Origin of the Moon

Beautiful, vast, mysterious, and unexplored. From the planets in our own solar system to those in the other 500 solar systems, there are endless possibilities to what is beyond our terrestrial existence.

But not all the mysteries of the heavens beckon from so far away. Standing in your own backyard, you can easily see one of the most mysterious objects of all. Only 384,393 kilometers away, illuminating our nights and moving our tides, silently spins one of the most puzzling objects in the skies—our Moon. As familiar to our eyes as an old friend, our nearest neighbor in space shouldn't be there at all!

But there it is. Most moons revolve around their planets' equators. Not ours. Our Moon has a mass about one-eightieth that of Earth. That may seem pretty small, but no other planet except Pluto, if Pluto is a planet, has a moon so near its own size! To put it plainly, our Moon is an oddball in the Solar System—oversized and out of place!

Before the Apollo missions to the Moon, scientists had developed a number of theories about its origin. Many thought we could settle the question if only we could go there. But even today, after visits to six separate places on its surface and after collecting hundreds of kilograms of the Moon's soil and rocks for study, no one knows why our Moon is where it is—or where it came from!

The Apollo missions did give us some solid clues. We now believe that the Moon is about the same age as the Earth. Examination of rocks from the Moon's surface tells us that, like the Earth, the Moon is about 4.6 billion years old. We have also learned that the Moon rocks are somewhat different from rocks found on Earth. The rocks from the Moon have about six times more aluminum, calcium, and titanium than Earth rocks, but much less sodium, magnesium, and iron. But where did our Moon come from? There are some major theories and their opponents.

Child of the Earth

The first theory says that the origin of the Moon is really a child of the Earth, a chunk thrown away from our planet in its very early childhood, or protoplanet stage. If that happened, it must have been a colossal event! Imagine what it might have looked like if anyone had been alive to see it. Some scientists even think that the Pacific Ocean may be a “scar” in the Earth, left when the Moon was hurled away from us. It's an exciting idea, but it has a few problems.

Opponents of the theory point out that if the Moon had been thrown into space by the Earth's rapid spinning, it should have freed itself from the place on Earth where the centrifugal force is the greatest—the equator. And it should be orbiting close to Earth's equator today. But the Moon actually orbits closer to the plane of the ecliptic, the apparent path of the planets and the Sun as viewed from the Earth.

Another serious problem is that the Earth probably never was spinning fast enough to dislodge the Moon. Calculations show that the Earth would have to turn on its axis once every two and a half hours to create enough force to hurl the Moon free. Try to imagine a day only two and a half hours long! It’s a dizzying idea! Our best evidence, based on the present rotation of the Earth and Moon, tells us that in those very early stages the Earth was probably spinning at a rate of once every five hours—still a short day and quite a head-turner, but much too slow a spin to have thrown off the Moon.

Because the ocean basins are always changing, it is impossible to prove or disprove the theory by looking at today’s ocean floors. However, studies seem to show that the Pacific Ocean is too shallow to be the Moon's old home.

And, as the Apollo studies showed us, the chemical makeup of the Earth and the Moon is somewhat different. How, then, could they originally have formed parts of the same planet?

Captured by Earth

A second major theory can explain the Moon's odd orbit—and the chemical differences between Earth and Moon rocks. According to this theory, the Moon started out as a small runaway planet somewhere else in the Solar System. As it passed near Earth on its way around the Sun, it was captured by the Earth's gravity and trapped in its present orbit.

Studies have shown that this is possible. And there is some evidence that the Moon did experience strong gravitational tidal forces after it formed. But once again we find some problems.

Some scientists feel a close pass of Earth might well have changed the Moon's orbit around the Sun, but it is very unlikely that Earth could actually capture an object as large as our Moon. Other scientists admit that the “capture theory" is possible but maintain it is very unlikely because it depends upon just too many big coincidences.

A variation of the capture theory holds that the Moon was not captured in one piece, but was formed by small objects swept into Earth's orbit. These materials, called planetesimals, were probably plentiful in the early Solar System. As they passed close to the Earth, they broke up. Their lighter surface material was captured into Earth's orbit, while their heavier core material escaped. As more and more of the light material was robbed by the Earth and collected in Earth's orbit, frequent collisions and gravitational attraction eventually caused everything to lump together in one great mass to form the Moon.

Unfortunately, this Moon would orbit near Earth's equator.

Double Planet

The third major theory says that the Earth and the Moon formed as a double planet from the same rapidly rotating dust cloud. The double-planet theory has a good number of supporters. Its major problem is that it can't explain the chemical differences between the Earth and the Moon. These differences shouldn't exist, as Earth and Moon were formed at the same time in the same dust cloud. Some scientists have tried to get around this problem by having the Moon form at a more remote edge of the dust cloud, where slightly different chemicals might have been present.

As we can see, most of our ideas about the origin of the Moon have some problems—they each answer only some of the questions.

Blast Formation

Still, scientists work with theories because theories provide a starting point for their investigations. Perhaps the best (and fourth) theory on the origin of the moon we now have is that the Moon formed from material blasted off the Earth's surface by a collision with a small asteroid. If this collision occurred after most of Earth's iron sank into the core, the material thrown into orbit would resemble that found on the Moon today. Such a collision could have knocked the Earth so that its equator became tilted to the ecliptic. The material itself would have gathered where the Earth's equator used to be near the ecliptic. Collisions and gravitational attraction would eventually lump the material together to form the Moon. This theory accounts for the chemical differences between the Earth and the Moon, doesn't require rapid rotation, and explains why the Moon's orbit is tilted to Earth's equator.

Although this idea sounds pretty good, there still aren't enough facts to close the case. And so the detective work goes on as we look for further evidence to solve the mystery.

Recommended Reading

While there are many mysteries regarding the origin of the moon, there is still very much that we do know. Along with Mercury, Venus, Mars and Earth, the Moon is often considered a terrestrial “planet” as a result of its size and composition. The terrain of the Moon is basically put into two types: the cratered and old highlands and the smoother, younger maria. The Moon is responsible for the tides, and perhaps the insanity that is compelled by a full moon. Learn more about the mysterious Moon in The Moon by Michael Carlowicz.

Journey to our nearest neighbor in The Moon by Michael Carlowicz. Full of spectacular photos, this book explores the past, present and future affects of the Moon on our history, culture, planet, and science.