Guidepost

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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 and Mercury Airless Worlds

• Chapter 21

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Mare
Highlands
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2. Impacts rework surface (late heavy bombardment)
1. Original crust forms
4. 3.2 billion years volcanism stops
cratering only since then
3. Lunar mare form (volcanism)
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Formation of Maria Lava flows 3.8 to 3.2 billion
years ago
Impacts of massive objects broke the crust and
produced large basins that were flooded with lava
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The Lunar Surface
Older heavily cratered
Younger surface flooded with lava
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Impact Cratering
Ejecta from the impact bright rays around young
craters
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History of Impact Cratering
Rate of impacts decreased rapidly after the solar
system formed.
Most craters seen on the moons (and Mercurys)
surface were formed within the first 1/2
billion years.
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Apollo Landing Sites
First Apollo missions landed on safe, smooth
terrain.
Later missions explored highlands.
Apollo 17 Taurus-Littrow lunar highlands
Apollo 11 Mare Tranquilitatis lunar lowlands
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The Moon landing was not faked! Moon rocks were
brought back Black sky no stars expected to show
up Weird shadow angles caused by wide-angle
shots www.badastronomy.com
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Moon Rocks Igneous
No sedimentary rocks gt No sign of water ever
present on the moon.
Vesicular bubbles from gases in lava, basalts
(mare)
Older rocks are pitted with small micrometeorite
craters (highlands)
Breccias ( broken) highlands
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LCROSS impact plume
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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

• Visited by
• Mariner 10 (1974-75)
• Messenger (NOW!)

View from Earth
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Mercury is more massive than the Moon, so an
impact does not alter Mercurys surface as much
as the Moons. Result Mercury holds a better
record of solar system impact history
Mercury easier to count craters because they
dont overlap as much as the moons
Moon lots of overlapping craters
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day 700 K night 100 K
Mercury, Mariner 10 spacecraft similar to Moon
but no mare
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Lobate Scarps
Curved cliffs, probably formed when Mercury
shrank while cooling down
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Discovery scarp 500 km long, 2km high Mercurys
crust split and cracked as the planet cooled and
shrank
Lobate scarps
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Scarps unique to Mercury
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(Extinct) Volcanoes on Mercury

• Mercurys geology
• heavily cratered. There are no large volcanoes
  like Mars Olympus Mons, but there are many
  smooth, flat plains with few craters
• Ancient plains are caused by volcanic activity
• The latest closeup images by NASAs MESSENGER
  support the volcano theory

MESSENGER false color image of Caloris impact
basin (light orange is the basin interior).
Extinct volcanoes were imaged in several of the
bright orange regions just inside the southern
crater rim.
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Evidence for Volcanoes

• MESSENGER has found shield volcanoes and vents
  suggesting explosive volcanism inside the large
  Caloris basin
• The Mercury volcanoes smaller versions of the
  Hawaiian Islands or Olympus Mons on Mars
• Lava appears to have partly filled impact craters
  both inside and far from Caloris basin (not shown)

MESSENGER image (left) of a shield-like volcanic
dome, multiple vents and associated bright
deposits, and partially buried nearby features.
Shield volcanism formed the island of Hawaii
(right).
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The Big Picture

• Mercurys widespread plains formed by volcanism
• Mercurys volcanic style was more similar to the
  Moon than Mars or Earth
• MESSENGER will enter orbit around Mercury in 2011

Volcanic features in the inner solar system
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For more information

• Press Releases
• space.com - 7/3/08 - Volcanoes on Mercury Solve
  30-year Mystery
• http//www.space.com/scienceastronomy/080703-mercu
  ry-messenger.html
• Images
• Global view of Caloris basin and Mercury shield
  volcano courtesy of Science / AAAS
• http//messenger.jhuapl.edu/gallery/sciencePhotos/
  pics/caloris_color_MB.jpg
• http//messenger.jhuapl.edu/gallery/sciencePhotos/
  pics/Head_Fig1.jpg
• Aerial view of Hawaii courtesy of NASA/JSC STS61A
• http//tinyurl.com/maunaloashieldvolcano
• Aerial view of erupting Mauna Loa in Hawaii
  courtesy of HVO/USGS
• http//hvo.wr.usgs.gov/
• Image of Alaskas Redoubt Volcano courtesy of
  AVO/USGS, taken by Heather Bleick
• http//www.avo.alaska.edu/image.php?id17872
• Image of Olympus Mons on Mars and Maat Mon on
  Venus courtesy of NASA/JPL
• http//pds.jpl.nasa.gov/planets/captions/mars/olym
  pus.htm
• http//photojournal.jpl.nasa.gov/catalog/PIA00106
• Source Article (on-campus login may be required
  to access journals)

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Fig. 10.6
Caloris basin 1300 diameter!OUCH!
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Caloris Basin - 1300 km across
Discovery Scarp
Geological features on Mercury scarps are long,
steep cliffs, not found anywhere else
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The Plains of Mercury
intercrater plains, no large mare
Marked by smaller craters (lt 15 km) and impacts
from ejecta
Smooth plains
Even younger than intercrater plains
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Intercrater plains lots of small craters, no
large ones
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Smooth plains similar to mare. Result of ancient
volcanic flooding? We can learn about the timing
of the volcanic activity relative to cessation of
heavy bombardment
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Smooth plains near Caloris Basin Smooth plains
younger than intercrater
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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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Why does Mercury have such a large
core? Massive impacts may have blown away much
of its mantle after the planet differentiated
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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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• Outstanding questions about Mercury
• Why is it so dense?
• What is that stuff at its poles?
• What can it tell us about planetary evolution?

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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.