Earth's Moon

General Remarks

Diameter3,475 km, or 1/4 of Earth's
Mass1.2% of Earth's
Density3.3 g per cc, 3 times denser than water
Gravity17% of Earth's
Escape Speed2.4 km/s, 1/4 of Earth's
Rotation Period27.3 days

The Moon is a small, rocky, geologically inactive, airless satellite of the Earth. Its lack of geological and atmospheric weathering means that its surface appearance is dominated by impact cratering. The Moon is differentiated, meaning that at one point in its history the Moon was completely molten, so that heavier elements sunk into its center and lighter elements settled near the surface.

Its low density indicates that the Moon has a low abundance of metals. Moon rocks returned by the Apollo missions showed that the Moon's elemental composition resembles that of the Earth, but there is a lack of water and other ices (e.g., ammonia, sulfur dioxide, carbon dioxide), and organic compounds (i.e., carbon-based). This result is a vital clue to the Moon's origin.

The Moon is large enough to eliminate any voids in its interior, so it is probably constructed of rocky material throughout, unlike asteroids and comets.

The Moon's rotation period is identical to its revolutionary period about the Earth. Therefore the Moon has one side always facing inward. The inward-facing side is called the "lunar nearside", and the outward-facing side is called the "lunar farside", or sometimes inaccurately called the "dark side of the Moon". This coupling of spin and orbit times is not restricted to our Moon. We see spin-orbit couples in Mercury, and in many moons of the gas giant planets.

Visual Appearance

Near the 1st and 3rd quarter Moon phases, we can see shadows from mountain ranges and crater walls. In other words, in these phases the topography of the Moon is apparent. Two types of terrain are apparent:

  • Lunar highlands - are heavily cratered
  • Lunar maria (singular = mare, pronounced MAH-ray) - are flat lowlands with few craters (1/20 as many craters as the highlands)

Near the full Moon phase, the reflectivity of the Moon's surface is best studied. Again, we can distinguish between the lunar highlands and lowlands:

  • Lunar highlands - are brighter and reflect light better
  • Lunar maria - are darker and reflect light less better

The differences in color and reflectivity indicate that lunar highlands and maria are composed of different compositions of rock. The differences in elevation between the two indicate that there are different eras in the history of the shaping of the Moon's surface.

Lunar Nomenclature

The word "mare" means "sea" in Latin, due to the superficial similarity in appearance of the dark rock to pools of liquid water. The names of the maria are also Latin, and are descriptive; an example is the Mare Tranquillitatis, "Sea of Tranquillity". One also sees maria with names other than "sea", such as oceanus (ocean), lacus (lake), and sinus (bay).

Craters are named after people, both real and mythical. Examples are Archimedes, Cook, Hercules, Tycho, Julius Caesar, and Schiller.

Mountain ranges, which are really the walls of the largest impact basins, are named for terrestrial counterparts, e.g., Alps, Carpathian, and Pyrenees.

Apollo Missions

The Apollo missions to the Moon, although politically-motivated, were able to accomplish some amazing feats of science and engineering. A short list of Apollo mission accomplishments would include: collection and return of rock and dust from the Moon's surface down to a depth of 2 meters, extensive orbital mapping, surface photography, the implacement devices to give us an idea of the Moon's interior (seismometers, magnetometers, radioactivity-detection devices), and the implacement of devices to measure the intensity of the light and particle emission of the Sun.

It is surprising that the names of the Apollo astronauts are not common knowledge. Below is a list of those missions that successflly touched down on the Moon's surface.
MissionDateCrewLanding Site
Apollo 11 20Jul69 Neil A. Armstrong, Commander
Michael Collins, Command Module Pilot
Edwin E. Aldrin, Lunar Module Pilot
Mare Tranquillitatis
Apollo 12 19Nov69 Charles Conrad, Jr., Commander
Richard F. Gordon, Jr., Command Module Pilot
Alan L. Bean, Lunar Module Pilot
Oceanus Procellarum
Apollo 14 5Feb71 Alan B. Shepard, Jr., Commander
Stuart A. Roosa, Command Module Pilot
Edgar D. Mitchell, Jr, Lunar Module Pilot
Fra Mauro
(ejecta from Mare Imbrium)
Apollo 15 30Jul71 David R. Scott, Commander
James B. Irwin, Command Module Pilot
Alfred M. Worden, Lunar Module Pilot
Foot of Appennine Mts
in Mare Imbrium
Apollo 16 21Apr72 John W. Young, Commander
Thomas K. Mattingly II, Command Module Pilot
Charles M. Duke, Jr., Lunar Module Pilot
Crater Descartes
Apollo 17 11Dec72 Eugene A. Cernan, Commander
Ronald B. Evans, Command Module Pilot
Harrison H. Schmitt, Lunar Module Pilot
Taurus-Littrow, highlands
and valley, on periphery
of Mare Serenitatis

Additionally, three robot missions from the former Soviet Union returned rock samples from the Moon: Luna 16, 20, and 24.

Impact Cratering

Craters are basins or bowl-shaped depressions made in a surface by the impact and subsequent detonation of the incident object, like a meteorite. Impact cratering is the dominant process to have shaped the Moon's surface. Craters range in size from microscopic in size to hundreds of km in diameter. The largest craters, called "impact basins", are also usually filled with newer, dark rock, and thus are also called maria. Impact basins are 300 km or more in diameter and are ringed with mountain ranges. All craters are circular in shape.

Some craters have bright lines of rock, like spokes, pointing toward their centers and extending hundreds or thousands of km from their centers. These craters are called "rayed craters", of which crater Tycho, in the Moon's southern hemisphere, is the most prominent.

In the lunar highlands, the craters are adjacent to one another, and there are obvious areas where craters lie on top of craters. These areas have reached a saturation of crater density - any new craters that form will simply erase older ones. We can therefore discover the relative ages of craters simply by seeing which craters lie "on top" of the local distribution. The rocks in these regions are called "breccias". A breccia is a rock which is formed when fragments of rock re-cement themselves together.

The lunar mare are less heavily cratered, and many of these craters are filled with the dark mare rock. We conclude that teh emplacement of the dark mare rock came after the formation of the maria.

A typical crater has a steep inner wall, a gently-sloping outer wall called an ejecta blanket, a central peak, and a number of smaller, secondary craters surrounding the main bowl.

A crater forms when an incident piece of rock strikes the Moon's surface. There is so much energy associated with the rock that it detonates on contact with the surface. The force of the detonation spreads out in all directions producing a circular basin. The impacting meteorite itself is destroyed on impact: it heats and vaporizes very quickly. The expanding vapor help to dig out the crater.

The rock removed from teh impact site is called "ejecta". The ejecta fall from the crater rim outwards, to form a gentle slope on the outside of the crater called an ejecta blanket. Some of the removed rock fly up and out from the inmpact site, creating smaller secondary when they land. Occasionally, long thin streamers of ejecta squirt out of the impact site. These streamers will fall in long straight lines away from the impact site, creating "rays", or bright spokes of shattered rock all pointing away from the impact site.

Most crater walls are too steep for the rock to support itself, so slumping of the inner wall can occur. The visual effect of slumping is to form what appear to be steps or tiers around the innner rim of the crater.

At the impact site itself, the rock has been shattered and may relax upwards. This relaxation is like a rebound. Heavy material has been removed from the surface, so the newly exposed sub-surface layer can lift upward. A central peak forms in the crater.

Cratering and surface age

Craters accumulate over time, if there are no processes by which they are erased. By counting craters, we can estimate the age of the surface of a planet. The Apollo astronauts collected rocks from a number of sites of different ages. By comparing the local crater density with the ages of the rocks, we have put together a picture of the impact history of the Moon, and by inference, of the solar system.

The planets formed about 4.5 billion years (Gy) ago. The first 700 million years (My) after their formation, the planets experienced heavy bombardment. A large crater was placed on the Moon every few thousand years. This time is often called the "era of impacts". Afterward, the impact rate tapered off quite considerably, and has remained constant over the last 3.8 Gy.

The observed density of rock in our solar system today matches the historical impact rate over the last 3.8 Gy. The rate of impact cratering depends upon the size of the meteorite. For example, a rock 1 meter in diameter burns up in Earth's atmosphere every day. Such a rock would leave a 10 m crater on the surface of the Earth were it to strike. Every century, a rock of about 30 m in diameter enters the atmosphere. A rock of this size was probably responsible for the famous Tunguska Event. In this case, the meteorite vaporized in the air and did not strike the ground. Every few hundred My a rock of size 10 km strikes the Earth. We feel that a meteorite of this size is responsible for the mass extinction that included the dinosaurs. The crater has been discovered in the Gulf of Mexico close to the Yukatan Peninsula.

Lunar Volcanism and Maria Formation

Lava from volcanic vents filled low-altitude areas on the Moon from 3.9 to 3.2 Gy ago. These lowest areas were the largest impact basins. Maria are found almost exclusively on the lunar nearside.

The centers of the terrestrial or Earth-like planets are heated by the radioactive decay of elements. The most important of these are uranium (isotopes with atomic mass numbers 235 and 238), thorium (232), and potassium (40). The heating is enough to keep the center of the Earth molten, and at one time all of Mercury, Venus, Earth, the Moon, and Mars were completely liquified.

After the Moon's surface froze to solid rock, liquid rock in the interior periodically flowed to the surface. This liquid rock did not form volcanoes, but rather bubbled up through cracks and flowed in large sheets. A similar process occurred in eastern Washington state. Each sheet would have been 30-50 m deep, and the total depth of this type of rock in the lunar maria is thought to be about 5 km.

As this rock cooled, it settled and changed size, producing a ripple-like effect. Long-lasting lava rivers carved thin curving channels in the rock called "rilles". Hadley Rille was visited via lunar rover by the Apollo 15 astronauts. At 1 km in width, it is one of the largest rilles on the Moon.

Lunar maria are found almost exclusively on the lunar nearside, probably because the average altitude is lower on this hemisphere.

Surface conditions

Before the Apollo landings, a number of unmanned missions were sent to the Moon to determine the structure of its surface. There was a worry that billions of years of impacts might have resulted in the Moon being covered in a thick layer of dust. It was discovered that there is a layer of very fine dust, but it is only a few cm deep in most places.

Underneath this dust is a layer of shattered rock called "regolith". Most of the dust and regolith in any location is the result of very local impacts, because most impacts are from small meteorites. The smallest meteorites, called "micrometeorites" after their size, can be found all over the Moon.

The surface temperature of the Moon varies from +110° C in full sunlight to -170° C in full darkness. The Apollo astronauts landed in twilight conditions when the surface temperature was moderate.

The craters and mountains themselves are rounded, not sharp. Atmospheric sculpting causes sharpness in mountains.

The Moon is a differentiated body, meaning that is spent some time in a liquid state, which enabled heavy elements to sink to the Moon's center and lighter elements to float to the surface. Moon rocks from the surface indicate that there is a strong similarity to Earth rocks, but Moon rocks are depleted in light elements and compounds that are easily evaporated away, such as carbon, sulfur, and water. These materials are called "volatiles".

Moon formation

Before the return of Moon rocks there were four theories as to the origin of the Moon. After the return of Moon rocks, three of these theories were eliminated. This result is one of the greatest of the Apollo program. The theories are (i) spin-off, (ii) co-formation, (iii) capture, and (iv) collision-ejection.

SPIN-OFF - Often called the "daughter" or "fission" theory, the spin-off theory states that a rapidly-spinning molten Earth can eject part of itself and form a double-planet. This theory predicts that Moon rocks would resemble those of the Earth's surface and mantle (sub-surface) and be rich in volatiles. In fact, Moon rocks are poor in volatiles. The prediction and observation do not match.

CO-FORMATION - Sometimes called the "sister" theory, this theory states that the Moon and Earth collapsed together from the same local region of the dust cloud that formed the solar system. This theory predicts that Earth rocks and Moon would be identical. They are not. The prediction and observation do not match.

CAPTURE - This theory predicts that the Earth and Moon formed in separate locations in the progenitor dust cloud, and then captured each other gravitationally. The prediction is that Earth rocks and Moon rocks should be unrelated, but in fact Moon rocks are similar to Earth rocks. The prediction and observation do not match.

COLLISION-EJECTION - This theory states that a large, Mars-sized body struck the Earth in an indirect, glancing blow. The rock ejected from the Earth would have orbitted the Earth and quickly collected itself into the Moon. The theory predicts that Moon rocks would resemble Earth rocks in their compositions, but that volatiles would have boiled away during the collision. The theory matches the observations. We therefore conclude that the collision-ejection theory is the correct one.