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STUDY GUIDE FOR CHAPTER 7
The following is a list of topics and subtopics along with some questions and suggestions.
I.
Comparative Geology of the Terrestrial Worlds
A. Basic composition and structure
1. The basic composition is rock and metal.
2. The basic structure is a metal core surrounded by a mantle of fairly dense rock with a
crust of lighter rock on top. The basic shape is spherical.
a. Early in their history the terrestrial planets and the Moon were molten and thus the
heaviest elements sank to the center and the lighter ones rose to the surface.
b. Gravity made the shape spherical. Any object with a diameter of 500 km or more
will end up being spherical. This is not true for smaller objects if they have always
been solid.
3. Another way to look at the mantle and crust is through how strong the rock is.
a. The rock in the crust and upper mantle is strong and these regions are called the
lithosphere.
b. The rock in the rest of the mantle is weak and can flow.
B. Heating and cooling
1. Heating of the interior of a planet is due to
a. accretion when it formed.
b. differentiation via the conversion of gravitational potential energy to heat via
friction as the heavier elements sink to the bottom.
c. radioactive decay since the particles that come out have some kinetic energy and
collide with other particles.
2. Cooling of a planet occurs by
a. conduction: heat flows from one region to another because they are in contact. This
occurs for heat flow through the lithosphere.
b. convection: in a liquid or a gas or the aesthenosphere where rock can flow if you
have a heat source then the material heats up, expands, and becomes less dense. As
it does so it rises up and then cools off in one way or another. As it cools off it
becomes more dense and falls back down again to be reheated.
c. volcanic eruption.
3. Rate of cooling is slower for larger planets than for smaller ones. Which terrestrial
objects would you expect to be the least likely to be geologically active today?
C. Shaping of planetary surfaces
1. Impact Cratering
a. Occurs for all planets and moons.
b. Impact usually has enough energy to vaporize solid rock. Gas likes to take up a lot
more volume than a solid does so there is a huge amount of pressure when the rock
vaporizes. This causes an explosion and a crater is excavated.
c. The crater is usually 10 times as wide as the impacting object and 1 or 2 times as
deep.
d. Material us usually ejected from the crater and sometimes forms a series of rays that
go out from the crater.
2. Volcanism
a. When volcanoes erupt the magma resurfaces the surrounding region.
b. If it is runny then it can resurface very large regions creating volcanic plains.
c. If it is more viscous then shield volcanoes form. They look like the shield of an
ancient warrior – having very gentle slopes.
d. If it is quite viscous then stratovolcanoes form that are tall and steep.
3. Tectonics
a. In general internal stresses can cause the lithosphere to be deformed. It can
i.
bunch up to form a mountain range.
ii.
be pulled apart to form a rift valley.
iii.
crack due to the weight of a volcano.
b. Plate tectonics
i.
There are convection currents in the part of the mantle below the lithosphere.
ii.
As the material is cooling these move horizontally.
iii.
The lithosphere floats on top of these and thus moves with them.
iv.
Since there are many different convection currents the lithosphere breaks up
into plates that move around like “bumper cars”.
v.
When plates collide one may go under the other creating mountain ranges
and deep ocean trenches.
vi.
When plates are pulled apart you get a rift in between them.
vii.
Plates can also slide past each other as is happening in California.
4. Erosion
a. Liquids such as water clearly cause a lot of erosion.
b. If there is “snow” then solids such as ice can also cause erosion. Think about
glaciers.
c. If there is an atmosphere, then wind will cause erosion. The more atmosphere
and/or the stronger the wind, the more erosion.
d. If there is no atmosphere then erosion due to micrometeorites occurs.
II.
General Characteristics of Atmospheres
A. Basics
1. What is an atmosphere?
2. What happens as you go higher in an atmosphere?
B. Greenhouse effect
1. In general an object must emit as much energy in radiation as it receives or else it either
heats up (if it emits less energy than it receives) or cools down (if it emits more energy
than it receives).
2. When sunlight hits the surface of a planet it warms it. The surface then gives off
infrared radiation.
3. If an atmosphere exists and one or more greenhouse gases are present then some of the
radiation given off by the planet will be absorbed by the molecules in the gas. In the
process the gas heats up and emits infrared radiation itself. However this radiation is in
all directions so much of the radiation is trapped and heats the atmosphere and the
surface to a higher temperature than it otherwise would be.
4. As the temperature increases the amount of infrared radiation emitted by the surface
increases. Recall that the power per area emitted by a black body is proportional to T4,
with T the temperature of the black body.
5. Eventually the surface radiates so much energy that enough can get through the
atmosphere to balance the amount of energy coming from the Sun. At that point the
surface stops heating up. However its temperature is higher than it would be if no
greenhouse gases were present and infrared radiation could simply escape into space.
6. Clearly the larger the amount of greenhouse gas present the greater the surface will be
heated.
7. What are some greenhouse gases? Which occur in the Earth’s atmosphere? In Venus’
atmosphere? In Mars atmosphere?
C. Radiation Belts and the Aurora
1. Charged particles spiral around magnetic field lines.
2. Solar wind consists of ions and electrons streaming away from the Sun
3. Most of the charged particles get deflected when they encounter the Earth’s magnetic
field.
4. Some get caught in it for awhile and spiral around the field lines. These form the
Earth’s radiation belts. Same happens for the Jovian planets. Eventually the particles
leak out, but they are continuously replaced by new ones.
5. When the particles spiral in near the poles they hit the upper atmosphere and can collide
with atoms and molecules there. The electrons in these atoms and molecules can then
be knocked into excited states. They then emit photons as they go back to the ground
state and this creates an aurora if there are enough charged particles.
D. Retention of an Atmosphere
1. The key factors in the retention of an atmosphere are
a. how large the escape speed is. This depends on how massive the object is as well
as its radius. If, near the top of the atmosphere, a gas particle is moving at the
escape speed or faster and it headed away from the Earth, then it will leave and not
come back.
b. how far from the Sun the object is. This influences the temperature of the
atmosphere. That in turn influences how fast the gas particles are moving.
c. how massive the gas particles are. The more massive the gas particles are, the
slower they move at a given temperature and the easier it is to retain them.
2. To retain an atmosphere for billions of years you need to have the average speed of the
gas particles be less than 1/6 the escape speed.
E. Role of the greenhouse effect on Venus, Earth, and Mars
1. On Venus .
a. it may never have been cool enough for water to form oceans and lakes.
b. with water vapor and carbon dioxide in the atmosphere the temperature of Venus
may have been twice as hot as it is now.
c. ultraviolet light from the Sun would have gradually disassociated water molecules
into H2 molecules (made of two hydrogen atoms) and an oxygen atom. The
hydrogen would have escaped and the oxygen may have reacted with the surface
rocks.
2. On Mars the opposite may have occurred.
a. If lakes and oceans did exist they would have dissolved carbon dioxide from the
early Martian atmosphere and incorporated it into rocks.
b. If the carbon dioxide was not replaced then the planet would have gotten colder due
to the reduced greenhouse effect. Its atmosphere would also have become thinner.
c. Eventually liquid water could not exist on the surface due to the low pressure of the
atmosphere and Mars would have become the cold dry place it is today.
3. On Earth we have a balance between these two extremes. The mechanism is the
Carbon Dioxide Cycle. What are the elements of this cycle? What happens if the
Earth warms up a bit? What happens if it cools down a bit?
II. Individual planets and the Earth’s Moon
A. General characteristics to know
1. Relative sizes
2. Does the object have
a. an appreciable atmosphere?
b. an appreciable magnetic field?
3. Primary constituent of the atmosphere if there is one.
4. What clouds are made of if there are any.
5. How hot or cold is the surface relative to other objects?
6. Types of surface features including any that are unique
7. Anything else of importance about the object such as why the Moon always shows the
same face to the Earth.
8. What are the similarities and differences between various planets?
B. Atmospheres
What to remember about the specifics of each planet’s (and Moon in the case of Saturn’s
moon Titan and Neptune’s moon Triton) atmosphere.
1. Which objects have significant atmospheres? Of those that do:
a. What is the primary constituent of that atmosphere?
b. What are the clouds made of?
c. How thick is the atmosphere relative to that of other objects?
2. Winds
a. There are relatively weak winds on Venus near the surface so there is very little
surface erosion due to winds.
b. There are strong seasonal winds on Mars that can cause global dust storms. These
occur during summer in the southern hemisphere which is the time when Mars is
closest to the Sun in its orbit.
C. Exercises
1. Put the names of the terrestrial planets and the Moon on pieces of paper and put them in
a bowl. Randomly draw one out and say or write down everything you can remember about
that object. Check the book and your notes to see what you forgot or got wrong. This also
can work well in a study group.