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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).

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The Moon and Mercury: Airless Worlds

Chapter 21

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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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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:

Earth’s 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 Moon’s Craters


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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 moon’s (and Mercury’s) 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.


  • 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)


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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(= 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 Earth’s Moon

Early (unsuccessful) hypotheses:

Break-up of Earth during early period of fast rotation

Fission hypothesis:

Problems: No evidence for fast rotation; moon’s 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 Earth’s interior

Different chemical compositions of Earth and moon

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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

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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)

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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.

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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 (< 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 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.

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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




sinuous rille



secondary crater


multiringed basin

relative age

absolute age

vesicular basalt




jumbled terrain

fission hypothesis

condensation hypothesis

capture hypothesis

large-impact hypothesis


lobate 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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