Follow this link and learn about Apollo 11, the first landing of humans on an extraterrestrial body, on July 20, 1969. (logo by NASA).

 

Lick Observatory Image of the full moon provided by SEDS = Students for the Exploration and Development of Space hosted by the University of Arizona Chapter at the Lunar and Planetary Laboratory.

 

Astronaut, Lunar Rover and large boulder on the Moon (Apollo 17), provided by the National Space Science Data Center, Goddard Space Flight Center, Greenbelt, MD.

INTRODUCTION

Scientists may know more about our natural satellite than any other body in the solar system (besides the Earth). The Moon is the only other planetary body that humans have visited, and we have been able to gather much information from those visits, as well as from Earth-based telescopes and space probe imagery.

People have been looking to the Moon as long as they have been walking the Earth, gazing up at its luminance and beauty in wonder or in reverence. Recent discovery of water ice on the Moon has regenerated interest in the establishment of a permanent working colony. Hopefully, this module will give you a new appreciation and understanding of the Moon, so that you, too, may join the ranks of the "lunatics".

A close up view of the plaque which the Apollo 11 astronauts left on the moon in commemoration of the historic lunar landing mission. The plaque was attached to the ladder on the landing gear strut on the descent stage of the Apollo 11 Lunar Module. The plaque was covered with a thin sheet of stainless steel during flight.

(Image courtesy of NASA)

 

Visit the Lunar and Planetary Institute’s Exploring the Moon web page. Many questions in this module can be answered using information obtained at this site.

You might want to visit the Return to the Moon website to learn more about human expeditions to the Moon and ideas for permanent stations there.

Visit NASA's Vision for Space Exploration page and NASA's homepage to search out what recent discoveries about the Moon have been made. This site is an excellent Aeronautics and Space Resource for Educators – A MUST for Teachers of Planetary Geology !

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The Lunar Prospector, launched in January 1998, reached the Moon in four days and provided much information about the lunar surface in the form of high-resolution imagery. It used its complement of five science instruments to map the elemental components on the lunar surface. Prospector did not land on the lunar surface, but was intentionally crashed in order to gain additional information such as knowledge of potential water on the surface, which it did not confirm.

On the day humans first witnessed an "Earthrise" over another planetary body in 1969, we knew that planetary exploration and eventually settlement of other worlds is in our future. Actually, this is not too surprising because humans have ventured to new worlds, and traveled through harsh conditions (deserts, oceans, polar ice, etc.) since we began to trade and communicate. The only difference is that now our new worlds take us off this planet.

This view of the Earth rising over the Moon's horizon was taken from the Apollo 11 spacecraft. The lunar terrain pictured is in the area of Smyth's Sea on the nearside. Coordinates of the center of the terrain are 85 degrees east longitude and 3 degrees north latitude.

Image and caption courtesy of NASA.

NOTE: "Earthrise" is a misnomer! Why would it be impossible to watch the Earth rise if you were standing on the Moon? If the Apollo spacecraft was traveling west to east (like the shuttle orbits around Earth), would this scene be an apparent rising or an apparent setting Earth?

 

A Grumman Aircraft Engineering Corporation artist's concept depicting mankind's first walk on another celestial body. Here, Astronaut Neil Armstrong, Apollo 11 commander, is making his first step onto the surface of the Moon. In the background is the Earth, some 240,000 miles away.

Image and caption courtesy of NASA

 

 

Visit the Rice University website on The Galileo Project : and learn a bit about Galileo Galilei, born near Pisa in 1564 (the year Shakespeare was born, and Michelangelo and Calvin died). Note that Galileo made numerous astronomical discoveries, including mountains on the Moon:

 

Image provided by Los Alamos National Laboratory

 

 

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Lunar Facts:

• Mean distance from Earth 384,400 km
• Diameter at the equator 3,476 km
• Mass relative to Earth 0.0123
• Volume relative to Earth 0.02
• Surface gravity relative to Earth 0.165

Unlike Earth and our closest neighboring planets Venus and Mars, the Moon has no atmosphere and thus no chemical weathering. Lunar "soil" is actually a regolith made of breccia or pulverized lunar rock material (and some that was partially melted) during meteoroid impacts. Much of the surface is covered by regolith that is approximately 3 m thick.

The Moon Miner website has much information on how the Lunar regolith may be used for energy and mineral resources.

There is no wind (or water) to transport particulate matter, so transfer of matter across the lunar surface is enabled only by the forces related to relatively large impact events. Light is not diffused by air molecules into shadow regions either, so a stark contrast is apparent between lighted and non-lighted surfaces. All of these features can be observed in the following photograph of Buzz Aldrin on the Moon.

 

Astronaut Edwin F. Aldrin Jr., lunar module pilot, faces the camera as he walks on the Moon during Apollo 11 extravehicular activity. The plexiglass of his helmet reflects back the scene in front of him, such as the Lunar Module and Astronaut Armstrong taking his picture. Astronaut Neil A. Armstrong, Apollo 11 commander, took this photograph with a 70mm lunar surface camera. The astronauts footprints are clearly visible in the foreground.

Image and caption courtesy of NASA

 

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Phases of the Moon:

Phases of the Moon are particularly significant in our night sky. You should know what phase the Moon is in whenever you can see it, day or night. Examine the diagram of lunar phases and learn to interpret them relative to the position of the Moon with respect to the Sun and the Earth:

The lunar period of revolution around the Earth is 27.3 days, the same as the period of rotation. This means that the same side of the Moon faces toward Earth regardless of its position in the sky, so the Moon has a nearside and a farside.

There is no permanent "dark side of the Moon" (although this concept was popularized by the extraordinary British musical group Pink Floyd). The "dark side," like Earth’s night side, shifts regularly with rotation to produce night and day over the 27.3 day period, whereas darkness occurs at any place on Earth every 24 hours.

 

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Lunar Geology

The dark, smooth maria (seas) and the bright, rough, cratered highlands (terrae) of the Moon have been photographed extensively starting with Soviet spacecraft in the early 1960's. The NASA missions in the 1960's and 1970's gave us all a greater appreciation for this important component of our own planetary system.

Visit the Apollo Homepage to learn more about the fascinating Moon discoveries and check out the Top 10 discoveries made by these missions.

The Lunar and Planetary Institute’s The Apollo Landing Sites web page has much information on the locations of lunar landings. The URL for this page is: http://www.lpi.usra.edu/publications/slidesets/apollolanding/index.shtml

Moon Formation:

For centuries, people wondered how and why the Moon formed. Some thought it may have accreted in a fashion similar to that which formed the other planets, but chemical studies showed that the Moon is relatively poor in volatile elements (like K, Rb and Na) compared to its neighboring planets, so that idea didn't make sense.

Another possibility was that the Moon originally formed in the outer reaches of the solar system and was later captured by the Earth's gravitational field; however, the Moon’s current orbit is not consistent with physical laws of motion.

A third hypothesis is that the Moon was formed from material that was ejected out of the Earth early in its history. Earth supposed gave birth to the Moon when a giant, Mars-sized body collided with Earth and disintegrated on impact. The Earth had already begun differentiating, so some of the mantle was splashed out into space and re-solidified to form the Moon. The volatile elements largely would have remained in the form of vapor after this impact and not re-joined with the Moon material.

This collision theory is the most widely believed now, though there are still some unanswered questions about the exact details of the impact and early histories of the Earth and the Moon.

The Surface of the Moon:

The surface of the Moon is dominated by two main regions; the maria, or seas, and the highlands, or terrae.

Essentially every location on the Moon is covered by a thin layer of pulverized regolith, but this does not completely obscure the bedrock geology. Also, much information on major features is obtained by topography because the scale of observation (resolution) by telescopes or planetary probes is much wider than the thickness of the regolith.

Maria

The lunar maria are comprised of basaltic lava flows that lie within large impact basins. The maria are made up of large amounts of flood lava in thin layers that were, according to radiometric dating of samples brought back to the Earth, extruded several hundred million years after the basins formed. Occasionally lava overflowed the basins into adjacent basins or onto adjacent regions as evidenced by their continuous flow surfaces extending beyond the boundaries of the basin.

The melts were produced deep within the Moon during a large thermal event that lasted from ~4.3 to about 3.2 billion years ago. Radiometric ages of lunar basalts sampled in the maria range from ~3.8 to 3.1 billion years. Some breccias contain basalt that is older than this range, while basalts younger than this range are evident in stratigraphic relations determined by high-resolution photographs. Maria are predominantly seen on the near side of the Moon, with very few on the dark side (far). This may have something to do with the Moon's off-center core, which is displaced slightly towards the Earth.

Highlands

The lunar highlands make up about 2/3 of the near side of the Moon and dominate the far side.   They are characterized by rugged topography with up to 5000 meters of relief in some areas. Abundant craters vary from microscopic in size to nearly 900 kilometers in diameter. In contrast to the basaltic maria, the highlands are composed of anorthosite, a light-colored plagioclase feldspar-rich rock. These cratered areas are the oldest on the Moon and date back to the early intense bombardment associated with the beginnings of all the planets in our solar system. Also seen in the highlands are breccias, possibly formed by crater impacts.

Impact Craters:

By now, you know that the surface of the Moon is more heavily cratered in the highlands compared to the maria; however, much information about lunar geologic history can be obtained by close evaluation of craters.

Picture of Moon provided by SEDS - Students for the Exploration and Development of Space (many universities, hosted by the U. Arizona, Tucson chapter)

NOTE: The textbook, Exploring the Planets, by Eric Christiansen and Ken Hamblin (2^nd edition, Prentice-Hall, 1995) has an excellent discussion of crater formation in Chapter 4, The Moon. You will recall from the syllabus that this is a recommended second textbook for this course.

You also need to review Chapter 6 in your textbook, The Role of Collisions, by Gene and Carolyn Shoemaker.

 

Study Impact Crater Geology and Structure on the Lunar and Planetary website: http://www.lpi.usra.edu/expmoon/science/craterstructure.html

 

Impact Mechanisms - Kinetic energy is transferred by a compression shock wavethat spreads outward from the point of impact. A rarefaction (decompression) wave follows the compression wave and the overlying material is ejected upward, thus causing the crater to become excavated. The colliding meteoroid disintegrates to much smaller fragments, often melting and partially vaporizing as it is engulfed by the shock wave.

Detailed models and experiments show that craters will be roughly circular regardless of the angle of impact, unless it is less than about 20 degrees. Similar studies also show that the diameter of the crater is much greater than the diameter of the impactor.

Surrounding a large impact there may be secondary craters caused by impact of fragments ejected from the primary crater. An ejecta blanket made up of pulverized regolith and crust can be observed around relatively fresh craters. The youngest impact craters on the Moon, such as Tycho and Copernicus, are called ray craters in light of their distinct ray pattern of fresh ejecta.

Shock-melted material called impact melt sometimes fills small depressions and smoothes out the floors of larger-sized craters. This obviously reflects the tremendous amount of heat energy that is transformed from the initial kinetic energy of the meteoroid.

Subsequent modification - Once a crater has been formed, it will degrade over time as gravity and isostatic rebound modify the crater walls and floor. The central portion of a large crater will rise by isostatic uplift that results from removal of overburden. Walls may collapse by slumping to produce terraced crater walls which form the rings in multi-ring basins. Continued bombardment causes the crater to become less and less distinct from its surroundings.

Degradation also can be attributed to lava flows, atmospheric processes such as weathering and erosion, groundwater processes (sapping), and tectonism. Thus old craters may exhibit only remnants of the original crater walls.

Crater density. Refers to the number of craters, usually of a particular size range, that occur within a given area. Smaller craters are usually more abundant than large ones due to the inverse relation between impact frequency and size. Crater density is used to evaluate the age of a planetary surface, but many parameters (weathering, volcanism, etc.) have to be considered to apply this to an absolute age. Generally, relative ages are more important to determine the sequence of events that took place during planetary evolution.

Crater shape. Small craters, usually less than about 20 km diameter are typically simple bowl-like structures. Larger craters up to 200 km can be complex features with central peaks, and even larger ones can have concentric rings.

Examine the various types of craters observed on the Moon

Simple craters – bowl-shaped, range from microscopic to 20 kilometers in diameter. They are circular, less than 2 kilometers deep, and have a well-defined rim. The floor is either bowl shaped or flat. Moltke Crater – Image from NASA
Large basins with peaks and rings – have diameters between 200 and 300 kilometers (see Hamblin and Christiansen, 1995). There is no longer one central peak, but rather several peaks that form a ring about half way between the crater and the rim Impact structures larger than this are generally called Impact Basins   Picture of unnamed crater on farside of Moon provided by SEDS
Multiring basins – have diameters over 300 kilometers and often contain mare lava flows that filled the crater floor after the crater formation. These craters look like a bulls-eye, with low ridges radial around the crater. Examples include Orientale Basin, Nectaris Basin, and Imbrium Basin. Oblique view across Mare Imbrium (with 20 km diameter Pytheas crater near center) and Copernicus crater (near upper horizon) on the Moon.	Photo by the National Space Science Data Center, Goddard Space Flight Center, Greenbelt, MD.

 

• Study - Impact Basin Geology at the following website: http://www.lpi.usra.edu/expmoon/orbiter/orbiter-basins.html
• Study - the surface of the Moon using images shown above and see if you can evaluate the relative ages and types of impact features. This will become more apparent throughout the remainder of this module

After completing this part, go to The Moon Part II

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