Introduction to the MoonSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Introduction to the MoonPYTS 395B – Introduction to the Moon2Reading PapersPresenting TopicsClass DiscussionWhat Topics?The Origin of the MoonGeology and Geophysics of the MoonThe History of the MoonThe Lunar Atmosphere and Polar IceHuman ExplorationSpacecraft missionsEverything will be posted on:http://www.lpl.arizona.edu/~shane/PTYS_395_MOONPrevious examples at:http://www.lpl.arizona.edu/~shane/PTYS_395_MERCURYPYTS 395B – Introduction to the Moon3About 364 – 407 thousand km away~60 Earth radiiEccentricity of ~0.051:1 spin orbit resonance with the EarthSpins once per orbitVery long dayPYTS 395B – Introduction to the Moon4Earth and Moon comparedA small terrestrial planetPYTS 395B – Introduction to the Moon5Two major surface unitsHeavily cratered – light toned – TerraeLess cratered – dark toned – MariaCrater count = age (the devil is in the details)Apollo samples allow us to calibrate conversion between crater count and age Introduction to the MoonPYTS 395B – Introduction to the Moon6Morphology changes as craters get biggerPit → Bowl Shape→ Central Peak → Central Peak Ring → Multi-ring BasinMoltke – 1km10 micronsEuler – 28kmSchrödinger – 320kmOrientale – 970kmPYTS 395B – Introduction to the Moon7Regolith – “Rock Blanket”Produced by impactsModified by space weatheringIda (and Dactyl) – Galileo missionTycho craterBright Rays imply youthPYTS 395B – Introduction to the Moon8A very tenuous atmosphereMolecules on ballistic trajectories Trapping in the permanent shadows in lunar cratersPYTS 395B – Introduction to the Moon9How did things get this way?Lunar History (mostly 4.5 – 3.0 Ga)Formation from giant impactImpact basinsLate heavy bombardmentVolcanism A little tectonicsAtmospheres and polar volatilesSurface activity on the Moon and Mercury mostly died off about 3 GaSurface history of Venus is only available from ~1.0 Ga onward0.27 RE1.00 AU0.38 RE0.39 AUSurface activity and history of Mars spans its entire existence…as opposed to…History of the MoonPYTS 395B – Introduction to the Moon10Early Solar SystemA tough neighborhood! Last stages of planetary accretionMany planetesimals left overMost gone in a ~100 MyrWe’re still accreting the last of these bodies todayPYTS 395B – Introduction to the Moon11Previous theoriesCo-accretionFission of spinning EarthCapture of rogue planetisimalApollo results (and common sense)Moon depleted in volatile elementsMoon depleted in siderophile elementsOxygen isotope ratios similarCapture of a rouge planet would be a dynamical miracleFission questionable since Moon doesn’t orbit in equatorial planeCurrent paradigm is Giant impactEarth close to final sizeMars-sized impactorBoth bodies already differentiatedBoth bodies formed at ~1 AUFrom Robin Canup, SWRI BoulderFormation of the MoonPYTS 395B – Introduction to the Moon12Iron cores of both bodies stay in the Earth.About 1 lunar mass of material goes into orbit outside the Earth’s Roche limit.Most of the matter in the Moon is from the impacting body.Heat of debris-disk removes volatilesFrom Robin Canup, SWRI BoulderEarth’s spin and Moon’s orbit become locked in 1:1 Cassini stateMoon’s orbit expands by a few cm/yearEarth’s rotation slowsBulk composition and orbital statePYTS 395B – Introduction to the Moon13Accretion of lunar material into the Moon within a few years!High-accretion rates mean surface is moltenMagma ocean probably a few hundred km thickApollo 11 returned highland fragments, first suggestion of Magma oceanIdea since extended to other terrestrial planetsDifferent minerals condense at different timesPyroxene and Olivine sinkPlagioclase-feldspar floatsMoon gains global anorthosetic upper crustThe leftover stuff sandwiched between these layers finally condensesRich in incompatible elements such as potassium (K), rare-Earth elements (REE) and phosphorus (P)Collectively called KREEP – 4.3 GaCrust solidifies, sealing in radiogenic heat, which will become important ~0.5 Gyr laterCore?Magma OceanPYTS 395B – Introduction to the Moon14Crustal Thickness AsymmetryAverage crust 54-62km thick (45km at Apollo sites)Far-side crust is much (about 15km) thicker Crustal asymmetry is one the central questions in lunar scienceA core?Evidence both for and againstAnother basic piece on information we’re not sure ofFinal Internal StructurePYTS 395B – Introduction to the Moon15Bombardment of the Earth-Moon system continuesHeavily cratered lunar highlands formSaturation equilibrium reached i.e. new impacts remove previous cratersSeveral large basins form, which divide lunar stratigraphy into different epochsFracture lithosphere to great depthsExcavate lower crustal material e.g. KREEPLarge Basins FormPYTS 395B – Introduction to the Moon16Late heavy bombardmentSpike in impact rates ~3.8-4 Ga.Probably came from asteroid beltTriggered by migration of giant planets?TimeCratering rateH4.0-3.8 GaPYTS 395B – Introduction to the Moon17Crater saturationLithosphere homogenized to depths of 20kmRegolith generated to depths of 10’s of metersCratering rate declining dramaticallySeveral large basins formed – Aitken basin ~2200 KmWhole Moon has highland appearanceSmall amounts of mare material have appeared but were eliminated by basin impactsRadiogenic heat starting to produce large quantities of magmaThe Moon at 3.8 GaPYTS 395B – Introduction to the Moon18Mare material originates deep in the crustMaria lava fill pre-existing depressions (impact basins)High levels of pyroxene and olivine relative to the upper crustVery similar to terrestrial basaltExcept that it is completely devolatilizedAlso abnormally high in titaniumDarker color due to higher Fe contentAmounts are small…Most Maria 1-2km thick5km in Imbrium, 0.6km in OrientaleIndividual flows ~10-40m thickVERY low viscositySome maria material interacts chemically with the KREEP layer as it risesKnown as KREEP’y mariaMaria erupt mostly