Download Page 188 7.2 Structure of the Moon The Moon`s small size relative to

Page 188
7.2 Structure of the Moon
The Moon's small size relative to the Earth explains most of the differences between the two bodies.
Because its volume compared to its surface area is small relative to the Earth's, heat escapes far more
easily from the Moon. Thus, the Moon has cooled far more than the Earth has. (Think of how a french fry
cools much faster than a baked potato.) Thus, having a much cooler interior, the Moon lacks the
convection currents that drive plate tectonic activity on the Earth.
Without tectonic activity to recycle the debris from impacts that cratered the Moon, the surface has
become covered with a regolith—meaning “blanket of rock”—tens of meters deep. The regolith consists
of both rock chunks and fine powder, the result of successive impacts breaking rock into smaller and
smaller pieces. This powdery nature is easily seen in the crispness of the astronauts' footprints (fig. 7.7).
Samples of the regolith picked up by astronauts show that these surface rocks are typically the same type
as the underlying rock. That is, the regolith on maria is generally broken­up basalt, whereas that on the
highlands is broken­up highland material.
Figure 7.7
Footprint of an astronaut on the Moon.
Crust and Interior
The Moon's low overall density (3.3 grams per cubic centimeter) tells us its interior contains little iron.
Recall that in chapter 6 we saw that the Earth's high density (about 5.5 grams per cubic centimeter) is an
indication that it has a large iron core. In addition, the Moon lacks a magnetic field, suggesting that the
core is at most partially molten.
The Moon's interior can be studied by seismic waves just as the Earth's can. Apollo astronauts set up
seismic detectors on the Moon, which showed that the Moon's interior is essentially inactive and has a
simpler structure than the Earth's. Below the surface layer of rocky rubble is the Moon's crust, about 100
kilometers (60 miles) thick, on the average. The Moon's crust, like the Earth's, is composed of silicate
rocks relatively rich in aluminum and poor in iron. Beneath the crust is a thick mantle of solid rock,
extending down a little more than 1000 kilometers (600 miles). The Moon's mantle is probably rich in
olivine, the same type of dense, greenish rock that composes most of the Earth's mantle. Unlike the
Earth's mantle, however, it appears too cold and rigid to be stirred by the Moon's feeble heat.
The crust is much thinner (about 65 kilometers) on the side of the Moon that faces the Earth than on the
far side (about 150 kilometers), as shown in figure 7.8. The reason for this difference is not clear, but it
may have resulted from the Earth's gravity shifting the Moon's core slightly toward Earth billions of
years ago, when the Moon's interior was molten. The crust on the near side—being slightly closer to the
Moon's core because of that shift—might therefore have become hotter and as a result thinner than that
on the far side. Subsequently, the Moon cooled, leaving the crust thinner on one side than on the other.
Figure 7.8
An artist's impression of the Moon's interior. Notice the thinner near­side crust and the displacement
(exaggerated for clarity) of the core toward the Earth.
The internal asymmetry of the Moon helps to explain some of some of the differences between the
Moon's near and far sides. The far side was first seen in 1959 by a Soviet space probe that passed by the
Moon and sent back pictures. It is distinctly different from the near side, consisting entirely of very rough
terrain with no maria. Figure 7.9 shows an image made by Apollo astronauts. The Moon's far side is
heavily cratered like the highlands on the near side. The thinner crust on the near side made it much
easier for impact craters to be flooded by basalt, forming the maria. In fact, the largest impact feature,
Aitken Basin, is on the far side of the Moon as shown in the figure 7.10, a topographic map of the Moon.
Although this basin is more than 10 kilometers beneath the height of the surrounding terrain, it was not
flooded with basalt because of the thick crust there.
Figure 7.9
An image made by Apollo 16 astronauts showing some of the heavily cratered far side of the Moon. Part
of the near side is visible in the left portion of the image. Mare Crisium is on the edge of the picture at
the 9:30 position.
Figure 7.10
Topographic map showing the near (left half) and far (right half) sides of the Moon. The elevations were
mapped by the Clementine satellite and are shown in different colors. The maria are generally at lower
elevations, but the largest impact feature, the Aitken Basin, is at an even lower elevation. Several major
features and the locations of the six Apollo landing sites are labeled.
Q. Why don't we ever see one side of the Moon from the Earth?
answer
Page 189
The Absence of a Lunar Atmosphere
Lunar scientists have detected only tiny quantities of gas above the Moon's surface—less than one­
quadrillionth the density of our atmosphere. Most of this gas is helium, probably a by­product of
radioactive decay in the Moon's interior. There are also traces of hydrogen near the Moon's poles. Lunar
scientists suspected this came from the breakdown of frozen water mixed with rock in craters that remain
perpetually in shadow. A NASA space probe confirmed this in 2009 when it was crashed into a crater
near the south pole, raising a debris cloud containing water. The water may have originally come from
comets striking the Moon and vaporizing­the water vapor then condensed in the coldest places on the
Moon (the polar craters into which sunlight never shines). You may have seen this tendency for frost to
form in cold spots if you have taken something out of a freezer and left it for a while on a table.
The Moon lacks a significant atmosphere for two reasons. First, its interior is too cool to cause volcanic
activity, which as we saw in chapter 6 was probably an important source of Earth's atmosphere. Second,
and more important, even if volcanos or comets created an atmosphere in its youth, the Moon's gravity is
too weak to retain gas for long. In chapter 3 we learned that the Moon's escape velocity is only about a
fourth that of the Earth's (2.4 kilometers per second versus 11 kilometers per second), and so atoms in the
Moon's atmosphere would have found it easier to escape its gravity. With virtually no atmosphere to
absorb and trap heat, temperatures on the Moon soar during the day and plummet at night, and no wind
blows to stir the thick dust on its surface.