Chapter 9
1. Is the Earth completely solid inside? How can scientists tell? No, Earth has a
partly liquid core. The Earth has a magnetic field and it has geologically active
surface (very few visible craters)—these both indicate an active core. Also by
monitoring the waves from an Earthquake scientists can show that the core is
partly liquid.
2. What causes geologic activity? Heat from within a planet.
3. How is it possible for an entire continent to move across the face of the Earth?
The continents are floating on the asthenosphere. Convection currents in the
mantle cause the motion of the continents.
4. Describe various ways in which the Earth is unique among planets in the solar
system. The Earth has continental plates which are in motion. It has liquid water
over most of its surface and that water is in constant motion. The Earth’s
atmosphere has oxygen, and the Earth has a biosphere
5. Why are typical rocks found on the Earth’s surface much younger than the Earth
itself? The age of a rock is measured since it last solidified. Because the Earth
has a very active surface geologically, rocks are constantly getting “recycled.”
They are melted a reformed in various geological processes.
6. How do we know that the Earth was once entirely molten? Because the light
elements are on the outside and the densest elements are toward the center. This
means that the Earth must have been in a molten state so the light elements could
“float to the top” and the heavy elements could “sink to the bottom.”
7. Describe the interior structure of the Earth. The Earth has an outer crust.
Underneath this is the mantle which includes the asthenosphere in its outermost
region. Underneath the mantle is the outer core, which is liquid, then the inner
core, which is solid.
8. What is the evidence that iron is abundant in the Earth’s core? The density of the
Earth and the fact that it has a magnetic field are both evidence that the core is
iron.
9. What is the evidence that the outer core is molten and the inner core is solid? The
seismic (earthquake) studies of the Primary (“P”) waves, which can travel through
solids and liquids, and Secondary (“S”) waves which can travel through solids,
but not through liquids show the Earth has a solid inner core and a liquid outer
core.
10. Describe the process of plate tectonics. Give specific examples of features
created by plate tectonics. Plate tectonics are the motion of the tectonic plates in
the Earth’s crust as the float on the mantle (which is being heated and has
convection currents). Some features are: Mountain ranges at plate boundaries
(the Himalayas), volcanic mountain ranges (the cascades), Oceanic ridges such as
the mid-atlantic ridge. Subduction zones such as on the west coast of South
America where the pacific plate is going under the South American plate.
11. Is the moon completely covered with craters? No. There are areas of the Maria
which have relatively few craters.
12. Does the Moon have an interior structure similar to the Earth? The Moon has a
crust, a mantle, and an iron-rich core like the Earth, but the Moon’s core is only
about 2 or 3 percent of its mass whereas the Earth’s is about 32% of its mass. The
Moon’s core is 20 percent of its diameter; the Earth’s core is 55 percent of its
diameter. The lithosphere (crust plus the solid part of the mantle) is about 500
miles thick on the Moon, whereas on the Earth it is about 20 miles thick.
13. How do Moon rocks compare to rocks found on Earth? Moon rocks have
typically have a different chemistry than Earth rocks.
14. How did the Moon form? The current accepted hypothesis for how the Moon
formed is the “collision-ejection hypothesis” in which the Earth is struck offcenter by a Mars-sized object. This accounts for the differences in the types of
elements found on Earth and the Moon.
15. What kind of features can you see on the Moon with a small telescope? Maria
(areas which are dark from lava flows), craters, center peaks in some craters, and
terrae (light colored highlands).
16. Are the impact craters on the Moon the same size as the meteoroids that made the
craters? No, a fast moving meteoroid can create a crater that is much larger than
the meteoroid.
17. Describe the differences between the maria and the lunar highlands. The Maria
are about 2 miles lower in elevation, on average than the terrae (highlands). The
lowlands are darker, and younger (less heavily cratered). The maria had lava
flows between 3 and 3.8 billion years ago after the end of the intense
bombardment.
18. Does the Moon have continental plates? No, the Moon’s surface is solid and
unmoving and about 500 miles thick.
19. Could you use a compass to navigate on the Moon? No because the Moon has no
magnetic field.
20. How do the surface features on Mercury compare to those on the Moon? The
Moon and Mercury have very similar surfaces.
21. Is Mercury’s internal structure more like that of the Earth or the Moon?
Mercury’s internal structure is more like that of the Earth. Mercury has a large
iron core which is partly liquid and part solid, like Earth’s.
22. Explain why Mercury does not have a substantial atmosphere? Mercury is small
so it doesn’t have the gravitational force to hold much atmosphere. Also, it is
very hot, which means gases near its surface get very hot which gives them more
energy to escape.
23. Why do astronomers suspect that there are active volcanoes on Venus? First,
astronomers see lots of volcanoes. The sulfur content of the atmosphere indicates
that there has been recent geologic activity (suggesting volcanoes), particularly
since astronomers have seen rapid rising and falling of the levels of sulfur in the
atmosphere in the last 50 years. Also the reflectivity of the ground around the
volcanoes is very high, indicating recent eruptions.
24. Does Venus have the same kind of surface geology as Earth? No. While Venus
does have volcanoes like Earth does, Venus does not have plate tectonics. Most
of the surface of Venus is near the average elevation on the planet. On Earth the
continents are up high, and the sea floors are low, and there is little land that is
around the average elevation.
25. Why was it once thought there were canals on Mars? An Italian astronomer saw
objects that looked to him like channels. The Italian word for channels got
mistranslated into “canals.” There weren’t channels either, the astronomer got
fooled by surface features that only looked connected as channels, but were not.
26. How are the northern and southern hemispheres of Mars different from each
other? The northern hemisphere is at a lower elevation and has fewer craters, so it
is a younger surface. The southern hemisphere is higher and heavily cratered.
27. What is the evidence that there is water on Mars? Spacecraft have observed the
presence of hydrogen below the surface, and this could indicate water. Large
young craters have mud flows around them. There are sometimes gullies in
craters that indicate water flow. Teardrop-shaped islands indicate that water was
flowing very rapidly. Structures that appear like ancient dry riverbeds.
28. What causes the seasonal color changes on Mars? The appearance of the planet
changes over the course of the year as dust storms cover the planet at various
times. These storms will cover and/or uncover dark rocks with a then layer of red
Martian dust. So sometimes the planet will look mostly red, other times there will
be dark features on it. This is why some people though that Mars had civilization
with agriculture.
29. Why are there seasons on Mars? Because like Earth, Mars’ rotational axis is
tilted relative to the plane of its orbit.
30. Why do scientists think that the surface of Mars is old? Because so many of the
craters can still be seen on the surface.
31. What is the Martian crustal dichotomy? The northern lowlands are (about 3
miles) lower in elevation and have fewer craters than the southern highlands.
32. Compare the volcanoes on Earth, and Mars. Talk about hot-spot volcanism on
these worlds: a hot-spot is a hot region below the crust that causes a volcano at
the surface. On Earth the continents move (plate tectonics), so the hot-spot
produces a chain of volcanoes as the continent moves over the hot-spot. On Mars,
the surface does not move, so it gets enormous volcanoes over the hot spot.
33. If you were an astronaut on Mars, could you use a compass for navigation? No,
Mars no longer has a global magnetic field.
34. Why is it impossible for liquid water to exist on Mars today? Because the
atmospheric pressure is too low and the temperature is too low. Mars can only
have water in solid or gas form.
35. Why is Mars red? Because of iron in its surface.