The Moon Several theories have been proposed to explain the origin of the Moon.

Ejecting a Moon George Darwin (1878) suggested that the early earth ejects a blob as it spins. But if this were true the earth and moon would spin four times faster - the problem is angular momentum. beforeafter

Angular Momentum Angular momentum measures the spin of an object or a set of objects. The faster an object rotates, the more it contributes. The farther away an object is, the more it contributes. Newton’s laws require that angular momentum is conserved in the absence of an external force.

Accreting a Moon Accretion explains the planets, but can it explain the moon? The moon has rocks on the surface very much like earth’s rocks. In this picture, different minerals show up in different colors.

Lunar Composition If both earth and moon accrete from the same dusty orbit they should have the same composition. The moon is much less dense than the earth, it probably has no iron core. The moon could not accrete at the same time as the earth!

Capturing a Moon The moon could form elsewhere with lower density. The orbit could bring it near the earth where it would be captured. This is nearly impossible given the size of the moon.

Catastropic Impact The best theory is that an object larger than the moon collided with the earth. The collision left a cloud of debris that was blasted off of Earth and slowly coalesced into the Moon.

Formation of the Moon This theory explains the composition of the Moon and its orbital inclination.

Characteristics of the Moon Footprint of Apollo 11 astronaut in soft lunar regolith The Moon is about 1/4 the size of Earth. The surface is covered in impact craters. Crater sizes range from a few inches to several hundred miles across. The Moon has some dark, smoother areas and other rough brighter areas. Over billions of years impacts have pulverized the surface into a fine powder called regolith.

Near Side and Far Side The Moon is locked into a 1:1 spin orbit resonance with the Earth. The Moon spins 1 time on its axis for every 1 orbit around the Earth. Because of this the Moon always has one side facing the Earth. This is the “Near Side”. The side not facing the Earth is called the “Far Side”.

Types of Lunar Surfaces There are two major types of lunar surfaces: –Maria- dark, relatively smooth areas –Highlands - light colored, very rough cratered areas Highlands are made of less dense rock and are older. Maria are made of more dense rock and are younger

Formation of Maria and Highlands As the Moon formed, denser material sank towards the core while lighter material floated to the surface. The Highlands were formed after the Moon’s crust cooled. This surface was constantly being impacted during the early years of the solar system. Much later but before the Moon’s interior had cooled certain very large impacts occurred cracking through the crust. Denser rock flowed into the cracks and filled the large craters that formed. When this dark denser material cooled it formed the Maria.

The Moon’s Interior The Maria are located on the near side of the Moon. The crust of the moon is thinner on the near side of the Moon than on the far side. The Moon’s core is thought to be shifted slightly in the direction towards the Earth. This shift may have caused the crust on the far side of the Moon to cool more and become thicker than the crust on the near side