Introduction to the Moon PYTS 395B Introduction to the Moon Reading Papers Presenting Topics Class Discussion What Topics 2 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 MERCURY PYTS 395B Introduction to the Moon 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 day 3 PYTS 395B Introduction to the Moon Earth and Moon compared A small terrestrial planet 4 PYTS 395B Introduction to the Moon Introduction to the Moon 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 5 PYTS 395B Introduction to the Moon 6 Morphology changes as craters get bigger Pit Bowl Shape Central Peak Central Peak Ring Multi ring Basin 10 microns Moltke 1km Orientale 970km Euler 28km Schr dinger 320km PYTS 395B Introduction to the Moon Regolith Rock Blanket Produced by impacts Modified by space weathering Ida and Dactyl Galileo mission Tycho crater Bright Rays imply youth 7 PYTS 395B Introduction to the Moon A very tenuous atmosphere Molecules on ballistic trajectories Trapping in the permanent shadows in lunar craters 8 PYTS 395B Introduction to the Moon 9 History of the Moon 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 volatiles Surface activity on the Moon and Mercury mostly died off about 3 Ga 0 27 RE 1 00 AU 0 38 RE 0 39 AU as opposed to Surface history of Venus is only available from 1 0 Ga onward Surface activity and history of Mars spans its entire existence PYTS 395B Introduction to the Moon 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 today 10 PYTS 395B Introduction to the Moon Formation of the Moon Previous theories Apollo results and common sense Co accretion Fission of spinning Earth Capture of rogue planetisimal 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 AU From Robin Canup SWRI Boulder 11 PYTS 395B Introduction to the Moon Bulk composition and orbital state 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 volatiles From 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 slows 12 PYTS 395B Introduction to the Moon Magma Ocean 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 later Core 13 PYTS 395B Introduction to the Moon Final Internal Structure 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 of 14 PYTS 395B Introduction to the Moon Large Basins Form 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 KREEP 15 PYTS 395B Introduction to the Moon Late heavy bombardment Spike in impact rates 3 8 4 Ga Probably came from asteroid belt Triggered by migration of giant planets Cratering rate 4 0 3 8 Ga H Time 16 PYTS 395B Introduction to the Moon The Moon at 3 8 Ga 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 magma 17 PYTS 395B Introduction to the Moon Formation of the Maria 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 Darker color due to higher Fe content Amounts are small Except that it is completely devolatilized Also abnormally high in titanium 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 during the Imbrian period 3 8 3 1 Ga A little late Mare formation into the Eratosthenian period but not much 18 PYTS 395B Introduction to the Moon Evolution of the Maria Maria lava fill pre existing depressions impact basins Maria does not reach surface on the far side due to the thicker crust Cryptomaria some maria can be buried Very smooth on scales of 100 s of meters Weight of maria material causes subsidence Compression wrinkle ridges in the center Extension graben at the edges Edges of the Maria remain sharp Little lateral mixing from impacts 19 PYTS 395B Introduction to the