Transcript The Moon

The Moon
3.344 g/cm3
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
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.
• Meteoroid
impacts have
been the only
significant
“weathering”
agent on the
Moon
• The Moon’s
regolith, or
surface layer
of powdered
and fractured
rock, was
formed by
meteoritic
action
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
All of the lunar rock samples are igneous rocks
formed largely of minerals found in terrestrial rocks
Mare
basalt
• The lunar rocks contain no
water
• They differ from terrestrial
rocks in being relatively
enriched in the refractory
elements and depleted in
the volatile elements
Highlands
anorthosite
Impact
breccia
What about the other side?
The Moon’s airless, dry surface is covered
with plains and craters
• The Earth-facing side
of the Moon displays
light-colored, heavily
cratered highlands
and dark-colored,
smooth-surfaced
maria
• The Moon’s far side
has almost no maria
How can we explain the
difference?
Why do we have more maria on one side than the other?
Maria are lava flows. Why would you get more volcanics on one side?
Explore the interior of the moon.
Moonquake
Earthquake
The Moon has no global magnetic field but has
a small core beneath a thick mantle
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
The maria formed after the surrounding light-colored
terrain, so they have not been exposed to meteoritic
bombardment for as long and have fewer craters
• Virtually all
lunar
craters
were
caused by
space
debris
striking the
surface
• There is no
evidence
of plate
tectonic
activity on
the Moon
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.
Similar Processes to Earth?
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•
•
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•
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Volcanism 

Water

Cratering

Winds
Earthquakes  
Dynamo Effect
How did the Moon form?
• What are the constraints?
– BIG (for a moon)
– NO IRON CORE
– WAS CLOSER TO US IN PAST
– IGNEOUS ROCKS LACKING VOLATILES
– SIMILAR COMPOSITION TO EARTH’S
MANTLE
– CAN IT HAPPEN?
The Origin of Earth’s Moon
Early 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:
Sister hypothesis:
Capture of moon
that formed
elsewhere in the
solar system
Problem: Requires
succession of very
unlikely events
Condensation and accretion
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
•
•
•
•
The Moon probably formed from debris cast
into space when a huge planetesimal struck
the proto-Earth
The collisional-ejection theory holds that the
proto-Earth was struck by a Mars-sized
protoplanet and that debris from this collision
coalesced to form the Moon
This theory successfully explains most
properties of the Moon
The Moon was molten in its early stages, and
the anorthositic crust solidified from low-density
magma that floated to the lunar surface
The mare basins were created later by the
impact of planetesimals and filled with lava from
the lunar interior
Lunar rocks reveal a geologic history quite
unlike that of Earth
• The anorthositic crust
exposed in the highlands
was formed between 4.0
and 4.3 billion years ago
• The mare basalts
solidified between 3.1
and 3.8 billion years ago
• The Moon’s surface has
undergone very little
change over the past 3
billion years
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
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
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
14. For what reasons do we reject the condensation (double
planet, sister) 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.
Answers
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