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.

Above 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.

  • The Moon/Earth mass ratio is 1/80, also unprecedented in the Solar System

  • The radius of the Moon is 1738 km with a mean distance from the Earth of 384,000 km.

  • The average density of the Moon is 3.37 g/cm3 which is consistent with basaltic silicates and not consistent with a large iron (Fe)/nickel (Ni) core like the Earth.

  • The lunar surface is grey-tan with a low albedo of 0.07.

  • The combination of the Moon's lower mass and its smaller radius means that its surface gravity is 1/6 the Earth's surface gravity. Walking on the Moon is much different from walking on the Earth, as is falling on the Moon.

    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.


    Mercury is the smallest of the major planets with a radii of only 2440 km (1590 miles). Since it is the closest planet to the Sun its greatest elongation is only 28 degrees (i.e. its only visible right after sunset or right before sunrise). Since it is located near the Sun at sunrise or sunset, the ancients thought that Mercury was two different planets, Lucifer and Hermes.

    From its radius (i.e. volume) and mass we calculate it has a mean density of 5.4 g/cm3 which implies a dense iron (Fe) core. All planets form as molten balls, then cool and solidify. The cooling rate is proportional to the amount of material (the planet's mass) and its surface area (its radius). Since Mercury is low in mass (less heat stored from formation), its core is probably solid rather than liquid.

    The rotation period of Mercury is 58.6 days and its orbital period (year) is 87.9 days. Notice that 2/3 times 87.9 is 58.6; thus, Mercury suffers from spin-orbit coupling where the tidal forces from the Sun has locked Mercury's rotation into a resonance number (1/2, 2/3, 4/5, 5/6, etc...).

    Daytime temperatures on the surface of Mercury hover around 700 degrees Kelvin (enough to melt lead). Whereas, at night the ground temperature plunge to a mere 100 Kelvins (air turns to liquid at 77 Kelvins). Dawn is ten times more brilliant than on the Earth, since the Sun is ten times larger. The lack of any significant atmosphere means that before dawn you can see the Sun's corona spreading over the horizon.

    The above image is of Mercury from the Messenger mission (2008) The north pole is at the top and the equator extends from left to right about two-thirds down from the top. Bright rayed craters are prominent in this view of Mercury. One such ray seems to join in both east-west and north-south directions.

    The above mosaic shows the Caloris Basin (located half-way in shadow on the morning terminator). Caloris is Latin for heat and the basin is named this because it is near the subsolar point (the point closest to the sun) when Mercury is at aphelion. Caloris Basin is 1,300 kilometers (800 miles) in diameter and is the largest know structure on Mercury. It was formed from an impact of a projectile with asteroid dimensions. The interior floor of the basin contains smooth plains but is highly ridged and fractured. North is towards the top of this image.

    The above ``weird terrain'' best describes this hilly region of Mercury. This area is at the antipodal point from the large Caloris basin. The shock wave produced by the Caloris impact was reflected and focused to this antipodal point, thus jumbling the crust and breaking it into a series of complex blocks. The area covered is about 100 kilometers (62 miles) on a side.

    The above picture is a Mariner 10 image shows Santa Maria Rupes, the sinuous dark feature running through the crater at the center of this image (note how is passes through the crater rim indicating that it was created after the impact crater). Many such features were discovered in the Mariner images of Mercury and are interpreted to be enormous thrust faults where part of the mercurian crust was pushed slightly over an adjacent part by compressional forces. The abundance and length of the thrust faults indicate that the radius of Mercury decreased by 1-2 kilometers (0.6 - 1.2 miles) after the solidification and impact cratering of the surface. This volume change probably was due to the cooling of the planet, following the formation of a metallic core three-fourths the size of the planet. North is towards the top and is 200 kilometers (120 miles) across.

    Surface features:

    In general, the surface of Mercury is similar to the Moon (i.e. heavily cratered due to a lack of a heavy atmosphere to erode away primordial impacts). However, there are some key differences:

    1) There are few maria on Mercury and they are small. No large impact era like the Moon. Therefore, Mercury must have cooled faster.

    2) Cratering is less heavy, more plain region between craters. Due to the higher surface gravity on Mercury (you weight more than on Mercury than the Moon). This means impacts did not throw debris as far, fewer secondary craters and more concentrated around primary crater.

    3) Long scarps or wrinkles are found on the crust and the tops of craters (i.e. after cratering epoch). After Mercury cooled, its crust solidified first. Mercury was still rotating quickly back then and had an equatorial bulge. As Mercury slowed in its rotation, due to tidal forces with the Sun, gravity pulled Mercury into a more spherical shape and the crust had to fold producing long scarps.

    Mercury's Magnetic Field:

    Although Mercury has a high density, implying it is rich in iron (Fe), almost no Fe is detected by spectroscopy of its surface. Thus, most of the Fe must has sunk into the core while Mercury was young and molten. And, that it must have stayed in a molten state for much longer than the other planets formed at the same time.

    These facts are confirmed by the fact that Mercury has a strong magnetic field, even though it rotates slowly. This implies a liquid core for Mercury, which is in contradiction with the theory that the core is solid from cooling arguments. One possible solution is that the core is a mixture of iron and some other material such as sulfur.

    Mercury's strong magnetic field also gives it a very weak atmosphere. Planet's as hot as Mercury quickly lose their atmosphere's as the high temperatures heat the atmosphere molecules to escape velocity. However, Mercury is very close to the Sun and is able to capture some of the Sun's solar wind (composed of protons and electrons) in its magnetic field, giving it a very thin and tenuous atmosphere.