Passage I

Students debate 4 hypotheses regarding the Moon's origin, based on the following observations.

Observations

Observation 1 - Earth and the Moon have the same proportions of various oxygen isotopes (forms of oxygen).

Observation 2 - The same minerals that compose terrestrial lavas are also found in lunar lava specimens.

Observation 3 - The Moon contains little or no water.

Observation 4 - Earth contains a much higher percentage of iron than does the Moon.

Observation 5 - Fossil records show that the length of Earth's day used to be shorter.

Observation 6 - Collisions between Mars and other objects have resulted in the ejection of material from Mars's surface. Some of the ejected material has reached Earth.

Hypothesis 1

All of the material that formed the Moon came from Earth. The rate of Earth's spin on its axis was high enough at one time to eject material from Earth's surface. This material was unable to escape Earth's gravity and went into orbit around Earth, forming the Moon.

Hypothesis 2

The material that composes the Moon came mostly from Earth's surface. A large object from another part of the solar system collided with Earth and threw the surface material into orbit around Earth. Some of the differences in composition between Earth and the Moon might be accounted for by the composition of matter from the large object.

Hypothesis 3

The Moon was never a part of Earth, because Earth's and the Moon's chemical compositions have too few similarities. The Moon and Earth probably formed within the same cloud of gas and dust, when large clumps of gas and dust in the cloud gravitationally attracted additional materials from the cloud. (The collapse of clouds of gas and dust also results in the formation of stars.)

Hypothesis 4

The Moon most likely came from somewhere in the solar system far from Earth. As the Moon approached Earth, it was attracted more strongly by Earth's gravitational field. Although the Moon speeded up as it approached Earth, it was not moving fast enough as it passed Earth to escape Earth's gravity. Consequently, it entered into orbit around Earth.