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.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

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

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

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

Three Basic Lunar Feature Types

Details of a Lunar Crater (Far Side)

Details of a Lunar Sea (Mare Imbrium)

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

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

Mare Orientale: Low Res. Image

Mare Orientale: High Res. Image

http://apod.nasa.gov/apod/image/1103/orientale_lro.jpg

Contrasting Lunar Hemispheres

The Rate of Lunar Crater Formation

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 covering 99.5% of 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

Crater Alphonsus: Up Close & Afar

From Ranger 9 From Earth

Manned Lunar Exploration• Orbiters

– Earth orbit– Lunar transfer orbit– Lunar orbit

• Landers– Apollo 11 Mare Tranquilitatis– Apollo 12– Apollo 13 Barely averted disaster– Apollo 14– Apollo 15– Apollo 16– Apollo 17

Eugene Cernan (Apollo 17)

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

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

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

Typical Lunar Rocks

Vesicular mare basalt Highland anorthositeImpact breccia

Clementine Maps the Lunar Surface

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

The Moon’s Internal Structure

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

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

Tidal Effects on the Earth & Moon

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

Hypothesis: Moon Impact Formation

Timeline: Moon Formation by Impact

• 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