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. Due to its size and composition, the Moon is sometimes classified as a
terrestrial "planet" along with Mercury, Venus, Earth and Mars. 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 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 get 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. 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 confirmed by Lunar Prospector.
There is apparently ice at the north pole as well. The cost of future lunar
exploration just got a lot cheaper! 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's
mantle, however, the Moon's is only partially molten. 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 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, 20
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 hydrogen 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. This lunar hydrogen may also be of use
someday as rocket fuel.