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\hspace*{3.0in}Name:{\hrulefill} \\
\hspace*{3.25in}Date:{\hrulefill}
\begin{Large}
\section{\bf Surface of the Moon}
\end{Large}

\subsection{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.\\
\\
\noindent The type of orbit our Moon makes around the Earth is called a synchronous orbit.  This phenomenon is shown graphically in Figure~\ref{syncorbit-fig} 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}[htb]
\centerline{{\includegraphics[width=9cm]{Moonfig1.pdf}}}
\caption{The Moon's synchronous orbit. (Not drawn to scale.)}
\label{syncorbit-fig}
\end{figure}
 
\noindent On the Moon, there are extensive lava flows, rugged highlands and many 
impact craters of all 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.

\begin{itemize}
\item Lab 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 Materials: Moon pictures, ruler, calculator
\end{itemize}

\subsection{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{mooncrater-fig}, 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{mooncrater-fig} also shows that the rock beneath the crater becomes fractured (full of cracks).
\begin{figure}[htb]
\centerline{{\includegraphics[width=10cm]{impactcrt.jpg}}}
\caption{Formation of an impact crater.}
\label{mooncrater-fig}
\end{figure}
 
\noindent 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 liquefy 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.  
 
\subsection{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 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 
cannot tell us the absolute age of the feature.  To determine this, 
we need to use radioactive dating or some other technique.

\subsection{Lab Stations}

In this lab you will be using a three-ring binder that contains 
images of the Moon divided into separate sections, or ``stations''. At some 
stations we present data comparing the Moon to the Earth.  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.\\
\\
\noindent You should work in groups of 2--4 people, with one binder for each 
group. The binders contain separate sections, or ``stations,'' with the 
photographs and/or images for each specific exercise. Each group must go 
through all 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.}
\vspace{0.5in}

\noindent {\bf Station 1:} Our first photograph {\bf (\#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 {\bf 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.\\

\noindent 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.  {\bf Image \#3} is one of these images of Mare Imbrium seen from almost overhead.\\

\begin{enumerate}

\item  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 3 points})\\
\vskip 1.7in

\noindent  {\bf 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. \\

\item  Using the diameters for the large craters noted above, and a ruler, what
is the approximate diameters of the smallest craters you can clearly see 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 3 points})\\
\vskip 1.7in

\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.\\

\item Estimate the coverage of the Organ Mountains that are located to the
east of Las Cruces. Estimate a width and a length, and assuming a
rectangle, what is the approximate area of the Organs? ({\bf 2 points})
\vskip 1.6in

\item  Roughly how much area (in km$^{\rm 2}$) does Mons Piton cover?  Compare 
it to the Organ Mountains. 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 2 points})\\
\vskip 2.0in
\noindent {\bf Station 2:} Now let's move to the ``highlands''. In {\bf 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.  {\bf Image \#7} shows 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.\\
\\
\noindent {\bf 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).\\
\item  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 7.3 of the introduction.] ({\bf 5 points})\\
\vskip 1.5 in
\noindent {\bf Station 3:} Comparing Apollo landing sites. {\bf 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! Recently, a new mission to map the Moon
with better resolution called the ``Lunar Reconnaissance Orbiter'' (LRO) sent
back images of the Apollo 11 landing site ({\bf image 11B}). In this image
LM is the base of the lunar module, LRRR and PSEP are two science experiments.
You can even see the (faintly) disturbed soil where the astronauts walked!\\
\\
{\bf Images \#12 and \#13} show the landing site of the last Apollo mission,
\#17. Apollo 17 landed on the Moon on December 11th, 1972. In {\bf image 13B}
is an LRO image of the landing site. Note that during Apollo 17 they
had a ``rover" (identified with the notation LRV) to drive around with. 
Compare the two landing sites.\\

\item  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 2.0in
\noindent The next two sets of images show photographs taken by the astronauts {\it while on} the Moon. The first three photographs {\bf (\#14, \#15, and \#16)} are scenes from the Apollo 11 site, while the next three  {\bf (\#17, \#18, and \#19)} were taken at the Apollo 17 landing site.\\
\item  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 1.5in
\noindent {\bf Station 4:} On the northern-most edge of Mare Imbrium sits the crater Plato (labeled in images \#2 and \#6). {\bf Image \#20} is a close-up of Plato.\\
\item  Do you agree with the theory that the crater floor has been recently flooded?  Is the maria 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 4 points})\\
\vskip 1.5in
\noindent {\bf Station 5:} {\bf 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.\\
\\
\noindent 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$^\circ$ latitude is the equator, just like on the Earth. Like the Earth, the North pole of the Moon has a latitude of +90$^\circ$, and the south pole is at $-$90$^\circ$. On the x-axis is the {\it longitude} of the Moon. Longitude runs from 0$^\circ$ to 360$^\circ$. The point at 0$^\circ$ latitude
{\it and} longitude of the Moon is the point on the lunar surface that is 
closest to the Earth.\\
\\
\noindent 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$^\circ$ N, and its longitude is 351$^\circ$. You can clearly see the outline of Plato if you look closely.\\
\item  Is Plato located in a high region, or a low region? Is Plato lower than Mare Imbrium (centered at 32$^\circ$N, 344$^\circ$)? [Remember that Plato is on the Northern edge of Mare Imbrium.]({\bf 4 points})\\
\vskip 1.5in
\noindent 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$^\circ$, passing through 0$^\circ$, and 
going all the way to 90$^\circ$ (remember, 0 is located at the center of 
the Moon as seen from Earth). {\bf Image \#23} is a more conventional 
topographical map of the Moon, showing the two hemispheres: near side, and
far side.\\
\item  Compare the average elevation of the near-side of the Moon to that of the far-side. Are they different? Explain. Can you make out the maria?  Compare the number of maria on the far side to the number on the near side. ({\bf 4 points})\\
\vskip 2.0in

\noindent [Why the far side of the Moon is so different from the near side remains a mystery!]\\
\\
\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 fewer 
craters than the surface of the Moon?  Discuss two differences between the 
Earth and the Moon that could explain this.  {\bf (5 points)}\\
\vskip 2.0in

\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 {\it 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 as with water, 
denser objects in molten rock sink to the bottom more quickly 
than less dense 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.\\
\\
\noindent {\bf 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. Image \#24 is
a map of the amount of iron on the surface of the Moon (``hotter'' colors
mean more iron than cooler colors). Image \#25 is the same type of map,
but for titanium.\\
\item  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 5 points})\\
\vskip 2.0in
\item  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 2.0in
\noindent  A cut-away diagram of the Earth is shown in the 
Figure~\ref{layers-fig}. 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.  As you may recall from the Density lab, the density of a material is simply its mass (in grams or kilograms) divided by its volume (in cubic 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}[htbp]
\centerline{{\includegraphics[width=9cm]{earth-layers.jpg}}}
\caption{The internal structure of the Earth, showing the dimensions of
the crust, mantle and core, as well as their composition and temperatures.} 
\label{layers-fig}
\end{figure}
\begin{flushleft}
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{earthmoon-tab}, we present a comparison of the compositions of the Moon and the Earth. The data for the Moon
comes from analysis of the rocks brought back by the Apollo astronauts.
\end{flushleft}
\begin{table}[htbp]
\begin{center}
\caption{Composition of the Earth \& Moon.}
\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}
\label{earthmoon-tab}
\end{center}
\end{table}

\item  Is the Moon composed of the same proportion 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 5 points})\\
\vskip 2.0in
\noindent  As you will learn in lecture, the terrestrial planets in our solar system (Mercury, Venus, Earth and Mars) have higher densities than the jovian planets (Jupiter, Saturn, Uranus and Neptune). One theory for the formation of the Moon is that it formed 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.\\
\item  Given this information, do you think it is likely that the Moon formed out near Mars? Why? ({\bf 5 points})\\
\vskip 2.0in
\noindent The currently accepted theory for the formation of the Moon is called the ``Giant Impact'' theory. In this model, a large body (about the size of 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. Table~\ref{mantle-tab} shows the composition of the Earth's crust and mantle compared to that of the Moon.\\
\begin{table}[htbp]
\begin{center}
\caption{Chemical Composition of the Earth (crust and mantle) and Moon.}
\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}
\label{mantle-tab}
\end{center}
\end{table}
\clearpage
\item  Given the data in Table~\ref{mantle-tab}, present an argument for why the giant impact theory probably 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 3 points})\\
\vskip 2.0in
\clearpage
~ ~ ~\\
\newpage
\hspace*{3.0in}Name:{\hrulefill} \\
\hspace*{3.0in}Date:{\hrulefill}\\

\end{enumerate}
\subsection{Take Home Exercise (35 points total)}

Answer the following questions in the space provided:
\begin{enumerate}
\item  What are the maria, and how were they formed? ({\bf 5 points})\\
\vskip 2.0in
\item  Explain how you would assign relative (``this is older than that'') ages to features on the Moon or on any other surface in the solar system. ({\bf 5 points})\\
\vskip 2.0in
\item  How can the Earth be older than the Moon, as suggested by the Giant Impact Theory of the Moon's formation, but the Moon's surface is older than the Earth's surface?  What do we mean by `old' in this context? ({\bf 10 points})\\
\vskip 1.75in
\item  The maria are present on the Earthward-facing portion of the Moon and not on the Moon's far side.  Since there is no reason to suspect that the impact history of the near side of the Moon is substantially different from that experienced by the far side, suggest another possible reason why the maria are present on the Earth-facing side only, using the below figure as a guide.  {\bf (15 points)}\\
\begin{figure}[htb]
\centerline{{\includegraphics[width=12cm]{moon_crust.jpg}}}
\end{figure}
\end{enumerate}
\clearpage
\subsection{Possible Quiz Questions}

\begin{enumerate}

\item What is an impact crater?

\item What are the Maria?

\item What is the difference between the words Mare and Maria?

\item Explain what a synchronous orbit is.

\item What is a topographical map?

\end{enumerate}

\subsection{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
~ ~ ~\\
\newpage
