History of the Lunar Interior Surface Activity Today ASTRO GEOL 5835 October 24th 2006 Kasandra Jorgensen Bonnie Meinke Margaret Mitter Kennda Lynch Overview Composition and Structure of Lunar Interior z Does The Moon Have a Core z Thermal History and Evolution z Surface Activity Today z Composition and Structure of Lunar Interior Observations and Experiments of Lunar Interior z Orbital Observations z z z Manned Missions Apollo z z z Lunar Prospector Clementine In Situ Experimentation Sample Return over 800 lbs of lunar material returned to earth Meteorites Lunar Crust z Crust z z z z z 10 of Moon Volume 20 110 km thick Average thickness 61 km Thickness differentially distributed towards Far side Interesting Factoid The South Pole Aitken Basin on Far side is thinner than nearside average Bulk Composition of Lunar Crust z Upper Crust z z z 40 of upper crust is pure Anorthosite 90 plagioclase 90 of Upper crust contains more than 80 Plagioclase Lower Crust z z z z More mafic than upper crust and lacks pure anorthosite 33 Norite and Gabbo Norite 30 Anorthositic Norite Lower crust composition supports theory of formation by cumulate floatation in a magma ocean The Mantle z No samples of Lunar Mantle have been unambiguously identified to date Therefore mantle composition studied by less direct means z Isotopic analysis of Mare Basalts and Volcanic Glasses z Seismology Lunar Seismic Experiments z Manned Landers Apollo Mission z z z 4 Seismic Stations 12 14 15 16 operated concurrently Data collection from 19721977 More than 12000 Events Recorded z z z z Deep Moonquakes Shallow Moonquakes Meteoroid Impacts Thermal Moonquakes P Wave Velocity Profile of Lunar Mantle Khan et al 2000 Bulk Composition of Lunar Interior z Mantle z 90 of Moon s Volume z Upper Mantle z 500 km thick z Composed predominantly of orthopyroxene z Smaller abundances of olivine clinopyroxene plagioclase and garnet z Lower Mantle z Predominantly Olivine z Some evidence that deep mantle below 1000 km is Molten First Signs of a Core Magnetometers on Apollo 15 and 16 find an induced magnetic moment that implies a core of radius 439 22km z Apollo missions installed seismometers z z One very weak detection puts an upper limit if a core exists at 360km Lunar Laser Ranging z z LLR gives precise distance measurements that are sensitive to lunar rotation and orientation Dissipation due to relative motion at a fluid core solid mantle boundary could be detected Dickey et al 1994 Williams et al 2001 Results from LLR z z z LLR gives a core radius of 335km if Fe Implies a mass of 13 of total Lunar mass LLR results require a liquid core or shell Other Evidence for a Core z The gravity field of the moon z gives R 220 450km if Fe 330 590km if FeS Magnetization of lunar rocks z Love number k2 is larger than expected by 20 z z implies a liquid core Other Evidence for a Core z The gravity field of the moon z gives R 220 450km if Fe 330 590km if FeS Magnetization of lunar rocks z Love number k2 is larger than expected by 20 z z implies a liquid core Core Composition Metallic z Fe or Fe alloys z Adding S and Ni will lower density of core and the melting point z z From z 1600 C to 940 C S would be 4 13 by mass Possibility for an Inner Solid Core z z If the core cooled below 940 C the Fe would precipitate out Resulting in a outer liquid shell 65 180km thick and an inner solid Fe core of 170285km z z No direct evidence for inner solid core Dissipation from differential rotation at liquid solid core boundary could be detected Core Formation z z z z If moon was all molten differentiation easily explains metal core If not entirely molten more difficult to from core Formed 500 My after formation of moon itself Core formation tied to origin of moon What s Next More accurate LLR to determine presence of solid inner core z Absolute verification of lunar core z z SELENE z LUNAR A Japan Thermal History and Evolution Overview z Initial thermal state z Heat source z Evidence Differentiation z Mantle and lithosphere evolution z Subsequent magmatism volcanism z Constraints from s c data and models z Clementine and Lunar Prospector gravity and topography data z z Impact related redistribution of crustal material Lateral heterogeneities in early melting and differentiation z P wave model z z z Compositionally homogeneous upper mantle Compositionally heterogeneous middle mantle Apollo 15 heat flow measurement 33 5 erg cm 2 s 1 caution as using this for entire moon Initial Thermal State of Moon Initially hot Heat source z z z Impact origin provided heat Interior heated quickly after accretion Melted to depth of 200km Magma ocean shallower than 100km and deeper than 300km discounted from models z Other mechanisms for heat z z z z Tidal heating Radioactive isotopes such as 26Al Electromagnetic heating Giant impact accretional heat currently best explanation Evidence for Initial Thermal State of Moon z z Pattern of trace element concentrations in lunar plagioclase suggests that lunar interior differentiated by 4 4 Ga when large part if not all of interior was molten Stable remnant magnetization of lunar rock suggests that a field of possibly 1 G existed at 4 Ga decreasing significantly by 3 2 Ga dynamo in iron rich lunar core Formation of mascons and tectonics of lunar highlands Differentiation z z Happened early first 1 5 Gyr Period of major igneous activity and differentiation Convection played important role produces lateral heterogeneities 3 major distinctive reservoirs on Moon as evidence of early differentiation 1 2 3 urKREEP residuum of a global lunar magma ocean w high 238U 204Pb Primordial deep mantle source May have melted to for the low Ti basalts Shallower mantle reservoir Involved in early processing and remelting in a lunar magma ocean and contaminated by late meteoritic infall Gave rise to high Ti basalts Heat flow from core z Flow decreases substantially during first billion years until core cools conductively Mantle Evolution z First 2 Gyr hot upwelling plumes from lower mantle cause pressure release partial melting in mid to upper mantle Volcanism Magmatism z pre mare volcanism obscured by subsequent events z z High Al basalts are oldest dated volcanic material 4 19 4 29 Ga Oldest unequivocal basalts from Apollo 11 site initiated intense phase of active global volcanism z z z Original crust differentiated before formation of KREEP basalts w in first few Myr Igneous activity resulted in filling of mare basins 60 km thick feldspathic crust