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The Moon is the
only natural satellite of Earth:
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orbit: |
384,400 km from Earth |
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diameter: |
3476 km |
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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.
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.
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.
The Moon has no atmosphere. But evidence from Clementine suggests that there may be water ice in some deep craters near the Moon's south pole. If this turns out to be right, it is of great importance to the future of lunar exploration.
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 farside. Below the crust is a mantle and possibly a small core. 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, scientist 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.
