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
Chapter 21
The Moon and Mercury:
Airless Worlds
QuickTime™ and a
decompressor
are needed to see this picture.
Mare
QuickTime™ and a
decompressor
are needed to see this picture.
Highlands
1. Original crust forms
3. Lunar mare form (volcanism)
2. Impacts rework surface
(late heavy bombardment)
4. 3.2 billion years:
volcanism stops; cratering
only since then
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
The Lunar Surface
Older: heavily cratered
Younger surface:
flooded with lava
Impact Cratering
Ejecta from the impact : bright
rays around young craters
QuickTime™ and a
decompressor
are needed to see this picture.
History of Impact Cratering
Rate of impacts decreased
rapidly after the solar system
formed.
Most craters seen on the
moon’s (and Mercury’s)
surface were formed
within the first ~ 1/2
billion years.
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
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
Moon Rocks: Igneous
No sedimentary rocks => No sign of water ever present on the moon.
Vesicular
= bubbles from gases in
lava, basalts (mare)
Breccias (= “broken”)
highlands
Older rocks are
pitted with small
micrometeorite
craters
(highlands)
LCROSS impact plume
QuickTime™ and a
decompressor
are needed to see this picture.
Mercury
Very similar to Earth’s
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
Mercury is more massive than the Moon, so an impact does not
alter Mercury’s surface as much as the Moon’s.
Result: Mercury holds a better record of solar system impact
history
Moon: lots of overlapping craters
Mercury: easier to count craters
because they don’t overlap as much as
the moon’s
day : 700 K
night : 100 K
Mercury, Mariner 10 spacecraft
similar to Moon but no mare
Lobate Scarps
Curved cliffs, probably formed when
Mercury shrank while cooling down
Discovery scarp: 500 km long, 2km high
Mercury’s crust split and cracked as the planet cooled and shrank
Lobate scarps
Scarps: unique to Mercury
(Extinct) Volcanoes on Mercury
Mercury’s 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
NASA’s 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.
Evidence for Volcanoes
• MESSENGER has found
shield volcanoes and vents
suggesting explosive
volcanism inside the large
Caloris basin
partly filled
crater
• 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)
vents
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).
The Big Picture
Alaska
Hawaii
• Mercury’s
widespread plains
formed by volcanism
• Mercury’s volcanic
style was more
similar to the Moon
than Mars or Earth
• MESSENGER will
enter orbit around
Mercury in 2011
Mars
Mercury
Volcanic features in the inner solar system
Venus
For more information…
Press Releases
•
space.com - 7/3/08 - “Volcanoes on Mercury Solve 30-year Mystery”
http://www.space.com/scienceastronomy/080703-mercury-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 Alaska’s Redoubt Volcano courtesy of AVO/USGS, taken by Heather Bleick
http://www.avo.alaska.edu/image.php?id=17872
•
Image of Olympus Mons on Mars and Maat Mon on Venus courtesy of NASA/JPL
http://pds.jpl.nasa.gov/planets/captions/mars/olympus.htm
http://photojournal.jpl.nasa.gov/catalog/PIA00106
Source Article
•
(on-campus login may be required to access journals)
Head et al., ‘Volcanism on Mercury: Evidence from the First MESSENGER Flyby’,
Science, 321(5885), p. 69, DOI: 10.1126/science.1159256, 2008.
http://www.sciencemag.org/cgi/content/abstract/321/5885/69
Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider
dpsdisc@aas.org - http://dps.aas.org/education/dpsdisc/ - Released 24 April 2009
Fig. 10.6
Caloris basin
1300 diameter!
OUCH!
Caloris Basin - 1300 km across
Discovery Scarp
Geological features on Mercury
“scarps” are long, steep cliffs, not found anywhere else
The Plains of Mercury
intercrater plains, no large
mare
Marked by smaller craters (<
15 km) and impacts from
ejecta
Smooth plains:
Even younger than intercrater
plains
Intercrater plains: lots of small craters, no large ones
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
Smooth plains near
Caloris Basin
Smooth plains:
younger than intercrater
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.
Why does Mercury have such a large core?
Massive impacts may have blown away much of its
mantle after the planet differentiated
History of Mercury
Dominated by
ancient lava
flows and heavy
meteorite
bombardment.
Radar image
suggests icy
polar cap.
Outstanding questions about Mercury:
1) Why is it so dense?
2) What is that stuff at its poles?
3) What can it tell us about planetary evolution?
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.