10. Our Barren Moon Lunar plains & craters Manned lunar exploration The lunar interior The Moon’s geologic history The formation of Earth’s 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 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 two 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 maria 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 shows only one “face” to Earth’s inhabitants The Moon seems to wobble left & right This occurs due to changing orbital speed along an elliptical orbit The Moon seems to nod up & down This occurs due to tilt of the Moon’s rotational axis Surface visibility Bright & dark areas Cratered bright lunar highlands Smooth dark lunar maria

Three Basic Lunar Features

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 maria 14 “seas” Lunar terrae (highlands) “lands” Extensively cratered The “far” side Very homogeneous Lunar maria 1 “sea”

Mare Orientale Most prominent feature on the Moon’s “far side” 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 Image

Contrasting Lunar Hemispheres

The Rate of Lunar Crater Formation

Unmanned Lunar Exploration 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 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

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-based 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 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 Lunar rock types All lunar rocks result from heating & cooling Heat is derived from impact processes Strong evidence of chemical differentiation Lunar rock types Igneous rocks Cooled from magma Basalt Rich in iron & magnesium Maria        Anorthosite Rich in quartz & feldspar Highlands Impact breccia Cemented by magma Only appreciable lunar mechanical weathering process Lunar regolith “Blanket of stone” Fragments of pre-existing lunar rock ~2 to 20 m thick Fragments of incoming meteorites

Moon Rock Ages Basic physical processes Basic results Radiometric age dating Radioactive starting isotope Parent isotope Stable ending isotope Daughter isotope Measure the decay rate of the parent isotope Measure the ratio of parent to daughter isotopes 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 anorthosite Impact breccia

Clementine Maps the Lunar Surface

The Lunar Interior Chemical differentiation did occur Lowest density materials floated to the lunar surface Highest density materials sank to the lunar center Moon does have a very small iron-rich core Only ~3% of the lunar mass

The Moon’s Internal Structure

Lunar Magnetism The past The present Ancient igneous rocks preserve a weak magnetic field Implies at least a partially molten core when surface solidified The present No appreciable magnetic field Implies an essentially solidified core Moonquakes Only ~ 3,000 per year Far fewer than on Earth 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 generated by Earth’s differential gravity Vary by a factor of 2 due to the highly elliptical lunar orbit

Earth–Moon Dynamics Some evidence Basic physical processes Reflectors placed 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 fast 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 axial 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 & the 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 1975 Only 1.23% of the combined masses became the Moon The absence of lunar volatiles supports this hypothesis Intense heating was an inevitable part of the impact The low average lunar density supports this hypothesis Very little of Earth’s iron core was ejected

Hypothesis: Moon Formation by Impact

Timeline: Moon Formation by Impact

Important Concepts Moon data The Moon as seen from Earth ~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