LUNAR GEOLOGY

INTRODUCTION

Geologists piece together the physical evolution and history of our planet by studying materials such as rocks and minerals , as well as the processes that operate far below ground.  But the science of geology is not limited to the earth.  Geologists also study our moon, other planets and their moons, asteroids , and meteorites .  While geologists can study meteorites without much difficulty, the study of other planets and their moons is done with space probes launched from earth.  The information geologists gather from these remote places adds to our understanding of the origin and evolution of the solar system.  But there are always mysteries.  The origin of our moon is one mystery that has plagued geologists and scientists for many years.  The successful Apollo Program, however, provided geologists and scientists with over 840 pounds of moon rocks that led to new discoveries and a new hypothesis on the origin of our moon.   This tutorial will explain what the scientific method is, how it is applied by geologists studying the origins of the moon, and how the Apollo Program significantly added to the growing body of knowledge about our planet and its moon.

THE SCIENTIFIC METHOD

Already in this tutorial, I have used the term theory.  A theory is a generally accepted explanation of one or more related natural phenomena�such as the origin of the solar system and the moon�that is supported by a large body of evidence.  Theories are formulated through a process called the scientific method.  The first step in the scientific method is to gather all relevant information on the phenomena of study.  This information is then used to formulate tentative explanations for the phenomena called hypotheses.  Each hypothesis, if true, makes certain predictions.  Each hypothesis is then tested to see if what they predict actually occurs.  Over time and through many tests, competing hypotheses are eliminated if their predictions do not occur.  If one hypothesis better explains the phenomena and all the predictions it makes are found to occur, that hypothesis is then proposed as a theory.  Once a hypothesis becomes a theory, its testing and refinement do not end.

ORIGIN OF THE MOON

Have you ever stared up at the moon and wondered how it got there?  If so, you�re not alone.  For thousands of years, humans have looked up and wondered what the moon was and why it dominated the black sky.  The moon was a mystery that people tried to solve.  The Iroquois Indians believed that the moon was created by the Sky Woman.  The Mayans believed that the moon inhabited the earth before changing into a celestial body.    Without science, people were left to speculate about a celestial body they could not fully comprehend from earth.  Ancient Greeks thought that the dark portions on the moon were seas (which they called maria) and that the light portions of the moon represented dry land (which they called terrae) (figure 1).

It wasn�t until the astronomer Galileo peered through his newly invented telescope in 1610 that the true nature of the moon�s vast, desolate landscape came into focus.  But the world was not yet ready for science to explain natural phenomena.  Galileo (figure 2) was tried for heresy for stating that the earth revolved around the sun instead of the other way around.  This prompted many scientists working in the 17 ^th century to keep their findings, hypotheses and theories to themselves.

It wasn�t until 1878 that the first real hypothesis of the origin of the moon was proposed.  George Howard Darwin (Charles Darwin�s son) believed that the moon was once part of the earth.  He stated that after the formation of the earth it spun so rapidly that it elongated.  The sun�s gravity then ripped off a chunk of the planet.  This chunk settled into orbit around the planet earth.  The deep Pacific Ocean basin was the area from which this chunk was ripped.  This hypothesis on the origin of the moon was accepted by most scientists well into the 20 ^th century.  The next hypothesis to gain scientific support was the �captured planet�.  This hypothesis, proposed by Thomas Jefferson Jackson See, stated that the moon was a small planet that was captured by the earth�s gravity.  The third hypothesis is coaccretion.  Many astronomers, including Edouard Roche, believed that the moon was created at the same time as the earth, that the matter that had begun to accrete to form the earth, was already being orbited by another cloud of matter that was accreting to become the moon.  By the time the Apollo Program began, the first hypothesis proposed by George Howard Darwin was no longer accepted by most scientists.  The second and third hypotheses, however, were the leading candidates that explained the origin of the moon.  But the Apollo Program was such a success that the information collected by the Apollo astronauts led to the proposal of a new hypothesis on the origin of the moon: the big whack. 

I should note here that some of these hypotheses are referred to as theories by articles and reports for the general public.  This is because most people do not understand the difference between a hypothesis and a theory.  So whenever you see someone using the word theory, always question whether or not it truly is a theory, or whether the word theory is being used in place of hypothesis.  

THE BIG WHACK

The major difference between the Apollo Program and all other explorations on other celestial bodies is that humans explored another world firsthand�they were there; they walked upon the moon�s surface; they bought back samples they had collected with their hands.  It should not be surprising that the samples collected by the Apollo astronauts provided scientists with the most valuable source of information regarding another world that has ever been collected.  Space probes and robotic rovers that gather information on other celestial bodies do provide scientists with important information, but these devices are limited.  Nothing can replace a human�s ability to evaluate and re-think a situation or to use intuition.  Sending humans to other worlds is expensive, but the information they could gather would be invaluable (figure 3).

The information gathered by the Apollo astronauts was just that: invaluable.  Before the Apollo Program, scientists thought they had narrowed down the origin of the moon to two possibilities.  But the samples collected by the Apollo astronauts proved otherwise.  The key factors geologists looked at regarding the moon�s relationship to the earth was density , volatile elements , and the size of the core.  The samples collected by the Apollo astronauts revealed that the moon was less dense than the earth and that the moon was lacking volatile elements.  The moon also has a relatively smaller core than the earth.  Yet, there were enough similarities between elements found in lunar samples with samples of rocks here on earth that scientists believed the earth and the moon were related, kind of like a cosmic DNA test.  None of the previous hypotheses could explain all these facts effectively.  Plus, there was one more factor that had not been considered by scientists investigating the origin of the moon: the tilt of the earth�s axis.  Most of the other planets in the solar system rotate with their poles perpendicular to their orbital planes around the sun.  If a planet has a tilt in its axis, it usually means that something quite dramatic had occurred in its history, like a collision with a large meteorite, asteroid, moon, or even another planet. 

Taking into account all these factors, scientists formulated the big whack hypothesis.  This hypothesis states that a large planet-sized celestial body collided with the earth soon after it�s formation.  This impact was so great that it destroyed the celestial body and nearly the earth itself.  A great cloud of matter swirled around the wounded earth.  Some of the matter fell back into the earth, some drifted off into space, and a small amount of matter began orbiting the earth.  This matter then accreted over time into a spherical planetoid: our moon.  This hypothesis seems to explain all the above differences.  The earth absorbed most of the impacting body�s core, which would explain the difference in core size; the earth gained its volatile elements through the bombardment of comets and meteorites, which would explain why the earth has more than the moon; and the impact would have sent much of the impactor�s remains into orbit along with large portions of the earth�s mantle, which would explain the difference in density.  This hypothesis also explains why the earth�s axis is tilted.  The collision was so great that the earth was simply tilted off its axis.  To view an animation of this hypothesis, click on the link below.

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With each Apollo mission on the moon, beginning with Apollo 11, the size and scope of the scientific investigations grew.  The astronauts stayed longer and ventured further away from their Lunar Modules with each progressive mission. Because these explorers were humans and not rovers or robots, the information they collected provided scientists with the clues they needed to begin to understand the origin and history of the moon, of our earth, and ultimately of ourselves.  The Apollo Program was born in the Cold War.  Were it not for the Soviet Union�s early exploits in space, the Apollo Program might not have ever got off the ground.  But the Apollo Program cannot be thought of as purely a battle in the Cold War.  Once it gained momentum, it became something else.  It transformed us from a species that roamed our planet, to a species that ventured into space and roamed on another world.  The Apollo Program marked a new step in the cultural evolution of our species.  Aside from this lofty perspective, the Apollo Program also should be thought of in terms of its scientific value.  While the program was criticized for its expense, the information gathered during its run has proven to be invaluable.  Hopefully, the adventurous and explorative spirit of the Apollo Program will continue as we look to explore other worlds in our solar system.  Imagine what we could finally learn about Mars were humans allowed to roam the landscape, evaluating, re-thinking tasks, and using their intuition as they searched for clues.  Exploration performed by machines is limited.  Exploration performed by humans can be limitless. 

PROCEDURES:

1. Figure 1 in the Lunar Geology tutorial clearly shows the two different regions of the moon:  the lighter colored highlands, and the darker colored expanses.  Geologists have confirmed that the darker colored regions are younger than the lighter colored regions.  Divide the class into four groups.  Have each group come up with at least two hypotheses explaining the difference in age between the two regions.  One person from each group should then propose the two hypotheses to the class.  Each hypothesis should be written on the board.  Then have the students conduct research on the two regions of the moon.  Once they've conducted their research, have the students re-evaluate each hypothesis.  As a class, have them vote for the hypothesis that best explains the difference in age.  Each student should then write a one to two page report on their scientific findings. 
2. To prepare for class, you'll need the rock samples listed.  If you are unable to acquire the rock samples, get images of the rock samples from a geology textbook, or download images online.  Geologists sometimes learn a great deal from a group of rocks collected in a particular region.  Such a collection of rocks is called a suite. The two types of rock samples the students will be working with           will help define the regions in which they were found.  The first group should be river or stream rocks.  The second group of rocks should be volcanic.  Students do not need to identify the rock types, but should concentrate on their appearance.   Break the students into four groups.  Each group should be able to study each rock group for five minutes.  Have the groups try to identify the environments or        regions in which the rocks must have been found. Have them focus on the shape and texture of each rock, and give clues to groups who need it.  Once they've finished this, have the class openly discuss their answers.  Each student should then write answers to the following questions:
    

• Do you think geologists learn more or less from studying a group of rocks from a region as opposed to studying one rock from a region?

• From the two groups of rocks you studied here, which group is not likely to be found on the moon?  Why?

• If you were a geologist working on the moon, would you collect one rock from one region, a group of rocks from one region,or a group of rocks from many different regions?  Which method would give you more information about the moon as a whole?  Explain your answers in detail.

EXTENSION:

To do this experiment, you will need an area of soft dirt and four shoeboxes.

Because there is no flowing water or wind on the moon, rocks and surface features are not eroded over time as they are here on earth.  To demonstrate these differences in environment, set up the following experiment:

You should find a place outside where other students will not disturb the area in which the experiment will be conducted.  The experiment should last for three days.  The experiment area should have a soft dirt surface.  Break the students into four groups.  Have each group go outside to the experiment area and have one student from a group step firmly into the soft dirt with one foot.  Now have each group fill one shoebox with a layer of soft dirt about three inches thick.  Have the same student who stepped onto the soft dirt in the experiment area step into the shoebox with the same foot.  They should leave a clear, firm footprint.  Once the print is made, place the lid on each shoebox.  Have the students carefully carry the shoeboxes into the classroom.  They should be placed in an area of the room in which they will not be disturbed.  The following day, have the students go out to the experiment area and write a small paragraph describing any erosion of the footprints.  They should note if there had been any rain or wind.  Then have the students record the status of the prints inside the shoeboxes.  After the students have written the report for day three of the experiment, have them take their reports home and write a formal report explaining how and why the two prints are different.  In their reports, students should address how this experiment applies to the study of the moon.  Are the astronaut's footprints still on the moon?  Have the rocks on the moon changed much over time?  How well is the moon's surface preserved?

EVALUATION:

You may evaluate student's work on any of the above exercises using the following three point rubric: Three Points: The reports and answers were filled with good, accurate details and demonstrated an overall understanding of the concepts involved.

Two Points:  The reports and answers contained some accurate information and demonstrated a slight understanding of the concepts involved.

One Point:  The reports and answers were mostly inaccurate and demonstrated a lack of understanding of the concepts involved.

• Albedo - The percentage of the incoming sunlight that is reflected by a natural surface.

• Asteroids - One of the thousands of small planets between Mars and Jupiter.

• Basin - A large impact crater, usually with a diameter in excess of 100 kilometers. Most basins have been modified by degradation of the original basin relief through downslope movement of debris and flooding of the basin interior by lavas.

• Crater - A typically bowl-shaped or saucer-shaped pit or depression, generally of considerable size and with steep inner slopes, formed on a surface or in the ground by the explosive release of chemical or kinetic energy; e.g., an impact crater or an explosion crater.

• Density - The mass of a substance or body.

• Ejecta - The material thrown out of an impact crater by the shock pressures generated during the impact event. Ejecta generally covers the surface around an impact crater to a distance of at least one crater diameter, with individual streamers of material extending well beyond this distance (see rays). The ejecta blanket of a crater becomes less visible with increasing age of the crater.

• Highlands - The densely cratered portions of the Moon that are typically at higher elevations than the mare plains. The highlands contain a significant proportion of anorthosite, an igneous rock made up almost entirely of plagioclase feldspar.

• Lava - A volcanic rock protruded by the eruption of molten material.

• Mare - The low albedo plains covering the floors of several large basins and spreading over adjacent areas. The mare material is comprised primarily of basaltic lava flows, in contrast to the anorthosites in the highlands.

• Massif - A massive topographic and structural feature, especially in an orogenic belt, commonly formed of rocks more rigid than those of its surroundings. These rocks may be protruding bodies of basement rocks, consolidated during earlier orogenies, or younger plutonic bodies. Examples are the crystalline massifs of the Helvetic Alps, whose rocks were deformed mainly during the Hercynian orogeny, long before the Alpine orogeny.

• Meteorite - A small particle in the solar system (called a meteor) which falls through the atmosphere and reaches the surface of the earth without being completely vaporized.

• Mineral - A naturally occurring, inorganic crystalline material with a unique chemical structure.

• Phase angle - The angle between the incident sunlight and the viewing direction when looking at an illuminated surface. Low phase angles result in relatively few shadows being cast by the surface relief.

• Ray - A streamer of ejecta associated with an impact crater. Rays are most often of higher albedo than their surroundings. The albedo contrast may result from either disruption of the local surface by the ejecta or by emplacement of ejecta on the surroundings, or both.

• Rille - One of the several trench-like or crack-like valleys, up to several hundred kilometers long and 1-2 km wide, found on the Moon's surface. Rilles may be extremely irregular with meandering courses ("sinuous rilles"), or they may be relatively straight ("normal rilles"); they have relatively steep walls and usually flat bottoms. Rilles are essentially youthful features and apparently represent fracture systems originating in brittle material.

• Rocks -  A consolidated mixture of minerals.

• Scarp - A change in topography along a linear to arcuate cliff. The cliff may be the result of one or more processes including tectonic, volcanic, impact-related, or degradational processes.

• Secondary craters - Craters produced by the impact of debris thrown out by a large impact event. Many secondary craters occur in clusters or lines where groups of ejecta blocks impacted almost simultaneously.

• Volatile Elements � typically gases, such as water vapor and carbon dioxide, which can make volcanoes erupt violently.
  
  

• Title: Book of the Moon: A Lunar Introduction to Astronomy, Geology, Space, Physics and Space Travel
  Author(s): Thomas A. Hockey
  Published: 1986

• Title: Geology on the Moon
  Author(s): John E. Guest and Ronald Greeley
  Published: 1977

• Title: Pieces of Another World: The Story of Moon Rocks
  Author(s): Franklyn Mansfield Branley
  Published: 1972

• Title: Project Apollo
  Author(s): Hal Marcovitz
  Published: 2000