Giant Impact Hypothesis

Long ago, when the solar system had just formed, many would-be planets were whizzing around in crowded orbits. About 4.5 billion years ago, a giant object about the size of Mars came toward the young Earth's orbit. Scientists call this imagined object Theia, named after the mother of Selene, the Greek goddess of the Moon.

Theia raced toward Earth and hit it at a slant. The energy of the crash was beyond imagination. Theia was shattered, and the surface of the young Earth melted into an ocean of magma.

A huge amount of rock debris and vaporized gas was thrown out and formed a giant disk-shaped ring around Earth. Like the rings of Saturn, the debris circled Earth. Soon the pieces began pulling together with their own gravity, and at last they were reborn as the Moon that lights up our night sky today.

Until the Apollo missions brought back Moon rocks, nobody knew how the Moon formed. One idea said Earth captured a passing Moon with its gravity. Another said Earth spun so fast that a chunk flew off. Neither matched the physics.

Then scientists studied the Moon rocks closely. Their oxygen isotope ratios turned out to be almost identical to Earth's, like twins. These ratios work like a fingerprint that differs from one object in the solar system to the next. So the Moon and Earth must have been made from very similar material. Still, in the basic impact model the Moon should be made mostly of Theia's material, so why the two look so alike is a puzzle researchers are still working on.

We also learned that the Moon has almost no core made of heavy iron. The giant impact idea explains this well. During the crash, the heavy iron sank and merged with Earth's core. Only the lighter rocky outer layers were thrown out, and they became the Moon.

If Theia had hit Earth head-on, most of it would have merged with Earth. Not enough debris would have reached orbit to make a Moon. If it had barely grazed Earth, the debris would have flown far off into space, and again no Moon. Because Theia struck Earth at a slant of about 45 degrees, the debris could stay safely in orbit around Earth.

Even more surprising is how fast it happened. Calculations suggest the debris took only a few months to a few hundred years to gather into the Moon. That is a blink of an eye in cosmic time. One 2022 simulation even found that the seed of the Moon may have formed within just a few hours of the impact.

This huge crash may have shaped life on Earth today, too. It is named as one of the causes of Earth's tilted axis. Afterward, the Moon's gravity helped hold that tilt fairly steady, which is why the four seasons come back in a regular rhythm.