The Moon:

The Moon has fascinated mankind throughout the ages. By simply viewing with the naked eye, one can discern two major types of terrain: relatively bright highlands and darker plains. By the middle of the 17th century, Galileo and other early astronomers made telescopic observations, noting an almost endless overlapping of craters. It has also been known for more than a century that the Moon is less dense than the Earth. Although a certain amount of information was ascertained about the Moon before the space age, this new era has revealed many secrets barely imaginable before that time. Current knowledge of the Moon is greater than for any other solar system object except Earth. This lends to a greater understanding of geologic processes and further appreciation of the complexity of terrestrial planets.

The Moon is 384,403 kilometers (238,857 miles) distant from the Earth. Its diameter is 3,476 kilometers (2,160 miles). Both the rotation of the Moon and its revolution around Earth takes 27 days, 7 hours, and 43 minutes. This synchronous rotation is caused by an unsymmetrical distribution of mass in the Moon, which has allowed Earth's gravity to keep one lunar hemisphere permanently turned toward Earth. The above full disc of the Moon was photographed by the Apollo 17 crew during their trans-Earth coast homeward following a successful lunar landing mission in December 1972. Mare seen on this photo include Serentatis, Tranquillitatis, Nectaris, Foecunditatis and Crisium.

This image shows Earthrise over the Moon's limb.

Above is what the lunar surface was thought to look like in the 1800's.

Now we know it looks like the above image taking from the Apollo missions.

Many people have also seen imaginary faces on the Moon such as the Man on the Moon, the Lady on the Moon and the Bunny on the Moon.

This is a movie of Lunar rotation taken from the Clementine mission in 1994. Notice how the farside of the Moon does not have the large, smooth-looking maria that the nearside has.

The most obvious feature on the Moon is its many craters, due to impacts by space debris onto the surface. An Apollo 16 astronaut stands near the rim of Plum Crater (30m, or over 200 yards, in diameter).

Although Earth has experienced many meteorite impacts throughout its history, the action of wind and water quickly erases the resulting craters.

Earth-Moon system:

The Earth-Moon system forms one of the lowest ratios of primary to secondary diameters in the Solar System. The ratio of Earth to Lunar diameter is 3.6 to 1. For comparison, the next lowest is Saturn and its largest moon, Titan, with a ratio of 25 to 1. The Earth-Moon system is special in this regard and has an impact on the evolution of life.

Surface features:

The major surface features on the Moon are craters, highlands and maria.




The minor features are:

  1. wrinkle ridges
  2. scarps
  3. domes
  4. rilles


The typical features to an impact crater are shown below:

Craters range in size from microscopic to large basins of order 1000's km (see Orientale Basin below).

Normal, round craters are due to impacts from objects up to a couple thousand meters. Object larger (asteroids) will typically crack the crust and form impact basins.

Erosion is slow on a world without an atmosphere and is caused by:

  1. slumping (gravity)
  2. other impacts
  3. temperature changes
  4. moonquakes

The result is that young craters have sharp edges (usually less than 2x108 years old) and old craters are rounded, smoother (with ages of order a billion years old).


The lighter colored, heavily cratered regions are called the lunar highlands. The bare, chaotic terrain indicates that these regions are primordial and one would expect the oldest rocks in these regions. Mountains on the Moon are not due to tectonic activity, but rather are due to overlapping impact rims.


Dark-colored regions which turn out to be smooth plains of basaltic lava. They are the remnants of large impact events that cracked the crust and allowed the lava from the mantle to flow upward and erase early cratering. Note that the impacts must have occurred after the initial phase of cratering.

All the maria are on the nearside of the Moon, none on the farside. This is due to the fact that the nearside crust is thinner than the farside crust (easier to penetrate by impacts). The crust is thinner on the nearside due to tidal interaction with the Earth during the formation epoch.

Lunar Soil:

The lunar soil is a fine grained, cohesive sand/gravel containing glass spheres (impact ejecta), igneous dust and coarse breccia (cemented material from impacts).

Soil from the maria are 3 to 4x109 years old. Soil from the highland is 4.6x109 years old (from the early Solar System).

The soil composition for the Moon is similar to Earth soil but with very different element ratios. In particular, the lunar soil is rich in refractory elements (i.e. ones with high boiling points) and low in volatile elements (i.e. ones with low boiling points). The conclusion is that the Moon was formed from hotter materials than the Earth.

Apollo Lunar Program:

The effort to put men on the Moon was modivated by a patriotic/panicked reaction to the USSR launch of Sputnik in 1957, i.e. it was initially a political goal. The early space program was unique in that it represented leadership from the highest levels of government on down, with unpresendented support from Congress.

While the motivation was political, careful attention to the science goals were maintained. Those were:

The history of Lunar exploration is one of excellent planning and strategy on the part of NASA. The major components of this process were the following missions:

Altogether there were 6 landings on the Moon. The first three (Apollo's 11,12,14) returned lunar samples that were extremely local to the landing site (no one wanted to wander too far away from the lander vehicle). The last three (Apollos 15,16,17) took a dune buggy with them and drove around the lunar surface in order to perform more extensive sampling.

From the analysis of returned lunar samples, the following sequence of events regarding the geological history of the Moon have been determined:

The age dating of the lunar rocks have allowed us to identify four distinct periods in its geological history:

Origin of the Moon:

The Moon has long been an anamoly because its mass compared to the earth is 1/80 and there is very large for a planetary satellite

Possibilities for Lunar Origin:

Note that the Moon forms from Earth mantle material, which is low in density around 3 or so.

Since the Moon formed from the condensation of a debris ring in relatively near-earth orbit, the Moon was initially quite close to the earth. At this time (4.6 billion years ago) the earth was rotating quite rapidly (about once every 5 hours). The nearby Moon exerted large tidal forces on the spinning earth causing it to slow down. This process continues to day. To conserve total system angular momentum, the response of the Moon is then to move farther away from the earth.