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The Earth’s Moon
The Moon is the only natural satellite of Earth:
orbit:
384,400 km from Earth
diameter: 3476 km
mass:
7.35e22 kg
Called Luna by the Romans, Selene and Artemis by the Greeks, and many other names in other
mythologies.
The Moon, of course, has been known since prehistoric times. It is the second brightest object
in the sky after the Sun. As the Moon orbits around the Earth once per month, the angle between the
Earth, the Moon and the Sun changes; we see this as the cycle of the Moon's phases. The time between
successive new moons is 29.5 days (709 hours), slightly different from the Moon's orbital period
(measured against the stars) since the Earth moves a significant distance in its orbit around the Sun in
that time.
The Moon was first visited by the Soviet spacecraft Luna 2 in 1959. It is the only
extraterrestrial body to have been visited by humans. The first landing was on July 20, 1969 (do you
remember where you were?); the last was in December 1972. The Moon is also the only body from
which samples have been returned to Earth. In the summer of 1994, the Moon was very extensively
mapped by the little spacecraft Clementine and again in 1999 by Lunar Prospector.
The Moon is about one-quarter the diameter of the Earth---if placed on the United States, it
would extend from Los Angeles to almost Washington D.C. The Moon has held a special place in
history. This is because it moves quickly among the stars and it changes---it goes through a cycle of
phases, like a cycle of birth, death, and rebirth. The Moon is also primarily responsible for the tides
you experience if you spend any time near the coast. When Galileo looked through his telescope, he
discovered a wondrous place. The Moon became a place to explore. Galileo discovered impact craters,
mountains and valleys. The Moon is rough like the Earth.
The Moon also has large, dark smooth areas covering about 17% of the Moon's surface that
people originally thought were seas of liquid water so they are called mare (Latin for ``seas''---they are
what make out the face on the Moon). Now it known that the mare are vast lava flows that spread out
over many hundreds of square miles, covering up many craters that were originally there. The mare
material is basaltic like the dark material on the Earth's ocean crust and that coming out many of our
volcanoes (e.g., the Hawaiian islands). Mercury also has maria but they are lighter in color because of
the different chemical composition and they do not stand out from its heavily cratered areas.
Liquid water cannot exist on the Moon because of the lack of an atmosphere---the Moon has
only 1/81 the Earth's mass and about 1/6th the Earth's surface gravity. If there is any water to be found
on the Moon, it will be in a frozen state in a place of constant shade such as deep craters near the poles.
Recent missions have discovered some of those ice blocks near the poles. The ice blocks will be the
source of water for any humans that decide to set up bases on the Moon.
Craters
The Moon's surface is almost as old as the Earth. The rough highland regions are 3.8 to 4.2 billion
years old and the younger maria are between 3.1 and 3.8 billion years old. All of the planets and moons
experienced a period of heavy bombardment about 3.8 billion years ago that lasted for about 500
million years as most of the remaining chunks of rock left over from the formation of the solar system
pelted the planet and moon surfaces. The Moon, some of the moons of the giant planets, and Mercury
preserve a record of this bombardment. The record of this heavy cratering was erased on the Earth long
ago because of erosion and geologic activity that continues to this day. The Moon has no erosion
because of the lack of liquid water and an atmosphere and it is small enough that its interior cooled off
long ago so geologic activity has essentially ceased (an occasional very small moonquake can still
occur). The small size of the Moon meant that not much heat could be stored from its formation and its
small size also means that any remaining heat can easily escape to space (the ratio of its volume to its
surface area is smaller than that for the Earth).
Volcano craters are above the surrounding area on mountaintops while the craters from impacts
are below the surrounding area with raised rims. The craters on all of the moons except Io, Mercury,
and most of the ones on Mars are from impacts. The kinetic energy of the impacting meteorite or
asteroid is converted into heat, sound, and mechanical energy---the projectile explodes on impact. The
explosion is what carves out the crater so almost all craters are round (otherwise the great majority
would be oblong in shape). The rock on the surface of the planet or moon is bent backward, upward,
and outward so the amount of material ejected is much larger than the projectile. Large craters will
have a central peak formed by the rock beneath the impact point rebounding upward and they may also
have terracing of the inner walls of the crater from the collapsing of the crater rim inward. The size of
the craters having central peaks depends on the gravity of the planet or moon: on the Moon craters
larger than about 60 kilometers in diameter have central peaks while the crater diameter on the Earth
needs to be larger than just 1 to 3 kilometers.
The number of craters per unit area on a surface can be used to determine an approximate age
for the planet or moon surface if there is no erosion. The longer the surface has been exposed to space,
the more craters it will have. If you know how frequently craters of a given size are created on a planet
or moon, you can just count up the number of craters per unit area. This assumes, of course, that the
cratering rate has been fairly constant for the last few billion years. The heavy bombardment of about
3.8 billion years must be taken into account when using the crater age dating technique. For example,
the highland regions on the Moon have ten times the number of craters as the maria, but radioactive
dating shows that the highlands are approximately 500 million years older than the maria, not ten times
older. Careful studies of how the craters overlap other craters and other features can be used to develop
a history or sequence of the bombardment on the moons and planets.
Interior and Composition
The strong tides from the Earth pulled the early Moon's liquid interior toward the Earth, so the far
side's crust is now about 130 kilometers thick while the near side's crust is about 65 kilometers thick.
The thinness of the near side's crust is also why there are more mare on the near side than the far side.
The near side was thin enough to be cracked apart when large asteroids hit the surface and formed the
mare but the far side crust was too thick.
Our knowledge about the Moon took a huge leap forward during the Apollo missions. One main
science reason for going to the Moon was to return rock samples to find about their ages and
composition. Using their knowledge of geology gained from the study of Earth rocks, scientists were
able to put together a history for the Moon. The Apollo astronauts also left seismometers on the Moon
to detect moonquakes that can be used to probe the interior using seismology.
The Moon's density is fairly uniform throughout and is only about 3.3 times the density of
water. If it has an iron core, it is less than 800 kilometers in diameter. This is a sharp contrast from
planets like Mercury and the Earth that have large iron-nickel cores and overall densities more than 5
times the density of water. The Moon's mantle is made of silicate materials, like the Earth's mantle, and
makes up about 90% of the Moon's volume. The temperatures do increase closer to the center and may
be high enough to partially liquify the material close to the center. Its lack of a liquid iron-nickel core
and slow rotation is why the Moon has no magnetic field.
Lunar samples brought back by the Apollo astronauts show that compared to the Earth, the
Moon is deficient in iron and nickel and volatiles (elements and compounds that turn into gas at
relatively low temperatures) such as water and lead. The Moon is richer in elements and compounds
that vaporize at very high temperatures. The Moon's material is like the Earth's mantle material but was
heated to very high temperatures so that the volatiles escaped to space.
Formation
There have been a variety of scenarios proposed to explain the differences between the Moon and
Earth. The one that has gained acceptance after much study is the giant impact theory. Earlier theories
came in a variety of flavors.
1. The capture (pick up) theory proposed that the Moon formed elsewhere in the solar system and was
later captured in a close encounter with the Earth. The theory cannot explain why the ratios of the
oxygen isotopes is the same as that on the Earth but every other solar system object has different
oxygen isotope ratios. The theory also requires the presence of a third large body in just the right place
and time to carry away the extra orbital motion energy.
2. The double planet (sister) theory said that the Moon formed in the same place as the Earth but it
could not explain the composition differences between the Earth and Moon.
The spin (daughter or fission) theory said that the Earth rotated so rapidly that some of its mantle flew
off to the form the Moon. However, it could not explain the composition differences. Also, the spunoff mantle material would more likely make a ring, not a moon and it is very unlikely that the Earth
spun that fast.
The giant impact theory proposes that a large Mars-sized object hit the Earth and blew mantle
material outward which later recoalesced to form the Moon. The Earth had already differentiated by
the time of the giant impact so its mantle was already iron-poor. The impact and exposure to space got
rid of the volatiles in the ejecta mantle material. Such an impact was rare so is was not likely to have
also occurred on the other terrestrial planets. The one ``drawback'' of the theory is that it has a lot of
parameters (impactor size, speed, angle, composition, etc.) that can be tweaked to get the right result.
A complex model can usually be adjusted to fit the data even if it is not the correct one (recall
Ptolemy's numerous epicycles). But the giant impact theory is the only one proposed that can explain
the compositional and structural characteristics of the Moon.
Tides
The gravitational forces between the Earth and the Moon cause some interesting effects. The most
obvious is the tides. The Moon's gravitational attraction is stronger on the side of the Earth nearest to
the Moon and weaker on the opposite side. Since the Earth, and particularly the oceans, is not perfectly
rigid it is stretched out along the line toward the Moon. From our perspective on the Earth's surface we
see two small bulges, one in the direction of the Moon and one directly opposite. The effect is much
stronger in the ocean water than in the solid crust so the water bulges are higher. And because the
Earth rotates much faster than the Moon moves in its orbit, the bulges move around the Earth about
once a day giving two high tides per day.
But the Earth is not completely fluid, either. The Earth's rotation carries the Earth's bulges
slightly ahead of the point directly beneath the Moon. This means that the force between the Earth and
the Moon is not exactly along the line between their centers producing a torque on the Earth and an
accelerating force on the Moon. This causes a net transfer of rotational energy from the Earth to the
Moon, slowing down the Earth's rotation by about 1.5 milliseconds/century and raising the Moon into
a higher orbit by about 3.8 centimeters per year. (The opposite effect happens to satellites with unusual
orbits such as Phobos and Triton).
The asymmetric nature of this gravitational interaction is also responsible for the fact that the
Moon rotates synchronously, i.e. it is locked in phase with its orbit so that the same side is always
facing toward the Earth. Just as the Earth's rotation is now being slowed by the Moon's influence so in
the distant past the Moon's rotation was slowed by the action of the Earth, but in that case the effect
was much stronger. When the Moon's rotation rate was slowed to match its orbital period (such that the
bulge always faced toward the Earth) there was no longer an off-center torque on the Moon and a
stable situation was achieved. The same thing has happened to most of the other satellites in the solar
system. Eventually, the Earth's rotation will be slowed to match the Moon's period, too, as is the case
with Pluto and Charon.
Actually, the Moon appears to wobble a bit (due to its slightly non-circular orbit) so that a few
degrees of the far side can be seen from time to time, but the majority of the far side (left) was
completely unknown until the Soviet spacecraft Luna 3 photographed it in 1959. (Note: there is no
"dark side" of the Moon; all parts of the Moon get sunlight half the time (except for a few deep craters
near the poles). Some uses of the term "dark side" in the past may have referred to the far side as
"dark" in the sense of "unknown" (eg "darkest Africa") but even that meaning is no longer valid
today!)
The Moon has no atmosphere. But evidence from Clementine suggested that there may be
water ice in some deep craters near the Moon's south pole which are permanently shaded. This has now
been reinforced by data from Lunar Prospector. There is apparently ice at the north pole as well. A
final determination will probably come from NASA's Lunar Reconnaissance Orbiter, scheduled for
2008.
The Moon's crust averages 68 km thick and varies from essentially 0 under Mare Crisium to
107 km north of the crater Korolev on the lunar far side. Below the crust is a mantle and probably a
small core (roughly 340 km radius and 2% of the Moon's mass). Unlike the Earth, however, the
Moon's interior is no longer active. Curiously, the Moon's center of mass is offset from its geometric
center by about 2 km in the direction toward the Earth. Also, the crust is thinner on the near side.
There are two primary types of terrain on the Moon: the heavily cratered and very old
highlands and the relatively smooth and younger maria. The maria (which comprise about 16% of the
Moon's surface) are huge impact craters that were later flooded by molten lava. Most of the surface is
covered with regolith, a mixture of fine dust and rocky debris produced by meteor impacts. For some
unknown reason, the maria are concentrated on the near side.
Most of the craters on the near side are named for famous figures in the history of science such
as Tycho, Copernicus, and Ptolemaeus. Features on the far side have more modern references such as
Apollo, Gagarin and Korolev (with a distinctly Russian bias since the first images were obtained by
Luna 3). In addition to the familiar features on the near side, the Moon also has the huge craters South
Pole-Aitken on the far side which is 2250 km in diameter and 12 km deep making it the the largest
impact basin in the solar system and Orientale on the western limb (as seen from Earth; in the center of
the image at left) which is a splendid example of a multi-ring crater.
A total of 382 kg of rock samples were returned to the Earth by the Apollo and Luna programs.
These provide most of our detailed knowledge of the Moon. They are particularly valuable in that they
can be dated. Even today, more than 30 years after the last Moon landing, scientists still study these
precious samples.
Most rocks on the surface of the Moon seem to be between 4.6 and 3 billion years old. This is a
fortuitous match with the oldest terrestrial rocks which are rarely more than 3 billion years old. Thus
the Moon provides evidence about the early history of the Solar System not available on the Earth.
Prior to the study of the Apollo samples, there was no consensus about the origin of the Moon.
There were three principal theories: co-accretion which asserted that the Moon and the Earth formed at
the same time from the Solar Nebula; fission which asserted that the Moon split off of the Earth; and
capture which held that the Moon formed elsewhere and was subsequently captured by the Earth.
None of these work very well. But the new and detailed information from the Moon rocks led to the
impact theory: that the Earth collided with a very large object (as big as Mars or more) and that the
Moon formed from the ejected material. There are still details to be worked out, but the impact theory
is now widely accepted.
The Moon has no global magnetic field. But some of its surface rocks exhibit remanent
magnetism indicating that there may have been a global magnetic field early in the Moon's history.
With no atmosphere and no magnetic field, the Moon's surface is exposed directly to the solar
wind. Over its 4 billion year lifetime many ions from the solar wind have become embedded in the
Moon's regolith. Thus samples of regolith returned by the Apollo missions proved valuable in studies
of the solar wind.
Open Issues
1. Why are the maria concentrated on the near side?
2. Why is the Moon's center of mass off center? Because of the tidal lock with the Earth?
3. Now that we've found water on the Moon, what are we going do to with it?
4. Only twelve men have ever walked on the surface of the Moon. Who will be the 13th? Who will be
the first woman?
The Earth’s Moon Worksheet
1. What did the Roman’s call their moon?
2. The time between successive new moons is _________________ days.
3. Who visited the moon first?
4. When was the first landing on the moon?
5. What spacecraft mapped out the moon?
6. The moon is primarily responsible for the ___________________.
7. ______________ discovered the impact craters, _____________ and _____________.
8. What are the large dark smooth surfaces on the moon called?
9. Describe the mare?
10. Liquid water can’t exist on the moon because it lacks an ________________.
11. How old are the highland regions?
12. The craters are created by _______________ or _____________________.
13. The craters on a surface of the moon can be used to determine the ____________ because there is
no __________________________.
14. Explain why the far side is thicker than the near side?
15. Most our knowledge of the moon came during the ____________________ _________________.
16. What is the main reason for going to the moon?
17. The moon is deficient in ________________ and _______________________.
18. The moon’s mantle is made of _________________________.
19. Explain why the moon rotates synchronously?
20. Why does the moon wobble?
21. Name 3 famous craters on the far side of the moon?
22. Explain the theories of the moon formation?