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%{\bf \Large Introduction to the Geology of the Terrestrial Planets}
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\hspace{3.0in}Name:{\hrulefill}\\
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\begin{Large}
\section{Introduction to the Geology of the Terrestrial Planets} \label{terrlab}
\end{Large}

\subsection{Introduction}

There are two main families of planets in our solar system: the Terrestrial
planets (Earth, Mercury, Venus, and Mars), and the Jovian Planets (Jupiter,
Saturn, Uranus, and Neptune). The terrestrial planets are rocky planets
that have properties similar to that of the Earth. In contrast, the Jovian 
planets
are giant balls of gas. Table \ref{pproperties} summarizes the main properties of the
planets in our solar system (Pluto is an oddball planet that does not fall into either
categories, sharing many properties with the ``Kuiper belt'' objects discussed
in the lab \# \ref{cometlab}). \\
 
\begin{table}[ht]
\caption{The Properties of the Planets}
\begin{center}
\begin{tabular}{|c|c|c|c|}\hline
Planet & Mass & Radius & Density\\
~ ~ ~  & (Earth Masses) & (Earth Radii) & gm/cm$^{\rm 3}$\\
\hline
Mercury& 0.055 & 0.38 & 5.5\\
\hline
Venus & 0.815 & 0.95 & 5.2 \\
\hline
Earth & 1.000 & 1.00 & 5.5\\
\hline
Mars  & 0.107 & 0.53 & 3.9\\
\hline
Jupiter& 318 & 10.8 & 1.4\\
\hline
Saturn & 95  & 9.0 & 0.7\\
\hline
Uranus & 14.5 & 3.93 & 1.3\\
\hline
Neptune & 17.2& 3.87 & 1.6\\
\hline
Pluto & 0.002 & 0.178 & 2.1 \\
\hline
\end{tabular}
\end{center}
\label{pproperties}
\end{table}

It is clear from Table \ref{pproperties} that the nine planets in our solar system span
a considerable range in sizes and masses. For example, the Earth has 18
times the mass of Mercury, while Jupiter has 318 times the mass of the
Earth. But the separation of the planets into Terrestrial and Jovian is
not based on their masses or physical sizes, it is based on their densities (the
last column in the table). What is density? Density is simply the mass of an 
object divided by its volume: M/V. In the metric system, the density of
water is set to 1.00 gm/cm$^{\rm 3}$. Densities for some materials you are 
familiar with can be found in Table \ref{densities}.\\

\begin{table}
\caption{The Densities of Common Materials}
\begin{center}
\begin{tabular}{|c|c|c|c|}\hline
Element or & Density & Element & Density\\
Molecule  & gm/cm$^{\rm 3}$ &~ ~ & gm/cm$^{\rm 3}$\\
\hline
Water& 1.0 & Carbon & 2.3\\
\hline
Aluminum & 2.7 & Silicon & 2.3 \\
\hline
Iron & 7.9 & Lead & 11.3\\
\hline
Gold  & 19.3 & Uranium & 19.1\\
\hline
\end{tabular}
\end{center}
\label{densities}
\end{table}
\vspace{0.1in}
If we examine the first table we see that the terrestrial planets all
have higher densities than the Jovian planets. Mercury, Venus and Earth
have densities above 5 gm/cm$^{\rm 3}$, while Mars has a slightly lower
density ($\sim$ 4 gm/cm$^{\rm 3}$). The Jovian planets have densities
very close to that of water--in fact, the mean density of Saturn is lower
than that of water! The density of a planet gives us clues about its
composition. If we look at the table of densities for common materials, we see 
that the mean densities of the terrestrial planets are about halfway between 
those of silicon 
and iron. Both of these elements are highly abundant throughout the Earth, and
thus we can postulate that the terrestrial planets are mostly composed
of iron, silicon, with additional elements like carbon, oxygen, aluminum 
and magnesium. The Jovian planets, however, must be mostly composed of
lighter elements, such as hydrogen and helium. In fact, the Jovian
planets have similar densities to that of the Sun: 1.4 gm/cm$^{\rm 3}$.
The Sun is 70\% hydrogen, and 28\% helium. Except for small, rocky cores, the
Jovian planets are almost nothing but hydrogen and helium.\\

\vspace{0.2in}
The terrestrial planets share other properties, for example they all rotate
much more slowly than the Jovian planets. They also have much thinner
atmospheres than the Jovian planets (which are almost {\it all} atmosphere!).
Today we want to investigate the geologies of the terrestrial planets to
see if we can find other similarities, or identify interesting differences.\\

\subsection{Topographic Map Projections}

In the first part of this lab we will take a look at images and maps of
the surfaces of the terrestrial planets for comparison. But before we do so,
we must talk about what you will be viewing, and how these maps/images
were produced. As you probably know, 75\% of the Earth's surface is covered by 
oceans, thus
a picture of the Earth from space does not show very much of the actual
rocky surface (the ``crust'' of the Earth). With modern techniques (sonar,
radar, etc.) it is possible to reconstruct the true shape and structure
of a planet's rocky surface, whether it is covered in water, or by very
thick clouds (as is the case for Venus). Such maps of the ``relief'' of the 
surface of a planet are called {\it topographic} maps. These maps usually color
code, or have contours, showing the highs and lows of the surface elevations.
Regions of constant elevation above (or below) sea level all will have the
same color. This way, large structures such as mountain ranges, or ocean 
basins, stand out very clearly.\\

There are several ways to present topographic maps, and you will see two
versions today. One type of map is an attempt at a 3D
{\it visualization} that keeps the relative sizes of the continents in
correct proportion (see Figure \ref{sphthreeD}, below). But such maps only allow you to see a small
part of a spherical planet in any one plot. More commonly, the entire
surface of the planet is presented as a rectangular map as shown in
Figure \ref{topo}. Because the
surface of a sphere cannot be properly represented as a rectangle, the
regions near the north and south poles of a planet end up being highly 
distorted in this kind of map. So keep this in mind as you work through
the exercises in this lab.\\

\begin{figure}[ht]
\centerline{{\includegraphics[width=9cm]{Terrestrial/usearth.pdf}}}
\caption{A topographic map showing one hemisphere of Earth centered on North
America. In this 3D representation the continents are correctly rendered.}
\label{sphthreeD}
\end{figure}

\begin{figure}[!ht]
\centerline{{\includegraphics[width=9cm]{Terrestrial/globalrelief.pdf}}}
\caption{A topographic map showing the entire surface of the Earth.
In this 2D representation, the continents are incorrectly rendered. Note
that Antarctica (the land mass that spans the bottom border of this map)
is 50\% smaller than North America, but here appears massive. You might
also be able to compare
the size of Greenland on this map, to that of the previous map.}
\label{topo}
\end{figure}

\subsection{Global Comparisons}

In the first part of this lab exercise, you will look at the planets 
in a {\it global} sense, by comparing the largest structures on the 
terrestrial planets. 
~ ~\\
\begin{flushleft}
{\bf Exercise \#1}: At station \#1 you will find images of Mercury, Venus,
the Earth, the Moon, and Mars. The images for Mercury, Venus and the Earth
and Moon are in ``false colors'' to help emphasize different features, including 
different types of rocks or large-scale structures. The image of Mars, however, 
is in ``true color''.
\end{flushleft}
~ ~\\
Impact craters can come in a variety of
sizes, from tiny little holes, all the way up to the large ``maria'' seen
on the Moon. Impact craters are usually round.
~ ~\\
\begin{flushleft}

1. On which of the five objects are large meteorite impact craters obvious? 
 {\bf (1 point)}

~ ~ \\

~ ~ \\

2. Does Venus or the Earth show any signs of large, round maria (like
those seen on Mercury or the Moon)?
 {\bf (1 point)}

~ ~ \\

~ ~ \\

3. Which planet seems to have the most impact craters?
 {\bf (1 point)}

~ ~ \\

~ ~\\

4. Compare the surface of Mercury to the Moon. Are they similar?
 {\bf (3 points)}

~ ~\\

\smallspace

\end{flushleft}
Mercury is the planet closest to the Sun, so it is the terrestrial planet
that gets hit by comets, asteroids and meteoroids more often than the other 
planets because the Sun's gravity tends to collect small bodies like comets 
and asteroids. The closer you are to the Sun, the more of these objects 
there are in the neighborhood. Over time, most of the largest asteroids on 
orbits that intersect those of the other planets have either collided with a 
planet, or have been broken into smaller pieces by the gravity of a close
approach to a large
planet. Thus, only smaller debris is left over to cause impact 
craters.
~ ~\\

\begin{flushleft}

5. Using the above information, make an educated guess on why Mercury does
not have as many large maria as the Moon, even though both objects have been
around for the same amount of time. [Hint: Maria are caused by the impacts
of {\it large} bodies.]
 {\bf (3 points)}

\smallspace

\end{flushleft}
Mercury and the Moon do not have atmospheres, while Mars has a thin atmosphere.
Venus has the densest atmosphere of the terrestrial planets.
~ ~\\
\begin{flushleft}

6. Does the presence of an atmosphere appear to reduce the number of impact
craters? Justify your answer.
 {\bf (3 points)}

~ ~\\

\smallspace

{\bf Exercise \#2:} Topography of Mercury, Venus, Earth, and Mars.
At station \#2 you will find topographic maps of Mercury, Venus, the Earth, and 
Mars. The data for Mercury has not been fully published, so we only have
topographic maps for about 25\% of its surface.
These maps are color-coded to help you determine the highest and lowest parts
of each planet. You can determine the elevation of a color-coded feature
on these maps by using the scale found on each map. [Note that for the Earth
and Mars, the scales of these maps are in meters, for Mercury it is in
km (= 1,000 meters), while for Venus it is in 
planetary radius! But the scale for Venus is the same as for Mars, so you
can use the scale on the Mars map to examine Venus.]
\end{flushleft}

~ ~\\
\begin{flushleft}
7. Which planet seems to have the least amount of relief (relief =  
high and low spots/features)?
 {\bf (2 points)}

~ ~\\

~ ~ \\

8. Which planet seems to have the deepest/lowest regions?
 {\bf (2 points)}

~ ~\\

~ ~ \\

9. Which planet seems to have the highest mountains?
 {\bf (2 points)}

~ ~\\

~ ~ \\

On both the Venus and Mars topographic maps, the polar regions are plotted
as separate circular maps so as to reduce distortion. 

~ ~\\
10. Looking at these polar plots, Mars appears to be a very strange planet.
Compare the elevations of the northern and southern hemispheres of Mars.
If Mars had an abundance of surface water (oceans), what would the planet look 
like?  {\bf (3 points)}

~ ~\\

\smallspace
\end{flushleft}

\subsection{Detailed Comparison of the Surfaces of the Terrestrial Planets}

In this subsection we will compare some of the smaller surface features of
the terrestrial planets using a variety of close-up images. In the following,
the images of features on Venus have been made using
radar (because the atmosphere of Venus is so cloudy, we cannot see its surface).
While these images look similar to the pictures for the other planets,
they differ in one major way: in radar, smooth objects reflect the radio
waves differently than rough objects. In the radar images of Venus, the
rough areas are ``brighter'' (whiter) than smooth areas.\\

In the Moon lab (lab \# \ref{moonlab}), we discussed how impact craters form. 
For large impacts, the center of the crater may ``rebound'' and produce a 
central mountain (or several small peaks).
Sometimes an impact is large enough to crack the surface of the planet,
and lava flows into the crater filling it up, and making the floor of the
crater smooth. On the Earth, water can also collect in a crater, while on
Mars it might collect large quantities of dust. 

\begin{flushleft}
{\bf Exercise \#3:} Impact craters on the terrestrial planets. At station
\#3 you will find close-up pictures of the surfaces of the terrestrial
planets showing impact craters. 

~ ~\\

11. Compare the impact craters seen on Mercury, Venus, Earth, and Mars.
How are they alike, how are they different? Are central mountain peaks
common to craters on all planets? Of the sets of craters shown, does one
planet seem to have more lava-filled craters than the others?  {\bf (4 points)}

~ ~\\

\smallspace

~ ~ \\

12. Which planet has the sharpest, roughest, most detailed and complex craters?
[Hint: details include ripples in the nearby surface caused by the crater
formation, as well as numerous small craters caused by large boulders thrown
out of the bigger crater. Also commonly seen are ``ejecta blankets'' caused
by material thrown out of the crater that settles near its outer edges.]
 {\bf (2 points)}

\smallspace

13. Which planet has the smoothest, and least detailed craters?
 {\bf (2 points)}

\smallspace

14. What is the main difference between the planet you identified in question
\#12 and that in question \#13? [Hint: what processes help erode craters?]
 {\bf (2 points)}

\smallspace
\end{flushleft}

You have just examined four different craters found on the Earth: Berringer,
Wolfe Creek, Mistastin Lake, and Manicouagan. Because we can visit these
craters we can accurately determine when they were formed. Berringer is
the youngest crater with an age of 49,000 years. Wolf Creek is the second
youngest at 300,000 years. Mistastin Lake formed 38 million years ago, while
Manicouagan is the oldest, easily identified crater on the surface of the
Earth at 200 million years old.\\

\vspace{0.2in}
\noindent
15. Describe the differences between young and old craters on the Earth. What
happens to these craters over time?  {\bf (4 points)}

\smallspace

\subsection{Erosion Processes and Evidence for Water}

Geological erosion is the process of the breaking down, or the wearing-away
of surface features due to a variety of processes.  Here we will be concerned
with the two main erosion processes due to the presence of an atmosphere:
wind erosion, and water erosion. With daytime temperatures above 700$^{\rm o}$F,
both Mercury and Venus are too hot to have liquid water on their surfaces.
In addition, Mercury has no atmosphere to sustain {\it water or a wind}. Interestingly,
Venus has a very dense atmosphere, but as far as we can tell, very little
wind erosion occurs at the surface. This is probably due to the incredible
pressure at the surface of Venus due to its dense atmosphere: the atmospheric
pressure at the surface of Venus is 90 times that at the surface of the
Earth--it is like being 1 km below the surface of an Earth ocean! Thus,
it is probably hard for strong winds to blow near the surface, and there
are probably only gentle winds found there, and these do not seriously
erode surface features. This is not true for the Earth or Mars.\\

On the surface of the Earth it is easy to see the effects of erosion by wind.
For residents of New Mexico, we often have dust storms in the spring. During
these events, dust is carried by the wind, and it can erode (``sandblast'')
any surface it encounters, including rocks, boulders and mountains. Dust can
also collect in cracks, arroyos, valleys, craters, or other low, protected
regions. In some places, such as at the White Sands National Monument, large
fields of sand dunes are created by wind-blown dust and sand.
On the Earth, most large dunefields are located in arid regions. \\

\begin{flushleft}
{\bf Exercise \#4:} Evidence for wind blown sand and dust on Earth and Mars.
At station \#4 you will find some pictures of the Earth and Mars highlighting
dune fields.

~ ~\\
16. Do the sand dunes of Earth and Mars appear to be very different? Do you
think you could tell them apart in black and white photos? Given that
the atmosphere of Mars is only 1\% of the Earth's, what does the presence of 
sand dunes tell you about the winds on Mars?  {\bf (3 points)}

~ ~\\

\smallspace

\end{flushleft}

\begin{flushleft}
{\bf Exercise \#5:} Looking for evidence of water on Mars. In this exercise,
we will closely examine geological features on Earth caused by the erosion
action of water. We will then compare these to similar features found
on Mars. The photos are found at Station \#5.

~ ~ \\

As you know, water tries to flow ``down hill'', constantly seeking the lowest
elevation. On Earth most rivers eventually flow into one of the oceans. In
arid regions, however, sometimes the river dries up before reaching the ocean,
or it ends in a shallow lake that has no outlet to the sea. In the process
of flowing down hill, water carves channels that have fairly unique shapes.
A large river usually has an extensive, and complex drainage pattern.

~ ~\\

17. The drainage pattern for streams and rivers on Earth has been termed
``dendritic'', which means ``tree-like''. In the first photo at this station
(\#23) is a dendritic drainage pattern for a region in Yemen. Why was the term
dendritic used to describe such drainage patterns? Describe how this pattern
is formed. {\bf (3 points)}

~ ~ \\

\smallspace

18. The next photo (\#24) is a picture of a sediment-rich river (note the brown
water) entering a rather broad and flat region where it becomes shallow
and spreads out. Describe the shapes of the ``islands'' formed by this
river. {\bf (3 points)}

\smallspace
\end{flushleft}

In the next photo (\#25) is a picture of the northern part of the Nile river
as it passes through Egypt. The Nile is 4,184 miles from its source to
its mouth on the Mediterranean sea. It is formed in the highlands of Uganda
and flows North, down hill to the Mediterranean. Most of Egypt is a very
dry country, and there are no major rivers that flow into the Nile, thus
there is no dendritic-like pattern to the Nile in Egypt. [Note that in this
image of the Nile, there are several obvious dams that have created lakes
and reservoirs.]

~ ~\\

\begin{flushleft}
19. Describe what you see in this image from Mars (Photo \#26). {\bf (2 points)}

\smallspace

~ ~ \\

20. What is going on in this photo (\#27)? How were these features formed? Why do
the small craters not show the same sort of ``teardrop'' shapes? {\bf (2 points)}

\smallspace

~ ~\\

21. Here are some additional images of features on Mars. The second one 
(Photo \#29) is a close-up of the region delineated by the white box seen in 
Photo \#28. 
Compare these to the Nile. {\bf (2 points)}

\smallspace

~ ~ \\

22. While Mars is dry now, what do you conclude about its past? Justify
your answer. What technique can we use to determine when water might have 
flowed in Mars' past? [Hint: see your answer for \#20.] {\bf (4 points)}

\smallspace

~ ~ \\

\end{flushleft}

\subsection{Volcanoes and Tectonic Activity}

While water and wind-driven erosion is important in shaping the surface of
a planet, there are other important events that can act to change the 
appearance of a planet's surface: volcanoes, earthquakes, and plate tectonics.
The majority of the volcanic and earthquake activity on Earth occurs near the 
boundaries of large slabs of rock called ``plates''. As shown in Figure 
\ref{cutaway},
the center of the Earth is very hot, and this heat flows from
hot to cold, or from the center of the Earth to its surface (and into space).
This heat transfer sets up a boiling motion in the semi-molten mantle
of the Earth. \\

As shown in the next figure (Fig. \ref{mantle}), in places where the heat rises,
we get an up-welling of material that creates a ridge that forces the plates
apart. We also get volcanoes at these boundaries. In other places, the crust
of the Earth is pulled down into the mantle in what is called a subduction
zone. Volcanoes and earthquakes are also common along subduction zone
boundaries. There are other sources of earthquakes and volcanoes which are
not directly associated with plate tectonic activity. For example,
the Hawaiian islands are all volcanoes that have erupted in the middle of the
Pacific plate. The crust of the Pacific plate is thin enough, and there is 
sufficiently hot material below, to have caused the volcanic activity which
created the chain of islands called Hawaii. In the next exercise
we will examine the other terrestrial planets for evidence of volcanic
and plate tectonic activity.\\

~ ~\\
\begin{figure}[ht]
\centerline{{\includegraphics[width=9cm]{Terrestrial/earth-layers.pdf}}}
\caption{A cut-away diagram of the structure of the Earth showing the hot
core, the mantle, and the crust. The core of the Earth is very hot, and is
composed of both liquid and solid iron. The mantle is a zone where the rocks
are partially melted (``plastic-like''). The crust is the cold, outer skin
of the Earth, and is very thin.}
\label{cutaway}
\end{figure}

\begin{figure}[ht]
\centerline{{\includegraphics[width=9cm]{Terrestrial/mantle.pdf}}}
\caption{The escape of the heat from the Earth's core sets-up a boiling motion
in the mantle. Where material rises to the surface it pushes apart the
plates and volcanoes, and mountain chains are common. Where the material is 
cooling, it flows downwards (subsides) back into the mantle pulling down on 
the plates (``slab-pull'). This is how the large crustal plates move around on 
the Earth's surface. }
\label{mantle}
\end{figure}

\noindent
{\bf Exercise \#6:} Using the topographical maps from station \#2, we
will see if you can identify evidence for plate tectonics on the Earth.
Note that plates have fairly distinct boundaries, usually long chains of
mountains are present where two plates either are separating (forming long
chains of volcanoes), or where two plates run into each other creating
mountain ranges. Sometimes plates fracture, creating fairly straight lines
(sometimes several parallel features are created). The remaining photos
can be found at Station \#6.\\

\begin{flushleft}
23. Identify and describe several apparent tectonic features on the 
topographic map of the Earth.
[Hint: North and South America are moving away from Europe and Africa].
 {\bf (2 points)}
~ ~\\

\smallspace

24. Now, examine the topographic maps for Mars and Venus (ignoring the
grey areas that are due to a lack of spacecraft data). Do you see any evidence
for large scale tectonic activity on either Mars or Venus?{\bf (3 points)}

\smallspace

\end{flushleft}

The fact that there is little large-scale tectonic activity present on the
surfaces of either Mars or Venus today does not mean that they 
never had any geological activity. Let us examine the volcanoes found
on Venus, Earth and Mars. The first set of images contain views of a number of
volcanoes on Earth. Several of these were produced using space-based radar
systems carried aboard the Space Shuttle. In this way, they  better
match the data for Venus. There are a variety of types of volcanoes on
Earth, but there are two main classes of large volcanoes: ``shield'' and 
``composite''.
Shield volcanoes are large, and have very gentle slopes. They are caused
by low-viscosity lava that flows easily. They usually are rather flat on top,
and often have a large ``caldera''
(summit crater).  Composite volcanoes are more explosive,
smaller, and have steeper sides (and ``pointier'' tops). Mount St. Helens is 
one example of a composite volcano, and is the first picture (Photo \#31)
at this station
(note that the apparent crater at the top of St. Helens is due to the
1980 eruption that caused the North side of the volcano to collapse, and
the field of devastation that emanates from there).
The next two pictures are also of composite volcanoes while the last
three are of the shield volcanoes Hawaii, Isabela and Miakijima (the last
two in 3D).\\

\vspace{0.2in}
\begin{flushleft}
25. Here are some images of Martian volcanoes (Photos \#37 to \#41). What one type of volcano
does Mars have? How did you arrive at this answer? {\bf (3 points)}

\bigspace


26. In the next set (Photos \#42 to \#44) are some false-color images of Venusian volcanoes.
Among these are both overhead shots, and 3D images. Because Venus was
mapped using radar, we can reconstruct the data to create images as
if we were located on, or near, the surface of Venus. {\it Note, however, that
the vertical elevation detail has been exaggerated by a factor of ten!}
It might be hard to tell, but Venus is also dominated by one main type
of Volcano, what is it? {\bf (3 points)}

\end{flushleft}

\smallspace
\clearpage
\subsection{Summary (35 points)}

As we have seen, many of the geological features common to the Earth can
be found on the other terrestrial planets. Each planet, however, has its
own peculiar geology. For example, Venus has the greatest number of volcanoes
of any of the terrestrial planets, while Mars has the biggest volcanoes.
Only the Earth seems to have active plate tectonics. Mercury appears to
have had the least amount of geological activity in the solar system and,
in this way, is quite similar to the Moon. Mars and the Earth share something
that none of the other planets in our solar system do: erosion features
due to liquid water. This, of course, is why there continues to be interest
in searching for life (either alive or extinct) on Mars.

\begin{itemize}
\item Describe the surfaces of each of the terrestrial planets, and 
the most important geological forces that have shaped their surfaces.

\item Of the four terrestrial planets, which one seems to be the least
interesting? Can you think of one or more reasons why this planet is 
so inactive?

\item If you were in charge of searching for life on Mars, where would you
want to begin your search?

\end{itemize}

\subsection{Possible Quiz Questions}

\begin{flushleft}

1. What are the main differences between Terrestrial and Jovian planets?\\

2. What is density?\\

3. How are impact craters formed?\\

4. What is a topographic map?
\end{flushleft}

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

Since Mars currently has no large bodies of water, what is probably the most 
important erosion process there? How can we tell? What is the best
way to observe or monitor this type of erosion? Researching the images
from the several small landers and some of the orbiting missions, is
there strong evidence for this type of erosion? What is that evidence?
