of the Moon
The surface of the Moon has two hemispheres with rather asymmetric properties;
as a consequence the nature of the Lunar surface that we can see from the Earth
is substantially different from the surface that is always hidden from the
The Near Side
The face of the Moon
turned toward us is termed the near side (image at right).
It is divided into light areas called the Lunar Highlands and darker
areas called Maria (literally, "seas"; the
singular is Mare). The
Maria are lower in altitude than the Highlands, but there is no water on the
Moon so they are not literally seas
(Recent evidence from the Clementine spacecraft suggests that there may be some
water on the Moon, contrary to previous assumptions).
The dark material filling the Maria is
actually dark, solidified lava from earlier periods of Lunar volcanism.
Both the Maria and the Highlands exhibit large craters
that are the result of meteor impacts. There are many more such impact craters
in the Highlands.
The Far Side
The side of the Moon unseen from the Earth is called 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.
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. Thus, the Earth has a
relatively young surface because it has few craters. This is because the Earth
is geologically active, with plate tectonics and erosion having obliterated
most craters from an earlier epoch. 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
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.
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 ("fire-formed rocks").
Geologically, the Lunar surface material has the following characteristics:
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
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
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
The Lunar rocks may also be examined according to the chemicals that they
contain. Such analysis indicates:
The high concentration of rare metals like Titanium, and the availability of
abundant amounts of Silicon and Oxygen has led to serious proposals about
mining and manufacturing operations in the future for the Moon.
They are rich in refractory elements, which are elements such as
calcium (Ca), Aluminum (Al), and Titanium (Ti) that form compounds having high
They are poor in the light elements such as hydrogen (H).
There is high abundance of elements like Silicon (Si) and Oxygen (O).
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:
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
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
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.