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Note that the following lectures include
animations and PowerPoint effects such as fly ins
and transitions that require you to be in
PowerPoint's Slide Show mode (presentation mode).
2
The Moon and Mercury Airless Worlds

• Chapter 21

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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.
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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.
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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
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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
Earths gravitation has produced tidal bulges on
the moon
Tidal forces have slowed rotation down to same
period as orbital period
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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
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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
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The Highlands
Saturated with craters
Older craters partially obliterated by more
recent impacts
or flooded by lava flows
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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
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The Moons Craters
(SLIDESHOW MODE ONLY)
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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 moons (and Mercurys)
surface were formed within the first 1/2
billion years.
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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

Lunar module (LM) of Apollo 12 on descent to the
surface of the moon

• leave everything behind that is no longer
  needed.

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The Apollo Missions
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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
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Apollo Landing Sites (2)
Selected to sample as wide a variety as possible
of different lowland and highland terrains.
Lowlands (maria)
Highlands
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Moon Rocks
All moon rocks brought back to Earth are igneous
( solidified lava)
No sedimentary rocks gt No sign of water ever
present on the moon.
Different types of moon rocks
Vesicular ( containing holes from gas bubbles
in the lava) basalts, typical of dark rocks found
in maria
Breccias ( fragments of different types of rock
cemented together), also containing anorthosites
( bright, low-density rocks typical of highlands)
Older rocks become pitted with small
micrometeorite craters
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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.
Alan Shepard (Apollo 14) analyzing a moon rock,
probably ejected from a distant crater.
Heavy meteorite bombardment for the next 1/2
billion years.
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Formation of Maria
Impacts of heavy meteorites broke the crust and
produced large basins that were flooded with lava
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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!)
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Origin of Mare Imbrium
Terrain opposite to Mare Imbrium is jumbled by
seismic waves from the impact.
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The Origin of Earths Moon
Early (unsuccessful) hypotheses
Break-up of Earth during early period of fast
rotation
Fission hypothesis
Problems No evidence for fast rotation moons
orbit not in equatorial plane
capture hypothesis
Capture of moon that formed elsewhere in the
solar system
Problem Requires succession of very unlikely
events
Condensation hypothesis
Condensation at time of formation of Earth
Problem Different chemical compositions of Earth
and moon
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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 Earths
  interior

? Different chemical compositions of Earth and
moon
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Mercury
Very similar to Earths 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
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Rotation and Revolution
Like Earths moon (tidally locked to revolution
around Earth), Mercurys rotation has been
altered by the suns 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)
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The Surface of Mercury
Very similar to Earths 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.
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Lobate Scarps
Curved cliffs, probably formed when Mercury
shrank while cooling down
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The Plains of Mercury
No large maria, but intercrater plains
Marked by smaller craters (lt 15 km) and secondary
impacts
Smooth plains
Even younger than intercrater plains
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The Interior of Mercury
Large, metallic core. Over 60 denser than
Earths moon
Magnetic field only 0.5 of Earths magnetic
field.
Difficult to explain at present
Liquid metallic core should produce larger
magnetic field.
Solid core should produce weaker field.
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History of Mercury
Dominated by ancient lava flows and heavy
meteorite bombardment.
Radar image suggests icy polar cap.
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New Terms
tidal coupling terminator limb mare sinuous
rille ejecta ray secondary crater micrometeorite m
ultiringed basin relative age absolute
age vesicular basalt anorthosite breccia regolith
jumbled terrain fission hypothesis
condensation hypothesis capture
hypothesis large-impact hypothesis resonance lobat
e scarp intercrater plain smooth plain
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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 Earth-facing 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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Answers
1. e 2. b 3. e 4. a 5. a 6. d 7. c 8. c 9. b 10. d
11. e 12. d 13. e 14. d 15. c 16. d 17. e 18. b 19
. a 20. e