|
Lunar
Geology |
 |
|
"The expedition round the moon had
enabled them to correct the many theories regarding
the terrestrial satellite. They knew which systems
should be rejected, what retained with regard to the
formation of that orb, its origin, its habitability.
Its past, present and future had even given up it's
last secrets."
-Jules Verne, (Round the Moon,
1865)
Moon Facts
The Moon is the only natural satellite of the Earth. It is
384,400 kilometers from Earth and has a diameter of
3,476 kilometers. The moon was 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, and 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 (as measured against the stars) since
the Earth moves a significant distance in its orbit
around the Sun in that time. |
 |
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. For an overview of the moon visit The
Three-Minute Guide to the Moon. |
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. 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 asymmetric nature of this gravitational
interaction is also responsible for the fact that
the Moon 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 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
noncircular 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.
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). |
 |
The
Moon has no atmosphere. But evidence from the
Clementine spacecraft suggested that there may
be water ice in some deep craters, which
are permanently shaded near the Moon's south pole.
This has also been confirmed by the Lunar Prospector
spacecraft. There is apparently ice at the north
pole as well.
|
The
Moon's crust averages 68 km thick and varies from
essentially 0 km 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 mantle 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 lunar
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.
|
Hadley
Rille flooded by molten lava |
 |
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 of the
moon 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 huge craters South Pole-Aitken,
on the far side which are 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. Read more about
lunar impacts.
|
|
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, decades
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.
Explore the moon's terrain at the interactive Lunar
Atlas site.
Origin of the Moon |
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 theories worked 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 the
Moon formed from the ejected material. |
At the time Earth formed 4.5 billion
years ago, other smaller planetary bodies were
also growing. One of these hit earth late in Earth's
growth process, blowing out rocky debris. A fraction
of that debris went into orbit around the Earth
and aggregated into the moon.
Two scientists, Dr. William K. Hartmann and Dr. Donald
R. Davis, were the first to suggest the leading
modern hypothesis of the moon's origin in a paper
published in 1975 in the journal Icarus. For more
about this theory click
here.
|
Computer
simulation of the formation of the Moon |
|
The Moon has no global magnetic field.
But, some of its surface rocks exhibit remnant 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.
|
Read an interesting
article about the origin of the moon by planetary
scientist Jeff Taylor with computer simulations!
Here are
the traditional names given to each month's full Moon
from the "Old Farmer's Almanac":
- January - Wolf Moon
- February - Snow Moon
- March - Worm Moon
- April - Pink Moon
- May - Flower Moon
- June - Strawberry Moon
- July - Buck Moon
- August - Sturgeon Moon
- September - Harvest Moon
- October - Hunter's Moon
- November - Beaver Moon
- December - Cold Moon
Next... Recap:
Apollo (pg. 3 of 9) |
|
|
|
