Transcript The Milky Way - UNT Department of Political Science

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Chapter 21
The Moon and Mercury:
Airless Worlds
Guidepost
The two preceding chapters have been preparation for
the exploration of the planets. In this chapter, we begin
that detailed study with two goals in mind. First, we
search for evidence to test the solar nebula hypothesis
for the formation of the solar system. Second, we
search for an understanding of how planets evolve
once they have formed.
The moon is a good place to begin because people
have been there. This is an oddity in astronomy in that
astronomers are accustomed to studying objects at a
distance. In fact, many of the experts on the moon are
not astronomers but geologists, and much of what we
will study about the moon is an application of earthly
geology.
Guidepost (continued)
While no one has visited Mercury, we will recognize it
as familiar territory. It is much like the moon, so our
experience with lunar science will help us understand
Mercury as well as the other worlds we will visit in the
chapters that follow.
Outline
I. The Moon
A. The View From Earth
B. Highlands and Lowlands
C. The Apollo Missions
D. Moon Rocks
E. The History of the Moon
F. The Origin of Earth's Moon
II. Mercury
A. Rotation and Revolution
B. The Surface of Mercury
C. The Plains of Mercury
D. The Interior of Mercury
E. A History of Mercury
The Moon: The View from Earth
From Earth, we
always see the
same side of the
moon.
Moon rotates around
its axis in the same
time that it takes to
orbit around Earth:
Tidal coupling:
Earth’s gravitation has
produced tidal bulges
on the moon;
Tidal forces have
slowed rotation down to
same period as orbital
period
Lunar Surface Features
Two dramatically
different kinds of terrain:
• Highlands:
Mountainous terrain,
scarred by craters
• Lowlands: ~ 3 km lower
than highlands; smooth
surfaces:
Maria (pl. of mare):
Basins flooded by
lava flows
Highlands and Lowlands
Sinuous rilles =
remains of ancient
lava flows
May have been lava
tubes which later
collapsed due to
meteorite
bombardment.
Apollo 15
landing site
The Highlands
Saturated with craters
Older craters partially
obliterated by more
recent impacts
… or flooded by
lava flows
Impact Cratering
Impact craters on the moon
can be seen easily even
with small telescopes.
Ejecta from the impact can be
seen as bright rays originating
from young craters
The Moon’s Craters
(SLIDESHOW MODE ONLY)
History of Impact Cratering
Rate of impacts due
to interplanetary
bombardment
decreased rapidly
after the formation
of the solar system.
Most craters
seen on the
moon’s (and
Mercury’s)
surface were
formed within the
first ~ 1/2 billion
years.
Missions to the Moon
Major challenges:
Need to carry enough fuel for:
• in-flight corrections,
• descent to surface,
• re-launch from the surface,
• return trip to Earth;
need to carry enough food and
other life support for ~ 1 week
for all astronauts on board.
Solution:
• only land a small, light
lunar module;
• leave everything behind that
is no longer needed.
Lunar module (LM) of Apollo 12
on descent to the surface of the
moon
The Apollo Missions
Apollo Landing Sites
First Apollo missions landed on safe, smooth terrain.
Later missions explored more varied terrains.
Apollo 17: Taurus-Littrow;
lunar highlands
Apollo 11: Mare Tranquilitatis;
lunar lowlands
Apollo Landing Sites (2)
Selected to sample
as wide a variety as
possible of different
lowland and
highland terrains.
Lowlands
(maria)
Highlands
Moon Rocks
All moon rocks brought back to Earth are igneous (= solidified lava)
No sedimentary rocks => No sign of water ever present on the moon.
Different types of moon rocks:
Vesicular
Breccias (= fragments of
(= containing holes
different types of rock
from gas bubbles in cemented together), also
the lava) basalts,
containing anorthosites (=
typical of dark rocks bright, low-density rocks
found in maria
typical of highlands)
Older rocks
become pitted
with small
micrometeorite
craters
The History of the Moon
Moon is small; low mass 
rapidly cooling off; small
escape velocity  no
atmosphere  unprotected
against meteorite impacts.
Moon must have formed in a
molten state (“sea of lava”);
Heavy rocks sink to bottom;
lighter rocks at the surface
No magnetic field  small
core with little metallic iron.
Surface solidified ~ 4.6 – 4.1
billion years ago.
Heavy meteorite
bombardment for the next
~ 1/2 billion years.
Alan Shepard (Apollo 14)
analyzing a moon rock, probably
ejected from a distant crater.
Formation of Maria
Impacts of
heavy
meteorites broke
the crust and
produced large
basins that were
flooded with lava
Formation of Maria (2)
Major impacts forming maria might have ejected
material over large distances.
Apollo 14
Large rock probably ejected during the formation of Mare
Imbrium (beyond the horizon!)
Origin of Mare Imbrium
Terrain opposite to Mare
Imbrium is jumbled by seismic
waves from the impact.
The Origin of Earth’s Moon
Early (unsuccessful) hypotheses:
Fission
hypothesis:
Break-up of Earth during early period of fast
rotation
Problems: No evidence for fast rotation;
moon’s orbit not in equatorial plane
capture
hypothesis:
Condensation
hypothesis:
Capture of moon
that formed
elsewhere in the
solar system
Problem: Requires
succession of very
unlikely events
Condensation at time
of formation of Earth
Problem: Different chemical
compositions of Earth and moon
Modern Theory of Formation of the Moon
The Large-Impact Hypothesis
• Impact heated material enough to
melt it
 consistent with “sea of magma”
• Collision not head-on
 Large angular momentum
of Earth-moon system
• Collision after differentiation of
Earth’s interior
 Different chemical compositions of
Earth and moon
Mercury
Very similar to Earth’s
moon in several ways:
• Small; no atmosphere
• lowlands flooded by
ancient lava flows
• heavily cratered
surfaces
Most of our
knowledge based on
measurements by
Mariner 10 spacecraft
(1974 - 1975)
View from Earth
Rotation and Revolution
Like Earth’s moon (tidally locked
to revolution around Earth),
Mercury’s rotation has been
altered by the sun’s tidal forces,
but not completely tidally
locked:
Revolution period = 3/2 times
rotation period
Revolution: ≈ 88 days
Rotation: ≈ 59 days
 Extreme
day-night
temperature contrast:
100 K (-173 oC) – 600 K (330 oC)
The Surface of Mercury
Very similar to Earth’s moon:
Heavily battered with craters,
including some large basins.
Largest basin: Caloris Basin
Terrain on the opposite side
jumbled by seismic waves
from the impact.
Lobate Scarps
Curved cliffs, probably formed when
Mercury shrank while cooling down
The Plains of Mercury
No large maria, but
intercrater plains:
Marked by smaller
craters (< 15 km)
and secondary
impacts
Smooth plains:
Even younger than
intercrater plains
The Interior of Mercury
Large, metallic core.
Over 60% denser than Earth’s moon
Magnetic
field only
~ 0.5 % of
Earth’s
magnetic
field.
Difficult to
explain at
present:
Liquid metallic core
should produce
larger magnetic field.
Solid core should
produce weaker field.
History of Mercury
Dominated by
ancient lava
flows and heavy
meteorite
bombardment.
Radar image
suggests icy
polar cap.
New Terms
tidal coupling
terminator
limb
mare
sinuous rille
ejecta
ray
secondary crater
micrometeorite
multiringed basin
relative age
absolute age
vesicular basalt
anorthosite
breccia
regolith
jumbled terrain
fission hypothesis
condensation hypothesis
capture hypothesis
large-impact hypothesis
resonance
lobate scarp
intercrater plain
smooth plain
Discussion Questions
1. Old science-fiction paintings and drawings of colonies
on the moon often showed very steep, jagged
mountains. Why did the artists assume that the
mountains would be more rugged than mountains on
Earth? Why are lunar mountains actually less rugged
than mountains on Earth?
2. From your knowledge of comparative planetology,
propose a description of the view that astronauts would
have if they landed on the surface of Mercury.
Quiz Questions
1. Why does the same side of the Moon always face Earth?
a. The Moon does not rotate.
b. The Moon rotates in the same direction that it revolves.
c. The Moon's period of rotation is equal to its orbital period.
d. Sometimes the backside of the Moon is lit by the Sun.
e. Both b and c above.
Quiz Questions
2. How did the Moon achieve its synchronous rotation?
a. When the Moon formed it just happened to have this
synchronous rotation.
b. The Earth raises tidal bulges on the Moon. As the Moon
rotated through these bulges, internal friction slowed the
Moon's rotation until it achieved tidal coupling.
c. Competing gravitational tugs on the Moon by the Earth and
Sun set up this synchronous rotation.
d. The Moon pulls up a tidal bulge on Earth, and Earth rotates
so fast that it has locked the Moon into this synchronous
rotation.
e. As the Earth and Moon orbited their common center of mass,
the centrifugal forces sent the Moon outward until this
synchronous rotation was achieved.
Quiz Questions
3. How do we know that Copernicus is a young impact crater?
a. It is on the side of the Moon that faces Earth.
b. It has a central peak and raised rim.
c. It has scalloped slopes along its inner crater walls.
d. Blocks of material in its ejecta formed secondary craters.
e. It has bright rays that extend onto the surrounding maria.
Quiz Questions
4. How do we find the relative ages of the Moon's maria and
highlands?
a. By counting the number of impact craters.
b. By measuring the depth of the lunar regolith.
c. By measuring the lunar latitude and longitude.
d. By measuring the size of the smallest impact craters.
e. By measuring variations in the Moon's gravitational field.
Quiz Questions
5. Why do almost all impact craters have a circular shape?
a. High-speed projectiles vaporize explosively upon impact,
sending out spherical compression waves.
b. The impacting projectiles have a spherical shape and thus
punch out circular penetration holes.
c. Erosion has reduced the irregular craters to circular shapes.
d. Most impacts occur from directly overhead.
e. A circle is the most perfect form.
Quiz Questions
6. Why did the first Apollo missions land on the maria?
a. The most interesting geology is at these locations.
b. To maintain a continuous communication link with the
command module.
c. To search for fossils that are more likely to exist where water
was once present.
d. It was thought to be safer due to the smoother terrain and
thinner regolith.
e. The lunar air is thicker at low elevation.
Quiz Questions
7. Why do we suppose that the Moon formed with a molten
surface?
a. The Moon is covered with volcanic craters of all sizes.
b. Samples from the maria regions are basalt, a common
igneous rock.
c. The oldest lunar rock samples are about 4.4 billion years old
and composed of anorthosite, a mineral that crystallizes and
rises to the top of a lava ocean.
d. Both a and b above.
e. All of the above.
Quiz Questions
8. What are the characteristics of a rock that is a breccia?
a. Breccia is igneous rock, with large crystals that form by slow
cooling of magma deep beneath the surface.
b. Breccia is igneous rock, with small crystals that form by rapid
cooling of lava flows on the surface.
c. Breccia is rock consisting of broken rock fragments that are
cemented together by heat and pressure.
d. Breccia is a sedimentary rock composed of calcium and
magnesium carbonates.
e. Breccia is sedimentary rock formed by the evaporation of
salty shallow seas.
Quiz Questions
9. Why are so many lunar rock samples breccias?
a. The many violent volcanic eruptions have formed a lot of
breccia.
b. The numerous impact events produce a lot of brecciated
rock.
c. Slow evaporation of shallow seas in the maria regions left
breccia deposits.
d. Plate motion has pushed the deeply formed breccias to the
lunar surface.
e. Carbon dioxide dissolves in water, combines with calcium,
and precipitates onto the sea floor. These deposits are later
lithified by the heat and pressure that accompany deep burial.
Impact events bring the breccias to the lunar surface.
Quiz Questions
10. On the large scale, which of the four states of development
of a planetary body could be termed arrested development in
the case of the Moon?
a. Melting and differentiation.
b. Impact cratering.
c. Flooding of low-lying regions.
d. Slow surface evolution.
e. None of these stages took place on the Moon.
Quiz Questions
11. What single factor resulted in the Moon today being so very
much different than the Earth is today?
a. The long, continued period of occasional impacts.
b. The flooding of lowland basins with basalt.
c. The early torrential bombardment.
d. The late heavy bombardment.
e. The Moon's small size.
Quiz Questions
12. Why does the Moon have large maria on the Earth-facing
side, yet no large maria on the opposite side?
a. The maria regions are the same on both sides; we normally
don't see those on the far side.
b. The late heavy bombardment only occurred on the Earthfacing side.
c. The maria on the far side are not as dark as those on the
near side.
d. The Moon's crust is thicker (or elevations higher) on the far
side.
e. No large impact basins exist on the Moon's far side.
Quiz Questions
13. Which of the following is due to the Moon's small size?
a. The Moon has no atmosphere.
b. The Moon does not have a dipole magnetic field.
c. The Moon does not have plate tectonics.
d. The Moon's surface geology is dominated by impact craters.
e. All of the above.
Quiz Questions
14. For what reasons do we reject the condensation (double
planet) hypothesis of the Moon's origin?
a. The Moon has a much lower density than Earth.
b. The Moon is very low in volatiles, compared to Earth.
c. The Moon is much smaller and less massive than Earth.
d. Both a and b above.
e. All the above.
Quiz Questions
15. How does the large impact hypothesis explain the Moon's
lack of iron?
a. The impact occurred before either planetesimal had
differentiated and formed an iron core.
b. The ejected orbiting material that formed the Moon was
initially at a high temperature.
c. Both planetesimals were differentiated, and the two iron
cores went to Earth.
d. The impacting planetesimal was not differentiated and thus
had no iron core.
e. The Moon's lack of iron is the major problem of the large
impact hypothesis.
Quiz Questions
16. How is the planet Mercury similar to Earth's moon?
a. Their surfaces both appear heavily cratered by impacts.
b. Their lowland regions were flooded by ancient lava flows.
c. Their rotational periods are equal to their orbital periods.
d. Both a and b above.
e. All of the above.
Quiz Questions
17. How is the planet Mercury different than Earth's moon?
a. The lowland maria on Mercury are not much darker than the
cratered highlands.
b. Mercury has a much higher density.
c. Mercury has a dipole magnetic field.
d. Both a and b above.
e. All of the above.
Quiz Questions
18. How do we suppose that the lobate scarps on Mercury's
surface formed?
a. Lobate scarps are huge dormant lava tubes.
b. As Mercury cooled and shrank, the crust wrinkled.
c. Plate tectonics created a chain of folded mountains.
d. One side along a strike-slip boundary was forced upward.
e. As a chain of volcanic mountains along the edge of a
subduction zone.
Quiz Questions
19. What is the difference between the intercrater plains and
the smooth plains that are found on Mercury, in terms of time of
formation?
a. The intercrater plains are older than the smooth plains.
b. The intercrater plains are younger than the smooth plains.
c. These two types of plains formed at the same times at
different locations.
d. Their times of formation overlap due to the Sun's tidal
influence.
e. Their times of formation overlap due to the formation of the
Caloris Basin.
Quiz Questions
20. What evidence do we have that Mercury has a partially
molten, metallic core?
a. The rate at which the orbit of Mercury's moon precesses
indicates that Mercury has a high-density center.
b. The recent volcanic activity seen on Mercury's surface
indicates that it still has a molten interior.
c. The S waves created by the impact that formed Caloris
Basin did not appear on the opposite side of Mercury. And we
know that S waves cannot travel through liquids.
d. The peculiar tidal coupling of Mercury's spin to its orbit can
only be due to a partially molten, metallic core.
e. Mercury has a weak dipole magnetic field.
Answers
1.
2.
3.
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6.
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9.
10.
e
b
e
a
a
d
c
c
b
d
11.
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20.
e
d
e
d
c
d
e
b
a
e