Download Surface of the Moon

Lecture #13:
The Main Points
• Surface of the Moon:
(1) The Moon’s surface is divided into
two major kinds of geologic
terrain: mare and highlands.
– General Properties
of the Moon.
– Lunar Geology.
(2) Most of our detailed knowledge of
the Moon comes from the Apollo
human exploration missions.
• Reading:
– Chapters 2.3, 9.3, 10.3.
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Luna Lore
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Basic Properties of the Moon
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Our Moon has provided a source of
inspiration, beauty, and awe in literature
and the arts, as well as the sciences.
"Lunar Science" is a very young field. Prior to the 1960's only a few
practitioners:
– G.K. Gilbert: "father of lunar geology": proposed dominance of
impact processes vs. volcanic processes (crazy idea at the time!)
– H.C. Urey: "father of planetary science": lunar geochemistry and
implications for formation of the Earth and planets.
– E.M. Shoemaker: Founder of modern-day studies of impacts and
lunar evolution/stratigraphy.
Telescopic surveys and mapping of the Moon conducted in the
1950's, 1960's as a prelude to spacecraft exploration.
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Average Distance from Earth: 384,000 km.
Sidereal period around Earth (rel. to stars): 27.32 days.
Period of Spin around axis: 27.32 days.
Mass = 7.35x1022 kg = 1/81 ME.
Radius = 1738 km = 0.27 RE.
Density = 3.3 g/cm3 (recall that Earth ~5.5 g/cm3).
Surface Gravity = 1.62 m/sec2 (17% of Earth's).
Escape Velocity = 2.4 km/sec = 0.21 vE.
Atmospheric Pressure = 10-15 bar.
Most of surface covered by gray rocky regolith.
Abundant evidence of impact and volcanic processes!
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Other Properties
The Lunar Surface
• Two major types of terrains visible from Earth:
• Ratio of Moon:Earth radii (1:4) is large for planets in our
solar system.
• Orbit: a ≈ 384,000 km; e ≈ 0.05.
• Apparent orbital period
Moon
(New Moon to New Moon):
29.5 days (Lecture 5).
• This "synodic" period
Earth
is not equal to the
a(1-e)
a(1+e)
sidereal period (27.3 d).
• Synchronous spin state.
– Mare and Highlands.
• Mare (Latin for sea; plural: Maria):
– Dark "patches", often circular, cover ~16% of surface.
– Sparsely cratered.
– Topographically low and smooth.
• Highlands:
– Brighter regions.
– Heavily cratered.
– Topographically high and rough.
(eccentricity highly exaggerated!)
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Craters and Basins
Mosaics from
NASA/DoD
Clementine
Spacecraft.
• What are the circular depressions
that are found all over the Moon,
in all sizes?
• Much debate!
• Two options:
- Volcanic calderas.
- Impact craters.
• Volcanism was the favored view
until only about 40 years ago!
• Geologists on Earth have lots of
experience with volcanoes, little
with impact craters.
Mare
Highlands
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Volcano or Impact?
Impact Cratering Process
• Critical measurement: shape.
• Volcano crater: circular feature
appearing as a deep hole on
the top of a mountain.
• Impact crater: circular feature
appearing below elevation of
surroundings, possibly with a
raised central peak.
• G.K. Gilbert (1843-1918) experimented with impacts into
sand and mud, and could produce round features like impact
craters, but only if they impacted head on (Why?)
• As the role of impacts was discovered on Earth, their
importance on the Moon became widely accepted.
• Understanding developed that circular depressions
formed essentially as a result of high energy, high
velocity explosions on a planetary surface.
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Cratering Vocabulary
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Major Questions About the Moon
Central Peak: A mountain found in the center of
large craters. It is formed by a "rebound" of the
rock at the impact site.
Ejecta: Material tossed out of the crater.
Ejecta Blanket: Ejecta tossed out at low speed. The
material lies like a blanket around the crater.
Floor: The interior of the crater. It is flat in large
craters.
Rays: Ejecta tossed out of the crater at high speed.
The material forms long lines pointing directly
away from the crater.
Rim: The raised edge of the crater. It is formed by the
outwards and upwards compression of the crater walls,
not ejecta.
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• Is the Moon a "primitive" body, or has it
been processed or differentiated?
• How has the Moon evolved with time?
• Where did the Moon come from?
• Most of our current answers have been
provided by the Apollo missions.
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The Apollo Missions
Twelve names to know at a party
• 9 missions to the Moon: 1968-1972.
• 6 landings, 3 “flybys”.
• 12 astronauts explored the surface at 6 nearside landing sites.
• Hundreds of kg of lunar rocks and soils brought back.
• Cost in today's dollars: ~ $20 billion (about 10 B-2 bombers).
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Apollo 11
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Apollo 12
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Apollo 14
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Surface Conditions
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Apollo 16
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Apollo 17
David R. Scott
James B. Irwin
John W. Young
Charles M. Duke, Jr.
– Eugene A. Cernan
– Harrison H. "Jack" Schmitt
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The Lunar Regolith
• Average temperatures:
• Regolith = Smashed-up upper layer of the
lunar crust, composed of blocks and fine
debris resulting from a tortured history of impacts.
• Regolith on the Moon consists of:
– Day: + 107°C (~380 K).
– Night: -153°C (~120 K).
• Dark, gray surface:
– Average reflectivity: 7%.
– Rocks.
– Breccias = rocky fragments (many unrelated) cemented by impacts.
– Loose, fine-grained surface “Soils”.
• No water.
• Very porous regolith.
• Regolith = rubble layer
generated by impacts.
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Alan B. Shepard, Jr.
Edgar D. Mitchell
Apollo 15
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Charles "Pete" Conrad, Jr.
Alan L. Bean
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Neil A. Armstrong
Edwin E. "Buzz" Aldrin, Jr.
• Lunar Soil contains:
– Small rocky fragments.
– Small pieces of impact-generated glass.
– Clumps of glass-bonded fragments called agglutinates.
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Composition of the Moon
Age of the Moon
• The Moon has a lower density than Earth:
– Implies lighter rocks overall.
– But ample evidence for Earth-like volcanic rocks and minerals and
lava flows on the surface.
– So maybe the interior of the Moon has less heavy stuff?
• Confirmed! Seismometers placed on the Moon by the
Apollo astronauts and spacecraft orbital gravity data
provide a glimpse into the inside of the Moon: the Moon
is differentiated into crust, mantle, and core.
• But the metallic core is very small, maybe
even < 5% of the mass of the Moon.
• A clue: Earth and Moon are very different.
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• Because we have samples, we can do radioactive
dating on the rocks and soils.
• Typical rock ages: 3.3 to 4.4 billion years!
• Most rocks on the Moon are much older than most
rocks on the Earth:
– The Moon preserves a record of the early solar system.
• The oldest Moon rocks and the oldest meteorites
are about the same age:
– This is a big clue to the overall age of the solar system.
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Summary
Next Lecture...
• Origin of the Moon
• The Moon is a large natural satellite locked into
synchronous rotation around the Earth.
• Mare and Highlands: the two major lunar terrains.
• Circular impact craters are the dominant geologic feature
on the lunar surface on all scales.
• The lunar regolith consists of fragmented and cemented
rocks, breccias, and soil.
• The Moon is differentiated but less dense than Earth.
• The Moon is also ~4.5 billion years old.
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• Once part of the Earth, but separated at birth?
• Unrelated and then captured by the Earth?
• Formed by a giant impact into the Earth?
• Reading:
– Finish Chapters 2.3, 9.3, 10.3.
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