How did the Moon Form?
According to the “giant impact” theory, the young Earth had no moon. At some point in Earth’s early history, a rogue planet, larger than Mars, struck the Earth in a great, glancing blow. Instantly, most of the rogue body and a sizable chunk of Earth were vaporized. The cloud rose to above 13,700 miles (22,000 kilometers) altitude, where it condensed into innumerable solid particles that orbited the Earth as they aggregated into ever larger moonlets, which eventually combined to form the moon. By measuring the ages of lunar rocks, we know that the moon is about 4.6 billion years old, or about the same age as Earth. The distance between the Earth and its moon averages about 238,900 miles (384,000 kilometers). The diameter of the moon is 2,160 miles (3,476 kilometers). The moon’s mass the amount of material that makes up the moon is about one-eightieth of the Earth’s mass. Because the force of gravity at the surface of an object is the result of the object’s mass and size, the surface gra
The dominant theory explaining the creation of the Moon is the giant impact hypothesis, which argues that 4.533 billion years ago (only about 70 million years after the formation of the Earth itself), a Mars-sized body named Theia collided catastrophically with the Earth, throwing many trillions of tons of magma and rock into Earth’s orbit, which coalesced and cooled to become the Moon. This theory is not perfect, however — it predicts that the Earth had a magma ocean at one time, for which no evidence can be found. However, it explains the creation of the Moon better than any other extant theory. Early in the solar system’s history, when the planets were just forming, the solar system was a more crowded place. Orbits were still stabilizing, and many of the early asteroids degraded into unstable orbits that fell into the Sun and Jupiter or got ejected into the far reaches of the solar system. One gravitationally stable point for the coalescence of space rock, the L5 point, had a specia
Today, there are several main theories on the origin of the Moon. The most widely accepted of these theories states that the Moon formed after a Mars-sized protoplanet (a young planet not yet fully formed), collided with the early Earth over 4 billion years ago. This massive collision produced debris that eventually coalesced due to gravitational forces to form the Moon.
< Moonquakes Before astronauts went to the Moon there were several main theories about how the Moon formed. One of the goals of the Apollo missions was to try and choose which theory fit best, but in the end, a completely new theory was born. Before we look at all the different theories, we need to look at some of the differences and similarities between the Earth and Moon: • The Earth has a large iron core, but the Moon does not. • Earth has an average density of 5.5 g/cm3 and the Moon has an average density of 3.3 g/cm3 (because it is depleted in iron). • The Moon has exactly the same oxygen isotope composition as the Earth, whereas Mars rocks, and meteorites from other parts of the Solar System (e.g. the asteroid belt) have much different isotope compositions.
