made of cheese.
What else do we know about it?

LUNA 101

What we know for sure about Earth’s only natural satellite is that it’s a 2,159-mile-wide collection of minerals that orbits our planet about a quarter-million miles away. NASA visited the Moon six times between 1969 and ’72, and astronauts brought back hundreds of pounds of rocks, from which geologists determined that the Moon is about 10 percent iron, 50 percent oxygen isotopes, 20 percent magnesium, 20 percent silicon, and the rest trace elements like calcium, aluminum, and niobium.

But how did it get there? Throughout history, there have been four predominant theories.

1. COLLISION-CONDENSATION THEORY

Details: The early universe was a gaseous cloud of material, organized by gravity into vast collections of matter from which galaxies were created, as were flat spinning disks that later became solar systems. Almost all (99.9 percent) of the material in one such disk gravitated toward itself and became the Sun. The remaining 0.1 percent stayed in orbit around the Sun and separated into smaller spirals. Eight of the spirals were big enough to become planets and exert their own gravitational pull. Whatever material was still floating around the planets condensed into orbiting clumps of rock. There are at least 166 of these “clumps” in our solar system—our moon is one of them.

Supporting Evidence: Everything that formed from that first spiral of material is still spinning in the same direction it always has. Looking down from the North Pole, the Earth spins in a counterclockwise direction, and so does almost everything else in the solar system, including planets, moons, comets, and the asteroid belt.

Detracting Evidence: If the Moon and Earth condensed from the same cloud at the same time, the Moon should have an iron core similar to Earth’s. But Earth is 30 percent iron while the Moon is only 10 percent, which suggests that perhaps they did not form at the same time and place.

2. FISSION THEORY

Details: The Moon is made of material that was once part of Earth. The hypothesis is that a young, malleable Earth was spinning so fast that part of it detached and flew off into space, where some of it eventually came together as the Moon.

Supporting Evidence: A third of Earth is iron, but most of that is in the core, where heavier elements coalesced inside the young planet. The iron content near Earth’s surface is closer to what you would find on the Moon. Possible site of the detachment: the Pacific Ocean.

Detracting Evidence: While this may sound like a good theory, there isn’t much real evidence to back it up.

3. CAPTURE THEORY

Details: The Moon wasn’t made from, near, or along with Earth but was a wayward traveler spinning through space until it happened to get close enough for the planet’s gravitational field to grab it.

Supporting Evidence: Every meteor crater in the solar system is proof that rogue asteroids and planetoids were rampant throughout the early solar system. Billions of years of gravitational attraction (that is, meteors smashing into things) has “cleaned up” most of the debris, but the asteroid belt between Mars and Jupiter remains as a reminder.

Detracting Evidence: Unusual things do happen in the universe, so it’s hard to say this is impossible. But the specific weights, speeds, distances, and gravitational effects needed for this theory to work make it statistically unlikely.

4. GIANT IMPACTOR THEORY

Details: The world’s leading moon scientists held a conference in Hawaii in 1984, during which they devised this fourth theory. It goes like this: 4.5 billion years ago, a planetoid called Theia, about the size of Mars, crashed into a still-forming Earth. The impact was 100 million times more powerful than the asteroid believed to have killed the dinosaurs. It was probably only a glancing blow rather than a full-on collision, so it didn’t reach the iron in Earth’s core. Still, it sent enormous amounts of “earth” into space. As the ejected material orbited around Earth over the next few billion years, some of it rained back down onto the planet, some spun away into the solar system, and the rest settled into a single body: the Moon.

Detracting Evidence: About 40 percent of the Moon would be made of the remnants of Theia; the other 60 percent would have come from Earth. But evidence published in 2012 shows that certain titanium isotopes are just as abundant in Earth rocks as in Moon rocks, suggesting that they both formed from the same debris field.

GOING, GOING, GONE

While the question of the Moon’s origin is debated, one thing is certain. The satellite is moving away from Earth at about four centimeters per year, and in time will drift so far that Earth’s gravity won’t be able to hold it. The Moon will shrink in the sky until it disappears into the heavens. Don’t worry, though—you’ll be able to enjoy moonlit nights for at least a few more billion years.

Price of a box of Girl Scout Cookies when they debuted in 1936: 25¢.