The Origin of the Moon

Theories

Objectives

• SWBAT describe theories on the origin of the moon.

• SWBAT evaluate the theories based on evidence that has been gathered on the moon’s origin.

Earth and Moon (a Planetary Romance)

You cannot talk about the Earth without taking the Moon into account. The Moon has influenced the evolution of Earth – all living and non-living systems – since its birth 4.5 billion years ago.

The Earth-Moon system is dynamic, the Moon is pulling away from Earth, and has changed over time but it has always been there helping to mold Earth.

Four Theories (really three hypotheses and one theory)

• The “sister” or conformation theory.• The capture theory• The “daughter” or fission theory• Impact Theory – modern, accepted theory.

Sister Theory

• This theory suggests that the Moon formed as a separate object near Earth.– Its formation paralleled that of Earth’s and took

place at the same time.– The coalescing material that formed Earth, also

gave rise to the Moon. – Thus, they formed as, essentially, a “double-planet

system.”

Flaw in the Sister Theory

• The Moon differs in both density and composition from the Earth.– This brings into doubt that the idea that Earth and

Moon originated from the same “preplanetary” material.

• This once favored theory has largely been discounted due to non-conformity with the Moon’s basic physical characteristics.

Capture Theory

• The Moon formed far from Earth and was later captured by Earth.– This resolves the problem of the density and

composition of the Earth and Moon being very different.

• The main problem with this theory is the Moon’s mass – it is very large relative to that of the Earth’s (it is an abnormally large Moon).– Computer models show that capture might have been

physically impossible due to the Moon’s size.

Daughter or Fission Theory

• The Moon originated out of Earth itself.– This theory was first

formulated by Charles Darwin’s son George.

– George’s hypothesis was based on the Pacific Ocean’s basin being the source of “protolunar” material.

This is flawed since Earth could not have spun fast enough to eject an object the size of the Moon (but a good try).

Modern Impact Theory

• Formed by the collision of a Mars size planet, often called Thea, that occupied the same orbit as Earth. – Earth would have been still molten, in its Hadean period,

at the time of the collision.– Collisions of this type would have been common in the

early solar system. – Thea would have hit Earth with a glancing blow that

scattered matter from Earth into stable orbit around Earth.

– The matter would have coalesced to form the Moon.

Thea Side Swipes Earth

Modern Impact Theory - Evidence

• Physical evidence and computer modeling provide compelling evidence for this theory.

• The primary physical evidence was supplied by moon rocks brought back by the Apollo missions.

• This theory originated largely due to the data supplied by the Apollo missions.– Apollo missions made Moon origin hypotheses

testable.