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The Origin of the Moon

Moon rocks hold important clues to the origin of the satellite. The results of analyzing these Moon rocks have led scientists to conjecture that the Moon may have formed 4.5 billion years ago when the Earth collided with a very large object (the size of Mars or larger), ejecting raw materials that eventually became the Moon. This is known as the impact theory. Currently, the impact theory is conceivably the one most widely accepted by scientists, but other theories do exist and have significant support.

Coaccretion theory holds that the Moon formed in Earth's orbit along with all the other planets in the Solar System, so that there were two planets in the same orbit for some time before the moon was taken over by the earth's gravity.

Fission theory states that, when the Solar System was very young, the Earth was spinning so fast (nearly 10,000 miles per hour (16,000 km/hr), or ten times its current speed), that it threw off a large chunk of material in order to stabilize itself and slow down; that chunk, the theory holds, became the Moon.

Capture theory holds that the Moon was formed elsewhere in the Solar System and was seized by the Earth.

Of all these theories, the impact theory fits best with all the data that has thus far been accumulated about the Moon. Future lunar exploration, such as the determination of the bulk composition of the Moon, should help scientists settle this issue.

Is there ice on the Moon?

Comets and meteorites incessantly assail the moon. Some meteorites rich in water, and comets, which are made up of mostly ice, may deposit substantial amounts of water on the lunar surface as they bombard it. Energy from the sunlight evaporates most of this water; the evaporated water usually just flies off into space immediately.

Some water molecules, however, may have landed on the surface inside enormous craters at the lunar poles, trapped there. Due to the very slight "tilt " of the Moon, some of these deep craters never receive any light from the Sun - they are permanently shadowed.

It is in such craters that scientists expect to find frozen water if it is there any at all.

What is KREEP?

More than 4.5 billion years ago, the surface of the Moon was a liquid magma ocean. Scientists think that a component of lunar rocks, KREEP (K (potassium), Rare Earth Elements, and Phosphorous) represents the last chemical remnant of that magma ocean. Scientists believe that KREEP floated to the surface the magma because these elements are together "incompatible", that is, they do not fit together to form a compact molecule. KREEP is used by scientists to trace the volcanic history of the moon and to record the impacts by meteorites and other matter.

Are there Moonquakes?

Seismic activity on the Moon is very low, basically insignificant, due to the lack of plate tectonics. In eight years of monitoring, no large moonquake has ever been recorded. The largest recorded seismic activities are approximately equivalent to a 4 on the Richter scale, with 1-2 being typical. [Data through 1999]

Lunar seismic activity is usually caused from tidal forces and the effects of impacts. When there is a moonquake, the seismic activity can be clearly recorded a long distance away. Thus an astronaut was prompted to use the phrase "rang like a bell" after observing a discarded portion of the Saturn rocket strike the moon.

Some have speculated that this long-range effect could be harnessed and used for communication, while others believe that this is just one more piece of evidence that suggests that the moon is hollow.