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\section{Introduction}
One can learn a lot about the Moon by looking at the lunar surface.  Even before
 astronauts landed on the Moon, scientists had enough data to formulate theories
 about the formation and evolution of the Earth's only natural satellite.  
However, since the Moon rotates once for every time it orbits around the Earth, 
we can only see one side of the Moon from the surface of the Earth.  Until 
spacecraft were sent to orbit the Moon, we only knew half the story.

The type of orbit our Moon makes around the Earth is called a synchronous 
orbit.  This phenomenon is shown graphically in Figure \ref{fig:morbit}
 below.  If we 
imagine that there is one large mountain on the hemisphere facing the 
Earth (denoted by the small triangle on the Moon), then this mountain is 
always visible to us no matter where the Moon is in its orbit.  As the 
Moon orbits around the Earth, it turns slightly so we always see the same 
hemisphere.

\begin{figure}[h]
\centering
\leavevmode
\epsfxsize=0.5\textwidth
\epsfbox{MOON/Moonfig1.ps}
%\epsfbox{MOON/Moonfig1.ps}
\caption{The Moon's ``synchronous'' orbit (not drawn to scale). Note how
the Moon spins exactly once during its 27.3 day orbit around the Earth, but
keeps the same face pointing towards the Earth.}
\label{fig:morbit}
\end{figure}
 

On the Moon, there are extensive lava flows, rugged highlands, and many 
impact craters of all different sizes.  The overlapping of these features 
implies 
relative ages.  Because of the lack of ongoing mountain building processes,
or weathering by wind and water, the accumulation of volcanic processes and 
impact cratering is readily visible.  Thus by looking at the images of the 
Moon, one can trace the history of the lunar surface.
Most of the images in this lab were taken by NASA spacecraft or by 
Apollo Astronauts.

\begin{itemize}
\item{{\it Goals:}  to discuss the Moon's terrain, craters, and the theory of relative ages; to use pictures of the Moon to deduce relative ages and formation processes of surface features}
\item{{\it Materials:}  Moon pictures, ruler, calculator}
\item{\it Review:} Section 1.2.2 in Lab \#1
\end{itemize}

\section{Craters and Maria}

A crater is formed when a meteor from space strikes the lunar surface.  The force of the impact obliterates the meteorite and displaces part of the Moon's surface, pushing the edges of the crater up higher than the surrounding rock.
At the same time, more displaced material shoots outward from the crater, 
creating $rays$ of ejecta.  These rays of material can be seen as radial
streaks centered on some of the craters in some of the pictures you will be 
using for your lab today. As shown in Figure \ref{fig:crater}, some of the material
from the blast ``flows'' back towards the center of the crater, creating
a mountain peak. Some of the craters in the photos you will examine
today have these ``central peaks''. Figure \ref{fig:crater} also shows that the rock
beneath the crater becomes fractured (full of cracks).

\begin{figure}[h]
\centering
\leavevmode
\epsfysize=0.3\textheight
\epsfbox{MOON/impactcrt.eps}
%\epsfbox{MOON/impactcrt.eps}
\caption{Formation of an Impact Crater.}
\label{fig:crater}
\end{figure}

 
Soon after the Moon formed, its interior was mostly liquid.  It was continually being hit by meteors, and the energy (heat) from this period of intense cratering was
enough to liquify the Moon's interior.  Every so often, a very large meteor 
would strike the surface, and $crack~the~Moon's~crust$.  The over-pressured ``lava'' from the 
Moon's molten mantle then flowed up through the cracks made by the impact.  
The lava filled in the crater, creating a dark, smooth ``sea''. Such a sea is 
called a $mare$ (plural: $maria$). Sometimes the amount of lava that came out 
could overfill the crater.
In those cases, it spilled out over the crater's edges and could fill in other craters as well as cover the bases of the $highlands$, the rugged, rocky peaks on the surface of the Moon.  
\section{Relative Ages on the Moon}

Since the Moon does not have rain or wind erosion, astronomers can determine 
which features on the Moon are older than others.  It all comes down to counting the number of craters a feature has.  Since there is nothing on the Moon that can erase the presence of a crater, the more craters something has, the longer it must have been around to get hit.  For example, if you have two large 
craters, and the first crater has 10 smaller craters in it, while the second one
has only 2 craters in it, we know that the first crater is older since it has been there long enough to have been hit 10 times.   If we look at the highlands,
we see that they are covered with lots and lots of craters.  This tells us that
in general, the highlands are older than the {\it maria}, which have fewer
 craters.  We also know that if we see a crater on top of a mare, the mare is 
older.  It had
to be there in the first place to get hit by the meteor.  Crater counting
can tell us which features on the Moon are older than other features, but it can not tell us the absolute age of the feature.  To determine that, 
we need to use radioactive dating or some other technique.


\section{Lab Stations}
In this lab you will be using a 3-ring binder that has pictures organized
into separate sections, or ``stations''. 
At some stations we present data comparing the Moon to the Earth or Mars.
Using your understanding of simple physical processes here on Earth and 
information from the class lecture and your reading, you will make 
observations and draw logical conclusions in much the same way that a 
planetary geologist would.  

You should work in groups of two to four, with one notebook for each group.
The notebooks contain separate sections, or ``stations'', with the photographs
and/or images for each specific exercise.  Each group must go through all 
of the stations, and consider and discuss each question and come to a 
conclusion. {\bf Remember to back up your answers with reasonable explanations,
and be sure to answer {\it all} of the questions.}  While you should discuss 
the questions as a group, be sure to write down one group answer for each
question.  The take-home questions must be done on your own. {\bf Answers for 
the take-home questions that are exact duplicates of those of other members of 
your group will not be acceptable.}
\vskip 0.3in

\subsection{The Surface of the Moon}
\noindent
{\bf Station 1:} Our first photograph (\#1) is that of the full Moon. It is 
obvious that the Moon has dark regions, and bright regions. The largest dark
regions are the ``Maria'', while the brighter regions are the ``highlands''.
In image \#2, the largest features of the full Moon are labeled. 
The largest of the maria on the Moon is Mare Imbrium (the ``Sea of Showers''),
and it is easily located in the upper left quadrant of image \#2. Locate
Mare Imbrium. Let us take a closer look at Mare Imbrium.
\vskip 0.8in

Image \#3 is from the Lunar Orbiter IV. Before the Apollo missions
landed humans on the Moon, NASA sent several missions to the Moon to map
its surface, and to make sure we could safely land there. Lunar
Orbiter IV imaged the Moon during May of 1967. The technology of the time
was primitive compared to today, and the photographs were built up by making
small imaging scans/slices of the surface (the horizontal striping can be 
seen in the images), then adding them all together to make a larger photograph. 
Image \#3 is one of these images of Mare Imbrium seen from almost overhead.
\vskip 0.3in
\noindent
{\bf Question \#1:} Approximately how many craters can you see inside the dark
circular region that defines Mare Imbrium? Compare the number of craters in 
Mare Imbrium to the brighter regions to the North (above) of Mare Imbrium. 
({\bf 2 points})


\vskip 1.0in


\noindent
Images \#4 and \#5 are close-ups of small sections of Mare Imbrium. In image
\#4, the largest crater (in the lower left corner) is ``Le Verrier'' (named
after the French mathematician who predicted the correct position for
the planet Neptune). Le Verrier is 20 km in diameter. In image \#5, the
two largest craters are named Piazzi Smyth (just left of center) and Kirch
(below and left of Piazzi Smyth). Piazzi Smyth has a diameter of 13 km,
while Kirch has a diameter of 11 km. 

\noindent
{\bf Question \#2:} Using the diameters for the large craters noted above, and a
ruler, what is the approximate diameter of the smallest crater you can make out
in images \#4 and \#5? If
the NMSU campus is about 1 km in diameter, compare the smallest crater you
can see to the size of our campus. ({\bf 2 points})

\vskip 1.5in


\noindent
In image \#5 there is an isolated mountain (Mons Piton) located near Piazzi
Smyth. It is likely that Mons Piton is related to the range of mountains
to its upper right.
\noindent
\vskip 0.15in
{\bf Question \#3:} Roughly how much area (in km$^{\rm 2}$) does Mons Piton 
cover?  Compare it to the area of the Organ mountains that are located to
the east of Las Cruces (estimate a width and a length, and assuming a
rectangle, calculate the approximate area of the Organs). How do you think 
such an isolated mountain came to exist? [Hint: In the introduction to the lab
exercises, the process of maria formation was described. Using this idea,
how might Mons Piton become so isolated from the mountain range to the 
northeast?] ({\bf 5 points})

\vskip 2.0in

\noindent
~ \\
\vskip 0.5in
{\bf Station \#2:} Now let's move to the ``highlands''. In image \#6 (which
is identical to image \#2), the crater Clavius can be seen on the bottom
edge---it is the bottom-most labeled feature on this map. In image \#7,
is a close-up picture of Clavius (just below center) taken from the ground 
through a small telescope (this is similar to what you would see at the campus 
observatory).  Clavius is one of the largest craters on the Moon, with a 
diameter of 225 km. In the upper right hand corner is one of the best
known craters on the Moon, ``Tycho''. In image \#1 you can identify Tycho
by the large number of bright ``rays'' that emanate from this crater. Tycho
is a very young crater, and the ejecta blasted out of the lunar surface
spread very far from the impact site.

{\bf Question \#4:} Esitmate (in km) the distance from the center of the
crater Clavius to the center of Tycho. Compare this to the distance between
Las Cruces, and Albuquerque (375 km). ({\bf 3 points})


\vskip 1.2in

Images \#8 and \#9, are two high resolution images of Clavius and
nearby regions taken by Lunar Orbiter IV (note the slightly different
orientations from the ground-based picture). 

{\bf Question \#5:} Compare the region around Clavius to Mare Imbrium. Scientists
now know that the lunar highlands are older than the Maria. What evidence
do you have (using these photographs) that supports this idea? [Hint: review
section 2.3 of the introduction.] ({\bf 5 points})


\vskip 2.2 in

{\bf Station \#3:} Comparing Apollo landing sites. In images \#10 and \#11
are close-ups of the Apollo 11 landing site in Mare Tranquillitatis (the
``Sea of Tranquility''). The actual spot where the ``Eagle'' landed 
on July 20, 1969 is marked by the small cross in image 11 (note that three 
small craters near the landing site have been named for the crew of this 
mission: Aldrin, Armstrong and Collins). [There are also quite a number of
photographic defects in these pictures, especially the white circular blobs
near the center of the image to the North of the landing site.] The landing
sites of two other NASA spacecraft, Ranger 8 and Surveyor 5, are also labeled
in image \#11. NASA made sure that this was a safe place to explore!

Images \#12 and \#13 show the landing site of the last Apollo mission,
\#17. Apollo 17 landed on the Moon on December 11th, 1972. Compare
the two landing sites. 

\vskip 0.2in
{\bf Question \#6:} Describe the logic that NASA used in choosing the
two landing sites--why did they choose the Tranquillitatis site for the
first lunar landing?  What do you think led them to choose the Apollo 17 site?
({\bf 5 points})

\vskip 1.9in


The next two sets of images show photographs taken by the astronauts while
on the Moon. The first three photographs (\#14, \#15, and \#16) are scenes
from the Apollo 11 site, while the next three  (\#17, \#18, and \#19)
were taken at the Apollo 17 landing site.

{\bf Question \#7:} Do the photographs from the actual landing sites back-up
your answer to why NASA chose these two sites? How? Explain your reasoning.
({\bf 5 points})

\vskip 1.75in


{\bf Station 4:} On the northern-most edge of Mare Imbrium sits the crater
Plato (labeled in images \#2 and \#6). Photo \#20 is a close-up of Plato. Do 
you agree with the theory that the crater floor has been recently flooded?  Is
the mare that forms the floor of this crater younger, older, or approximately 
the same age as the nearby region of Mare Imbrium located just to the South 
(below) of Plato?  Explain your reasoning. {\bf (5 points)}

\vskip 1.75in

{\bf Station 5:} Images \#21 and \#22 are ``topographical'' maps of the
Earth and of the Moon. A topographical map shows the {\it elevation} of 
surface features.
On the Earth we set ``sea level'' as the zero point of elevation. Continents,
like North America, are above sea level. The ocean floors are below sea level.
In the topographical map of the Earth, you can make out the United States.
The Eastern part of the US is lower than the Western part. In topographical
maps like these, different colors indicate different heights. Blue and dark
blue areas are below sea level, while green areas are just above sea level.
The highest mountains are colored in red (note that Greenland and Antarctica
are both colored in red--they have high elevations due to very thick ice
sheets). We can use the same technique to map elevations on the Moon. Obviously,
the Moon does not have oceans to define ``sea level''. Thus, the definition
of zero elevation is more arbitrary. For the Moon, sea level is defined
by the {\it average} elevation of the lunar surface.

Image \#22 is a topographical map for the Moon, showing the highlands (orange, 
red, and pink areas), and the lowlands (green, blue, and purple). [Grey and
black areas have no data.] The 
scale is shown at the top. The lowest points on the Moon are 10 km below
sea level, while the highest points are about 10 km above sea level. On
the left hand edge (the ``y axis'') is a scale showing the latitude. 0$^{o}$
latitude is the equator, just like on the Earth. Like the Earth, the North
pole of the Moon has a latitude of +90$^{o}$, and the south pole is at
$-$90$^{o}$. On the x-axis is the {\it longitude} of the Moon. Longitude
runs from 0$^{o}$ to 360$^{o}$. The point at 0$^{o}$ latitude
{\it and} longitude of the Moon is the point on the lunar surface that is 
closest to the Earth.

It is hard to recognize features on the topographical map of the Moon because
of the complex surface (when compared to the Earth's large smooth areas).
But let's go ahead and try to find the objects we have been studying. First,
see if you can find Plato. The latitude of Plato is +52$^{o}$ 
N, and its longitude is 351$^{o}$. You can clearly see the outline of Plato if
you look closely.

{\bf Question \#8:} Is Plato located in a high region, or a low region? Is Plato
lower than Mare Imbrium (centered at 32$^{o}$N, 344$^{o}$)? [Remember 
that Plato is on the Northern edge of Mare Imbrium.]({\bf 2 points})

\vskip 1.25in

{\bf Question \#9:} Apollo 11 landed at Latitude = 1.0$^{o}$N, longitude = 
24$^{o}$. Did it land in a low area, or a high area? ({\bf 2 points})

\vskip 0.8in

As described in the introduction, the Moon keeps the same face pointed 
towards Earth at all times. We can only see the ``far-side'' of the Moon
from a spacecraft. In image \#22, the {\it hemisphere} of the Moon that we 
can see runs from a longitude of 270$^{o}$, passing through 0$^{o}$, and 
going all the way to 90$^{o}$ (remember 0, 0 is located at the center of 
the Moon as seen from Earth). In image \#23 is a more conventional 
topographical map of the Moon, showing the two hemispheres: near side, and
far side.

{\bf Question \#10:} Compare the average elevation of the near-side of the Moon
to that of the far-side. Are they different? Can you make-out the Maria?
Compare the number of Maria on the far side to the number on the near side.
({\bf 5 points})

\vskip 2.6in




\noindent
{\bf Station 6:} With the surface of the Moon now familiar to you, and your 
perception of the surface of the Earth in mind, compare the Earth's surface to 
the surface of the Moon.  Does the Earth's surface have more craters or less 
craters than the surface of the Moon?  Discuss two differences between the 
Earth and the Moon that could explain this.  {\bf (5 points)}


\vskip 1.8in


\subsection{The Chemical Composition of the Moon: Keys to its Origin}
{\bf Station 7:} Now we want to examine the chemical composition of the Moon
to reveal its history and origin. The formation of planets (and other large
bodies in the solar system like the Moon) is a violent process. Planets grow 
through the process of ``accretion'': the gravity of the young planet pulls on
nearby material, and this material crashes into the young planet, heating it,
and creating large craters. In the earliest days of the solar system, so
much material was being accreted by the planets, that they were completely
{\it molten}. That is, they were in the form of liquid rock, like the
lava you see flowing from some volcanoes on the Earth. Just like the
case with water, heavier objects in molten rock sink to the bottom more quickly 
than lighter material. This is also true for chemical elements. Iron is one of
the heaviest of the common elements, and it sinks toward the center of a
planet more quickly than elements like silicon, aluminum, or magnesium.
Thus, near the Earth's surface, rocks composed of these lighter elements
dominate. In lava, however, we are seeing molten rock from deeper in the
Earth coming to the surface, and thus lava and other volcanic (or 
``igneous'') rock, can be rich in iron,
nickel, titanium, and other high-density elements. 

\vskip 0.8in
Images \#24 and 25 present two unique views of the Moon obtained by
the spacecraft {\it Clementine}. Using special sensors, {\it Clementine}
could make maps of the surface composition of the Moon. In Image \#24 is
a map of the amount of iron on the surface of the Moon (redder colors
mean more iron than bluer colors). Image \#25 is the same type of map,
but for titanium.

{\bf Question \#11:} Compare the distribution of iron and titanium to the surface
features of the Moon (using images \#1, \#2 or \#6, or the topographical map in 
image \#23). Where are the highest concentrations of iron and titanium found?
({\bf 4 points})

\vskip 2.25in

{\bf Question \#12:} If the heavy elements like iron and titanium sank towards
the center of the Moon soon after it formed, what does the presence of large 
amounts of iron and titanium in the maria suggest? [Hint: do you remember how
maria are formed?] ({\bf 5 points})

\vskip 1.55in


The structure of the Earth is shown in the diagram, below. There are three
main structures: the crust (where we live), the mantle, and the core. The
crust is cool and brittle, the mantle is hotter, and ``plastic'' (it flows),
and the core is very hot and very dense. The density of a material is simply its
mass (in grams or kilograms) divided by its volume (in centimeters or
meters). Water has a density of 1 gm/cm$^{\rm 3}$. The density of the
Earth's crust is about 3 gm/cm$^{\rm 3}$, while the mantle has a density
of 4.5 gm/cm$^{\rm 3}$. The core is very dense: 14 gm/cm$^{\rm 3}$ (this is
partly due to its composition, and partly due to the great pressure exerted
by the mass located above the core). The core of the Earth is almost pure
iron, while the mantle is a mixture of magnesium, silicon, iron and oxygen.
The average density of the Earth is 5.5 gm/cm$^{\rm 3}$.

\begin{figure}[h]
\centering
\leavevmode
\epsfxsize=0.5\textwidth
\epsfbox{MOON/earth-layers.ps}
%\epsfbox{MOON/earth-layers.ps}
\caption{The internal structure of the Earth, showing the dimensions of
the crust, mantle and core, as well as their composition and temperatures.}
\end{figure}

Before the astronauts brought back rocks from the Moon, we did not have
a good theory about its formation. All we knew was that the Moon had
an average density of 3.34 gm/cm$^{\rm 3}$. If the Moon formed from the
same material as the Earth, their compositions would be nearly identical, as 
would their average densities. In Table \ref{table:earthmoon}, we present a comparison of
the composition of the Moon to that of the Earth. The data for the Moon
comes from analysis of the rocks brought back by the Apollo astronauts.

\vskip 0.5in
\begin{table}
\begin{center}
\caption{Composition of the Earth \& Moon}
\label{table:earthmoon}
\begin{tabular}{|c|c|c|}
\hline
Element & Earth & Moon \\
\hline
Iron & 34.6\% & 3.5\% \\
\hline
Oxygen & 29.5\% & 60.0\%\\
\hline
Silicon & 15.2\% & 16.5\% \\
\hline
Magnesium & 12.7\% & 3.5\% \\
\hline
Titanium & 0.05\% & 1.0\%\\
\hline
\end{tabular}
\end{center}
\end{table}
\vskip 0.5in
{\bf Question \#13:} Is the Moon composed of the same mixture of elements as the
Earth? What are the biggest differences? Does this support a model where
the Moon formed out of the same material as the Earth? ({\bf 3 points})

\vskip 1.7in


As you will learn in the Astronomy 110 lectures, the inner planets in
the solar system (Mercury, Venus, Earth and Mars) have 
higher densities than the outer planets (Jupiter, Saturn, Uranus and Neptune). One theory for the formation of
the Moon is that it formed out near Mars, and ``migrated'' inwards to be
captured by the Earth. This theory arose because the density of Mars,
3.9 gm/cm$^{\rm 3}$, is similar to that of the Moon. But Mars is rich in
iron and magnesium: 17\% of Mars is iron, and more than 15\% is magnesium.

{\bf Question \#14:} Given this data, do you think it is likely that the Moon
formed out near Mars? Why? ({\bf 2 points})

\vskip 1.25in

The final theory for the formation of the Moon is called the ``Giant
Impact'' theory. In this model, a large body (about the size of the planet
Mars) collided with the Earth, and the resulting explosion sent a large
amount of material into space. This material eventually collapsed 
(coalesced) to form the Moon. Most of the ejected material would have come from 
the crust and the mantle of the Earth, since it is the material closest to
the Earth's surface. In the Table (\ref{table:comp}) is a comparison of the composition
of the Earth's crust and mantle compared to that of the Moon.
\begin{table}
\begin{center}
\caption{Chemical Composition of the Earth and Moon}
\label{table:comp}
\begin{tabular}{|c|c|c|}
\hline
Element & Earth's Crust and Mantle & Moon \\
\hline
Iron & 5.0\% & 3.5\% \\
\hline
Oxygen & 46.6\% & 60.0\%\\
\hline
Silicon & 27.7\% & 16.5\% \\
\hline
Magnesium & 2.1\% & 3.5\% \\
\hline
Calcium & 3.6\% & 4.0\%\\
\hline
\end{tabular}
\end{center}
\end{table}

{\bf Question \#15:} Given the data in this table, present an argument for why
the giant impact theory is now the favorite theory for the formation of
the Moon. Can you think of a reason why the compositions might not be 
{\it exactly} the same? ({\bf 5 points})

\clearpage
\section{Summary}

({\bf 35 points})  Please summarize in a few paragraphs what you have learned 
in this lab.  Your summary should include:

\begin{itemize}
\item{Explain how to determine and assign relative ages of features on the Moon}
\item{Comment on analyzing pictures for information; what sorts of things would you look for? what can you learn from them?}
\item{What is a mare and how is it formed?}
\item{How does the composition of the Moon differ from the Earth, and how
does this give us insight into the formation of the Moon?}
\end{itemize}

\noindent
Use complete sentences and proofread your summary before handing it in. 

\noindent
\section{Possible Quiz Questions}

1. What is an impact crater, and how is it formed?

2. What is a Mare?

3. Which is older the Maria or the Highlands?

4. How are the Maria formed?

5. What is synchronous rotation?

6. How can we determine the relative ages of different lunar surfaces?

\section{Extra Credit (ask your TA for permission before attempting, 
{\bf 5 points})}

In the past few years, there have been some new ideas about the
formation of the Moon, and why the lunar farside is so different from
the nearside (one such idea goes by the name ``the big splat''). In addition,
we have recently discovered that the interior of the Moon is highly
fractured. Write a brief (about one page) review on the new computer 
simulations and/or observations that are attempting to understand the 
formation and structure of the Moon.\\
\clearpage

%\end{document}
