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MOON
PROPERTIES
The Near Side
It is divided into light areas
called the Lunar Highlands and
darker areas called Maria
(literally, "seas"; the singular is
Mare).
The Far Side
One of the discoveries of the
first Lunar orbiters is that
the far side has a very
different appearance than the
near side. In particular, there
are almost no Maria on the far
side, as illustrated in the
image shown to the left of a
portion of the far side
surface. In this figure a
number of meteor impact
craters are visible.
Cratering Density
The amount of
cratering is usually an
indication of the age
of a geological surface:
the more craters, the
older the surface,
because if the surface
is young there hasn't
been time for many
craters to form
In contrast the surface of the
Moon is much older, with much
more cratering. Further,
different parts of the surface
of the Moon exhibit different
amounts of cratering and
therefore are of different
ages: the maria are younger
than the highlands, because
they have fewer craters.
Different Types of Impact Craters
Most of the craters on
the Moon are circular.
The few craters that
are not circular, like
Messier and Messier A
(pictured at the left)
in the Mare
Fecunditatis, are an
enigma. Scientists do
not know exactly how
these oddly-shaped
craters were formed
Most craters on the Moon that have
diameters less than about 15
kilometers have a simple, bowl-like
form.
Simple Crater
Moltke crater, a simple crater with a
diameter of 4.3 miles (7 km).
Complex Crater
Euler crater, a complex
crater with a diameter of
17 miles (28 km) and a
depth of 1.5 miles (2.5
km).
Simple Impact Craters:
Simple impact craters have bowl-shaped
depressions, mostly with smooth walls. This
type of crater generally has a diameter less
than 9 miles (15 km). Their depth is about 20%
of the diameter.
Complex Impact Craters:
Complex impact craters have a single
or multiple peaks in the middle of the
crater. These craters have diameters
between about 12 and 110 miles (20
and 175 km), and the central uplift is
usually one or a few peaks. Craters
with a diameter over 110 miles (175
km) can have more complex, ringshaped uplifts within the crater.
Impact Basin: A impact basin is an
impact crater that has a rim diameter
greater than 185 miles (300 km). There
are over 40 impact basins on the Moon.
These catastrophic impacts produce
faulting and other crust deformations.
Material ejected from impact basins is
distributed over wide areas.
The oldest surfaces in the Solar
System are characterized by
maximal cratering density. This
means that one cannot increase the
density of craters because there
are so many craters that, on
average, any new crater that is
formed by a meteor impact will
obliterate a previous crater,
leaving the total number
unchanged. Some regions of the
moon exhibit near maximal
cratering density, indicating that
they are very old.
The Lunar Surface Material
The bulk density of the Moon is 3.4 g/cc,
which is comparable to that of (volcanic)
basaltic lavas on the Earth (however, the
bulk density of the Earth is 5.5 g/cc,
because of the dense iron/nickel core). The
Moon is coverered with a gently rolling
layer of powdery soil with scattered rocks
that is called the regolith; it is made from
debris blasted out of the Lunar craters by
the meteor impacts that created them.
Each well-preserved Lunar crater is
surrounded by a sheet of ejected material
called the ejecta blanket.
Geological Composition
One striking difference
between the Lunar
surface material and that
of Earth concerns the
most common kinds of
rocks. On the Earth, the
most common rocks are
sedimentary, because of
atmospheric and water
erosion of the surface. On
the Moon there is no
atmosphere to speak of
and little or no water, and
the most common kind of
rock is igneous ("fireformed rocks").
Geologically, the Lunar
surface material has the
following characteristics:
The Maria are mostly composed of dark basalts, which
form from rapid cooling of molten rock from massive
lava flows.
The Highlands rocks are largely Anorthosite, which is a
kind of igneous rock that forms when lava cools more
slowly than in the case of basalts. This implies that the
rocks of the Maria and Highlands cooled at different rates
from the molten state and so were formed under different
conditions.
Breccias, which are fragments of different rocks
compacted and welded together by meteor
impacts, are found in the Maria and the
Highlands, but are more common in the latter.
Lunar Soils contain glassy globules not commonly found on the Earth.
These are probably formed from the heat and pressure generated by
meteor impacts.
The Anorthosites that are common in the Lunar Highlands are not common on
the surface of the Earth (The Adirondack Mountains and the Canadian Shield
are exceptions). They form the ancient cores of continents on the Earth, but
these have largely been obliterated by overlying sedimentary deposits and by
plate tectonic activity.
Age of Lunar
Material
The abundances of radioactive elements in rock samples can be
used to tell the age of the rock in a process called Radioactive
Dating. When such techniques are applied to the Lunar rock
samples, one finds the following:
Samples from Mare Imbrium and the Ocean of
Storms brought back by Apollo 11 and Apollo 12
are about 3.5 billion years old, which is
comparable to the oldest rocks found on the
surface of the Earth.
The ejecta blanket from the Imbrium Basin (which
was formed by a gigantic meteor impact) was
returned by Apollo 14 and found to be about 3.9
billion years old.
Lunar Highlands rocks returned by Apollo 16 are about 4 billion years old. The
oldest Lunar rock found was located by Apollo 17 and appears to be about 4.5
billion years old.
Thus, the oldest material from the surface of the Moon is almost as
old as we believe the Solar System to be. This is more than a billion
years older than the oldest Earth rocks that have been found. Thus,
the material brought back from the Moon by the Apollo missions
gives us a window on the very early history of our Solar System that
would be difficult the find on the Earth, which is geologically active
and has consequently has obliterated its early geological history.
The Structure of the Interior
Although there is a small amount of
geological activity on the Moon, it is largely
dead geologically (the energy associated
with the Earth's seismic activity is about
10^14 times larger than that of the Moon).
Most Lunar seismic activity appears to be
triggered by tidal forces induced in the
Moon by the Earth.
Geological History of the Moon
The weight of the evidence is that the
Moon was active geologically in its
early history, but the general evidence
suggests that the Moon has been
essentially dead geologically for more
than 3 billion years. Based on that
evidence, we believe the chronology of
Lunar geology was as follows:
The Moon was formed about 4.6 billion years ago; maybe hot or maybe
cold. The surface was subjected continuously to an intense meteor
bombardment associated with debris left over from the formation of the
Solar System.
By about 4.4 billion years ago
the top 100 km was molten,
from original heat of formation
and from heat generated by
the meteor bombardment.
By 4.2 billion years
ago the surface was
solid again
As the intense meteor bombardment associated with debris
left over from the formation of the Solar System continued,
most of the craters that we now see on the surface of the
Moon were formed by meteor impact.