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: kg Earth Density:3.3. water0.61. Earth Orbit: km Earth Day:27.32 days 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 towobbleleft & right –Caused by changing orbital speed along an elliptical orbit The Moon seems tonodup & 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 nota mare inthe traditional sense It isnotflooded with dark basalt lava –Lunar “far side” crust was too thick to be penetrated It is a multi-ringed basin –Isa mare inone 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

Contrasting Lunar Hemispheres

The Rate of Lunar Crater Formation

Old Unmanned Lunar Missions Impacters –Ranger program3 of 9 spacecraft Precursors to unmanned lunar landings Transmitted TV pictures until impact Orbiters –Lunar Orbiter program5 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 program5 of 7 spacecraft Soft-landed at various locations on the lunar surface

Crater Alphonsus: Up Close & Afar From Ranger 9From Earth

Manned Lunar Exploration Orbiters –Earthorbit –Lunar transferorbit –Lunarorbit Landers –Apollo 11Mare Tranquilitatis –Apollo 12 –Apollo 13Barely 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 peaksare common in large craters –Upthrown crater rimsare 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 rockformation –All lunar rocks result from heating & cooling Heat is derived from impact processes –Strong evidence of chemical differentiation Lunar rocktypes –Igneous rocksCooled from magma BasaltRich in iron & magnesium Maria AnorthositeRich in quartz & feldsparHighlands –Impact brecciaCemented by magma Only appreciable lunar mechanical weathering process Lunarregolith“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 Radioactivestarting isotopeParent isotope Stableending isotopeDaughter isotope Measure the decay rate of the parent isotope Measure the parent to daughter isotope ratio Basic results –Marebasalts ~3.1 to 3.8 billion years old –Highlandanorthosites ~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 basaltHighland anorthosite Impact 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 ~ 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 FissionhypothesisDoubtful –Earth’s axial rotation was extremely fast CapturehypothesisDoubtful –Earth’s gravity captured a planetesimal Co-creationhypothesisDoubtful –Particles in Earth orbit accreted into the Moon Collisional ejectionhypothesisProbable –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 –Fissionhypothesis –Capturehypothesis –Co-creationhypothesis –Collisional ejectionhypothesis Important Concepts