THE MOON

Key Concepts

The Moon's surface is divided between smooth, dark, low maria and cratered, light highlands.
The Moon's surface was shaped by heavy bombardment, followed by lava floods.
The Moon may have been ejected when a giant (Mars-sized) planetesimal collided with the young Earth.

(1) The Moon's surface is divided between smooth, dark, low maria and cratered, light highlands.

The Earth is unique among the terrestrial planets in having a giant moon. The Earth's moon (commonly called THE Moon), is 3500 kilometers in diameter (27% of the Earth's diameter), and is 380,000 kilometers away, on average. As seen from Earth, the Moon has an angular size of half a degree, and dark and light markings can clearly be seen upon it, even without the aid of a telescope.

The joint portrait of the Earth and Moon seen below was taken by the Galileo spacecraft when it was 6.2 million kilometers from the Earth, on December 16, 1992. The Moon is in the foreground. (Contrast and color have been computer-enhanced in this image.)

[Image credit: NASA, JPL, Galileo Project]

The darker regions of the Moon's surface are called maria. (``Maria'' is the plural form of ``mare'', the Latin word for ``ocean'' or ``sea''. ``Maria'' is accented on the first syllable.) Despite their misleading name, maria are not liquid seas. They are solid. In addition to being darker than the rest of the Moon (they reflect only 7% of the visible light that strikes them), maria are relatively smooth and level, and have few craters.

The lighter regions of the Moon's surface are called highlands. In addition to being brighter than the maria (reflecting twice as much of the light that strikes them), they are higher in elevation, and heavily cratered.

[Image credit: Clementine, BMDO, NRL, LLNL]

Lunar craters were all formed by the impact of comets and asteroids; they are not volcanic craters. The Moon has about 100,000 craters over 1 kilometer across. Most of these are on the highlands, but a few craters are scattered over the maria as well. Earth has many fewer craters than the Moon. This is because old craters on the Earth have been erased by erosion: the result of flowing water, falling rain, and blowing wind. The Moon has no atmosphere and no liquid water, so erosion of lunar craters is very slow; they are only slowly obliterated by the impact of tiny micrometeoroids on the Moon's surface.

When a meteoroid strikes the Moon (or any other body in the Solar System), a great deal of energy can be released. When a meteoroid 10 meters across strikes the Moon, it releases the same amount of energy as a 100 kiloton bomb, and gouges out a crater 100 to 200 meters across.

(2) The Moon's surface was shaped by heavy bombardment, followed by lava floods.

The Moon's surface is quite different from that of the Earth. The Earth's surface is shaped mainly by:

plate tectonics
erosion

The Moon has a crust which is all one piece, not broken into plates. Further, as mentioned above, there is only a puny amount of erosion on the Moon. The Moon's surface is shaped mainly by:

impact craters
lava floods

We know a good deal about the history of the Moon's surface thanks to the Apollo missions to the Moon.

The Apollo program consisted of 6 landings on the Moon, starting with Apollo 11 in July 1969 and ending with Apollo 17 in December 1972. (Apollo 13 didn't land on the Moon; the mission was aborted after one of the spacecraft's oxygen tanks exploded.) If you are historically inclined, and want more information about the Apollo program, try this link from the National Air and Space Museum. The Apollo missions brought back Moon rocks from both maria and highland regions. On Earth, their chemical composition was analyzed and their ages were found from radioactive dating. The results:

Maria rocks: dense igneous basalts (like the basalts making the ocean floors on Earth)
mostly 3.1 to 3.9 billion years old.
Highland rocks: low-density igneous rock, called ``anorthosite''
mostly 4.0 to 4.3 billion years old (a few are even older).

Thus, highland rocks are older than maria rocks. However, both types of Moon rock are older than the average Earth rock.

From the information yielded by the Apollo rocks, it is possible to reconstruct the Moon's geological history.

4.5 to 3.8 billion years ago
Heavy bombardment by leftover planetesimals and other interplanetary rubbish. During this period, the highlands became heavily cratered.
3.8 to 3.1 billion years ago
Lava flowed upward through cracks in the Moon's crust, repaving the lowlands with fresh lava, and creating the smooth, relatively young maria.
3.1 billion years ago to NOW
Rapidly decreasing lava flows and decreasing bombardment with meteoroids.

The Moon is now geologically dead, and has been for the last 3 billion years or so. The Moon today looks almost the same as it did 3 billion years ago, with only a few additional craters added. The Earth, by contrast is geologically active, and has been for its entire history, with lots of volcanos, earthquakes, and plate motions to keep life interesting. The Earth today looks quite different from the way it looked 3 billion years ago; plate tectonics have totally reshuffled the Earth's continents around.

Why is the Moon geologically dead while the Earth is still active? The main reason is simply that the Moon is smaller than the Earth. The Earth and Moon started out the same temperature 4.6 billion years ago, but small objects cool faster than large objects. Thus, the Earth has only a thin, brittle lithosphere, which the convection currents in the plastic mantle are able to break up and carry to and fro, but the Moon has a thick, cold solid lithosphere, too strong to be broken into plates.

(3) The Moon may have been ejected when a giant planetesimal collided with the young Earth.

But how did the Earth get such a whopping big moon in the first place? Many theories for the Moon's formation have been proposed; all but one have been ignominiously shot down. One difficulty that all theories must address is the fact that the chemical composition of the Moon is different from that of the Earth. In particular, the Moon is low in iron.

The interior structure of the Moon, like that of the Earth, is differentiated; at some time in the past, the Moon was liquid. The layers of the Moon, working from the outside in:

Crust: solid rock, mainly anorthosite
averaging 80 kilometers thick.
Outer Mantle: solid rock, mainly basalt
about 700 kilometers thick.
Inner Mantle: plastic (semisolid) rock
about 600 kilometers in radius.
Core: iron
perhaps 300 kilometers in radius, making up a few percent, at most, of the Moon's mass.

The theory that best explains why the Earth has a large, iron-poor moon is the Collisional Ejection Theory . The best available theory for the formation of the solar system states that planets formed by the collision of smaller planetesimals. During the last stages of planet formation, planetesimals had grown to be quite large, and their collisions with planets were not gentle. The giant impact theory for the creation of the Moon states that about 4.6 billion years ago, the young Earth was struck by a gigantic planetesimal, about the size of Mars. (Mars is 9 times the mass of the present Moon and 1/9 the mass of the present Earth.) Both the Earth and the giant planetesimal were differentiated, containing rocky mantles over iron cores.

[Image credit: William K. Hartmann]

The collision was not head on, but oblique. During the collision, vast amounts of energy were released, melting both the Earth and the colliding planetesimal. Computer simulations reveal that the rocky mantle of the planetesimal was ejected into orbit, eventually cooling to form the Moon. The iron core of the colliding planetesimal sank to the center of the Earth and merged with the Earth's core. Thus, the Moon ended up being made almost entirely of rock, while the iron content of the Earth was actually enhanced by its cannibalism of the planetesimal's iron core.

updated: