The Origin of the Moon

Background Information:

A theory in science is an explanation of why something occurs or behaves the way that it does in nature. In order for a theory explaining the Moon’s formation to be accepted, it should account for everything we currently know about the Moon. The observations we can make about the moon include:

  • The Moon seems to be made of the same material as the Earth's upper mantle.

• The Moon has little or no iron in it, and is composed of material unlike the composition of the Earth as a whole.

• The Moon is rounded in shape, like the other planets and not like an asteroid or comet

• The Moon orbits in the same direction as the Earth

• The Moon is located in the same plane as the Earth (the ecliptic plane)

• Theory suggests that the Moon seems to have drifted away from the Earth over the length of geologic history.

• This means that the Moon was once much closer to the Earth

• This means that the Moon was once bigger in the sky and much brighter than at present.

Directions:

Your group will be assigned one of four theories regarding how the moon formed. It is up to your group to determine whether your assigned theory of the moon’s origin is acceptable or not and why. In order to make this decision you will read about your assigned theory and determine whether it accurately explains the observations of the Moon listed above.

You will create and present an informative handout to the class that includes the following information: description of your theory, evidence that supports your theory, and evidence that does not support your theory. From this you will conclude whether or not we should accept your theory as the one that best describes how the moon formed.

Presentation Notes:

Name of Theory / Description of Theory / Evidence that Supports the Theory / Evidence that Does Not Support the Theory / Should we accept the theory?
Co-Formation Theory / Moon formed in orbit around the Earth as the Earth formed.
Both the Moon and the Earth were born in the Solar Nebula. / -Moon is close to Earth
- Moon orbits the Sun in the same direction as the other planets / -Material that makes up the Earth and Moon is not exactly the same. If they formed from the same part of the nebula they should have same composition.
Capture Theory / Moon formed somewhere else in the Solar System.
The Moon then came too close to Earth and was caught in Earth’s orbit. / Moon does orbit the Earth.
Is possible that objects can be trapped in orbit around other planets (think moons of Mars) / The object would have to be going at exactly the right speed and angle.
Would expect the object to be irregularly shape, but the Moon is not.
Fission Theory / Early Earth spun so fast that material broke off and formed the Moon / Moon rocks have similar density and composition to Earth’s crust and mantle / No evidence that Earth ever spun that fast.
No way to explain the core of the Moon.
Doesn’t completely explain moon’s composition.
Giant Impact Theory / A large body collided with Earth when it was in final stages of formation.
Debris from the impact came together and formed the Moon. / Moon is composed of rocks with similar composition to Earth’s crust and mantle.
Moon rotates and revolves in the same direction as Earth.
It would have been so hot that water would have evaporated (explains why Moon rocks are dry)