The Earth's Moon
The Earth's one natural satellite, the Moon, is more than one quarter
the size of Earth itself (3,474 km diameter), making the Earth-Moon
system virtually a double-planet. Because of its smaller size, the Moon's
gravity is one-sixth of the Earth's gravity, as we saw demonstrated by the
gigantic leaps of the Apollo astronauts.
While there are only two basic types of
regions on the Moon's surface, there are many interesting surface
features such as craters, mountain ranges, rilles, and lava plains.
The structure of the Moon's interior is more difficult to study. The
Moon's crust, the top layer, is a rocky solid. Below the crust is the
mantle which consists of a denser rock. Together, the crust and
mantle form the Moon's lithosphere, which is very thick (perhaps 800
km) compared to the Earth's lithosphere. Beneath the lithosphere is a
partially molten zone. Although it is not known for
certain, many
lunar geologists believe the Moon may have a small iron
core, even
though the Moon has no measurable magnetic field. By studying the
Moon's surface and interior, geologists can learn about the Moon's geological history and its formation.
The footprints left by Apollo astronauts will last for centuries
because there is no wind on the Moon. The Moon does not possess any
atmosphere, so there is no weather as we are used to on Earth. Because
there is no atmosphere to trap heat, the temperatures on the Moon vary
dramatically over the course of a day, from 100° C at noon
to -173° C at night.
The Moon doesn't produce its own light, but looks bright because it
reflects light from the Sun. Think of the Sun as a light bulb, and the
Moon as a mirror, reflecting light from the light bulb. The lunar phase changes as the
Moon orbits the Earth and
different portions of its surface are illuminated by the Sun.
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Currently, the theory that best explains all the evidence is the collisional ejection theory. This theory holds that the Moon formed from debris ejected from the Earth when a large object (possibly as
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The picture shows the moon's crust, about 60=150 km thick, followed by the lithosphere, shown in blue. The moon's lithosphere is the major portion of the interior, about 1000 km thick. The moon's core
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