10. our barren moon
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10. Our Barren Moon. Lunar plains & craters Manned lunar exploration The lunar interior The Moon ’ s geologic history The formation of the Moon. Moon Data (Table 10-1). Moon Data: Numbers. Diameter :3,476 . km0.27 . Earth Mass :7.4 . 10 22 kg0.012 . Earth - PowerPoint PPT PresentationTRANSCRIPT
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10. Our Barren Moon• Lunar plains & craters• Manned lunar exploration• The lunar interior• The Moon’s geologic history• The formation of the Moon
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Moon Data (Table 10-1)
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Moon Data: Numbers• Diameter: 3,476.km 0.27 . Earth• Mass: 7.4 . 1022 kg 0.012 . Earth• Density: 3.3 . water 0.61 . Earth• Orbit: 3.8 . 105 km 0.0026 . Earth• Day: 27.32 days 27.32 . Earth
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E-M Sizes & Distances to Scale
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Earth & Moon to Scale
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Moon Data: Special Features – 1• The Moon is the Earth’s only natural satellite• The Moon is 1 of 7 large Solar System satellites• The Moon has essentially no atmosphere• The Moon’s near side has 2 different surfaces
– The lunar highlands are very heavily cratered– The lunar lowlands have 14 maria (i.e., “seas”)
• The Moon’s “far side” has only 1 mare
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Moon Data: Special Features – 2• The Moon’s interior has a very small iron core• The Moon’s differential gravity causes tides
– Gravity differences on opposite sides of the Earth
• The Moon is intimately involved with eclipses– Solar eclipses: the Moon is in the middle– Lunar eclipses: the Earth is in the middle
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Orbiting a Common Center of Mass
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The Moon As Seen From Earth• Synchronous axial rotation
– 1-to-1 spin-orbit coupling• 1 spin on its axis for every 1 orbit around its parent object
– The Moon points only one “face” toward Earth• The Moon seems to wobble left & right
– Caused by changing orbital speed along an elliptical orbit• The Moon seems to nod up & down
– Caused by the 5.15° tilt of the Moon’s rotational axis
• Surface visibility– Bright & dark areas
• Cratered bright lunar highlands• Smooth dark lunar maria
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Three Basic Lunar Feature Types
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Details of a Lunar Crater (Far Side)
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Details of a Lunar Sea (Mare Imbrium)
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The Moon’s Two Hemispheres• The “near” side
– Very diverse• Lunar maria14 “seas”• Lunar terrae (highlands)
“lands”– Extensively cratered
• The “far” side– Very homogeneous
• Lunar maria1 “sea”• Lunar terrae (highlands)
“lands”– Extensively cratered
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Mare Orientale• Most prominent feature on the Moon’s far side
– Is not a mare in the traditional sense• It is not flooded with dark basalt lava
– Lunar “far side” crust was too thick to be penetrated
• It is a multi-ringed basin
– Is a mare in one sense• It is a very large impact basin
• Probable cause– Impact by a large asteroid or comet
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Mare Orientale: Low Res. Image
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Mare Orientale: High Res. Image
http://apod.nasa.gov/apod/image/1103/orientale_lro.jpg
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Contrasting Lunar Hemispheres
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The Rate of Lunar Crater Formation
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Old Unmanned Lunar Missions• Impacters
– Ranger program
3 of 9 spacecraft• Precursors to unmanned lunar landings• Transmitted TV pictures until impact
• Orbiters– Lunar Orbiter program
5 of 5 spacecraft• Precursors to manned lunar landings• Returned 1950 images of 99.5% of the lunar surface
– Clementine mission• Mapped lunar surface in UV, visible & IR wavelengths
– Lunar Prospector mission• Evidence of up to 6 billion tons of lunar ice
• Landers– Surveyor program
5 of 7 spacecraft• Soft-landed at various locations on the lunar surface
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Crater Alphonsus: Up Close & Afar
From Ranger 9 From Earth
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Manned Lunar Exploration• Orbiters
– Earth orbit– Lunar transfer orbit– Lunarorbit
• Landers– Apollo 11 Mare Tranquilitatis– Apollo 12– Apollo 13 Barely averted disaster– Apollo 14– Apollo 15– Apollo 16– Apollo 17
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Eugene Cernan (Apollo 17)
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The Lunar Surface• Many craters visible from Earth telescopes
– ~30,000 craters > 1.0 km in diameter• ~ 85% of the lunar near side is covered with craters• ~ 98% of the lunar far side is covered with craters
– Millions of craters actually exist on the lunar surface• Craters are typically circular
– Angle of impact has very little significance– Central peaks are common in large
craters– Upthrown crater rims are common on large
craters• Maria are larger than craters
– Tension fissures & pressure ridges are common– Rest ~2.0 to 3.0 km below the average lunar surface
• Comparable to Earth’s ocean crust– Flood basalts similar to Columbia River basalt flows
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Moon Rocks• Lunar rock formation
– All lunar rocks result from heating & cooling• Heat is derived from impact processes
– Strong evidence of chemical differentiation• Lunar rock types
– Igneous rocksCooled from magma
• Basalt Rich in iron & magnesium Maria
• Anorthosite Rich in quartz & feldsparHighlands
– Impact brecciaCemented by magma
• Only appreciable lunar mechanical weathering process• Lunar regolith “Blanket of stone”
– Fragments of existing lunar rock ~2 to 20 m thick– Fragments of incoming meteorites
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Moon Rock Ages• Basic physical processes
– Radiometric age dating• Radioactive starting isotope
Parent isotope• Stable ending isotope
Daughter isotope• Measure the decay rate of the parent isotope• Measure the parent to daughter isotope ratio
• Basic results– Mare basalts
• ~3.1 to 3.8 billion years old– Highland anorthosites
• ~4.0 to 4.3 billion years old– Period of intense bombardment
• ~3.8 to 4.6 billion years ago
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Typical Lunar Rocks
Vesicular mare basalt Highland anorthositeImpact breccia
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Clementine Maps the Lunar Surface
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The Lunar Interior• Chemical differentiation did occur
– Low density materials floated to the lunar surface– High density materials sank to the lunar center
• The Moon does have a tiny iron-rich core– The Moon’s core is ~ 3% of the lunar mass– The Earth’s core is ~33% of the Earth’s mass
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The Moon’s Internal Structure
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Lunar Magnetism• The past
– Ancient igneous rocks retain a weak magnetic field• Implies a partially molten core when surface solidified
• The present– No appreciable magnetic field
• Implies an almost completely solidified core– Moonquakes
• Only ~ 3,000 per year– Earth has ~ 1.5 million earthquakes per year
• Magnitude from ~ 0.5 to 1.5– Far weaker than on Earth
• Originate ~ 600 to 800 km beneath the surface– Far deeper than on Earth
• Triggered by tides produced by Earth’s differential gravity– Vary by a factor of 2 due to the highly elliptical lunar orbit
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Earth–Moon Dynamics• Some evidence
– Reflectors put on lunar surface by Apollo astronauts– Extremely precise distance measurements
• Moon is moving away from Earth ~3.8 cm . yr–1
• Basic physical processes– Differential lunar gravity raises ocean tides– Earth’s axial rotation drags tidal bulge ahead ~10°
• This is caused by friction along ocean bottoms• This in turn causes two things
– Earth’s tidal bulge pulls the Moon into a higher orbit– Earth’s tidal friction slows Earth’s rotation ~0.000 02 sec . yr–1
• Some implications– The month will become progressively longer– The dream of really long days will at last be realized – One face of Earth will always face the Moon
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Tidal Effects on the Earth & Moon
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The Formation of Earth’s Moon• Fission hypothesis Doubtful
– Earth’s axial rotation was extremely fast• Capture hypothesis Doubtful
– Earth’s gravity captured a planetesimal• Co-creation hypothesis Doubtful
– Particles in Earth orbit accreted into the Moon• Collisional ejection hypothesis Probable
– Earth was obliquely impacted by a planetesimal• Only 1.23% of the combined masses became the Moon• Absence of lunar volatiles supports this hypothesis
– Intense heating was an inevitable part of the impact • Low average lunar density supports this hypothesis
– Very little of Earth’s iron core was ejected
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Timeline: Moon Formation by Impact
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Hypothesis: Moon Impact Formation
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• Moon data– ~27 % Earth’s diameter– ~0.23% Earth’s mass– ~60 % Earth’s density
• The Moon as seen from Earth– Radically different near & far sides
• Synchronous rotation (1-to-1 S.O.C.)– Cratered highlands & craterless maria
• Lunar exploration– Unmanned
• Impacters, orbiters & landers– Manned
• Orbiters & landers• The lunar surface
– Crater & maria visibility– Lunar rocks
• Basalt & anorthosite• Impact breccia
• The lunar interior– Chemical differentiation
• Asymmetrical lunar crust• Mantle-dominated• Minimal iron core
• Lunar magnetism– Weak ancient magnetic field– No appreciable present mag. Field
• Earth-long-term Moon tidal dynamics– Lengthening days & months– Increasing Earth–Moon distance
• Formation of Earth’s Moon– Fission
hypothesis– Capture
hypothesis– Co-creation
hypothesis– Collisional ejection
hypothesis
Important Concepts