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slight cracking of the crust from gravitational interactions Stage 4. The cold and quiet stage began 3.1 billion years ago
with Earth and the Sun. The movement of these seismic waves as the last lava flow cooled and solidified. Since that
through the Moon’s interior suggests that the Moon has an in- time, the surface of the Moon has been continually
ternal structure. The outer layer of solid rock, or crust, is about bombarded by micrometeorites and a few larger
65 km (about 40 mi) thick on the side that always faces Earth meteorites. With the exception of a few new craters,
and is about twice as thick on the far side. The data also sug- the surface of the Moon has changed little in the last
gest a small, partly molten iron core at a depth of about 900 km 3 billion years.
(about 600 mi) beneath the surface. A small core would account
3
for the Moon’s low density (3.34 g/cm ) as compared to Earth’s 16.5 THE EARTH-MOON SYSTEM
3
average (5.5 g/cm ) and for the observation that the Moon has
Earth and its Moon are unique in the solar system because
no general magnetic field.
of the size of the Moon. It is not the largest satellite, but the
ratio of its mass to Earth’s mass is greater than the mass ratio
HISTORY OF THE MOON
of any other moon to its planet. The Moon has a diameter of
The moon rocks brought back to Earth, the results of the lunar 3,476 km (about 2,159 mi), which is about one-fourth the diam-
seismographs, and all the other data gathered through the Apollo eter of Earth, and a mass of about 1/81 of Earth’s mass. This is a
missions have increased our knowledge about the Moon, leading small fraction of Earth’s mass, but it is enough to affect Earth’s
to new understandings of how it formed. This model pictures the motion as it revolves around the Sun.
present Moon developing through four distinct stages of events. If the Moon had a negligible mass, it would circle Earth
with a center of rotation (center of mass) located at the center
Stage 1. The origin stage describes how the Moon originally
of Earth. In this situation, the center of Earth would follow a
formed. The Moon is believed to have originated from
smooth path around the Sun (Figure 16.28A). The mass of the
the impact of Earth with a very large object, perhaps
Moon, however, is great enough to move the center of rotation
as large as Mars or larger. The Moon formed from
ejected material produced by this collision. The colli-
sion is believed to have vaporized the colliding body
as well as part of Earth. Some of the debris condensed
away from Earth to form the Moon. Path of
The collision that resulted in the Moon took place x Earth
after Earth’s iron core formed, so there is not much
iron in moon rocks. The difference in Moon and Moon
Earth rocks can be accounted for by the presence of (with negligible x
mass)
materials from the impacting body.
Stage 2. The molten surface stage occurred during the first Earth
200 million years after the collision. Heating from Center of
a number of sources could have been involved mass x
in the melting of the entire lunar surface 100 km
(about 60 mi) or so deep. The heating required to A
melt the surface is believed to have resulted from the
impacts of rock fragments, which were leftover debris
from the formation of the solar system that intensely
bombarded the Moon. After a time, there were fewer x Path of
fragments left to bombard the Moon, and the molten Earth
outer layer cooled and solidified to solid rock. The
craters we see on the Moon today are the result of me- Moon
(with mass)
teorite bombardment that occurred between 3.9 and x
4.2 billion years ago after the crust had formed.
Stage 3. The molten interior stage involved the melting of the Earth Center of
interior of the Moon. Radioactive decay had been mass
slowly heating the interior, and 3.8 billion years ago, or x
about a billion years after the Moon formed, sufficient
heat accumulated to melt the interior. The light and B
heavier rock materials separated during this period,
perhaps producing a small iron core. Molten lava FIGURE 16.28 (A) If the Moon had a negligible mass, the
center of gravity between the Moon and Earth would be Earth’s
flowed into basins on the surface during this period,
center, and Earth would follow a smooth orbit around the Sun.
forming the smooth, darker maria seen today. The (B) The actual location of the center of mass between Earth and
lava flooding continued for about 700 million years, Moon (identified with X) results in a slightly in-and-out, or wavy,
ending about 3.1 billion years ago. path around the Sun.

422 CHAPTER 16 Earth in Space 16-18