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Introducing the Moon!
A primer on lunar formation and evolutionLillian R. Ostrach
5 June 2012
A17
Se
T
LROC WAC, RGB = 689, 415, 320 nm [NASA/GSFC/ASU]
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Earth MoonSystem
• Formation of Moon may have resulted in Earth’s axial tilt of 23.5°
• Moon stabilizes Earth’s axial tilt and thus stabilizes climate (Mars’ tilt varies 0-60°)
• Moon may have enhanced early melting and differentiation of Earth (much closer)
• Raises tides in oceans – influence on life?• Lunar recession is lengthening our day over
time• Moon has influenced mythology, religion,
arts (music, painting, writing)
Earth 12,756 km Moon 3476 km
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The Moon
• Day 27.3 Earth Days• “Year” 27.3 Earth Days• Near Side• Far Side• Dark Side• Diameter ~1/4 Earth’s• Gravity 1/6 Earth’s• Moon’s mass 1% that of the Earth’s• Earth to Moon 384,400 km (230,640
miles)• Axis tilted ~1.5°• Surface equivalent to area of Africa
Earth 12,756 km Moon 3476 km
And we’ve been there in person9 times, 6 times on the surface! 3
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Origin of the Moon:Classic Theories
• Co-accretion– Earth and Moon formed
together from the nebula• Capture
– Moon formed elsewhere, then captured by Earth’s gravity
• Fission– Earth rotation so fast that a
portion of Earth was thrown off
• Untestable hypotheses until lunar samples were returned
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What do We Know?
• Angular momentum of Earth-Moon system very high– Moon’s orbit is not in the Earth’s plane of rotation– Moon’s spin axis differs grossly than the Earth’s
• Bulk composition– 3.3 g/cc vs 5.5 g/cc (compressed)– Moon must be depleted in Fe– O isotopes between Earth Moon identical– Moon extremely depleted in volatiles
• Not completely, Mn• What about CO and CO2?
– Moon enriched in refractory* elements, or is it?
5*refractory elements = vaporizes or condenses at high temperatures; ex: Ca, Al, Ti
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Testing the ModelsPost Apollo
Moon: low density, O isotopes identical to Earth, depletedin volatiles, depleted in some siderophiles (Ni, Co), highangular momentum
So how did the Moon form?
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Giant Impact
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Giant Impact Hypothesis
• Moon formed as Mars sized bolide hit proto-Earth
• Core of bolide became part of Earth, depleting proto-Moon of metallic iron
• Mantle and crust of bolide and part of Earth’s crust vaporized and went into orbit around Earth (lighter elements, volatiles boiled off into space)
• Consistent with Moon’s orbital configuration
• What about O isotopes? Bolide needed to form in nearly same orbit
• Density (5.5 vs. 3.3 g/cc)
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Problem Solved?
Giant Impact hypothesis has been around for >35 years…We need more samples!
• Moon does/did have some volatiles (vesicles, pyroclastic materials)
• Identical O isotopes (same place in Solar System)
• Small metallic core of Moon explained• How well do samples represent the
whole Moon? – Six Apollo locations all on central
nearside (381.7 kg), three robotic Soviet locations (321 g) also nearside
• What about refractory elements?• How well do we know the Moon? What
do we need to do? Can we learn about Earth from further study of the Moon?
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Breaking news! Titanium Isotopes Identicial for Earth and Moon!
• Titanium isotopes vary in other samples
• From impact hypothesis, would not expect isotopic ratios to be the same…equilibration?
• Models estimate 100 – 1000 years for lunar formation after impact
• New results do not disprove impact hypothesis!
• Another tool for continued testing of lunar formation by giant impact
http://www.psrd.hawaii.edu/May12/Ti-isotopes-EarthMoon.html 10
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How well do we know the Moon’s bulk composition?
• Not all that well!• Recent papers giving very
different results in terms of bulk lunar Al2O3 (Longhi, 2006; Taylor et al, 2006)
• Longhi: Moon is not refractory enriched
• Taylor et al: The Moon is refractory enriched
• Both valid results from very different assumptions
http://www.psrd.hawaii.edu/April07/Moon2Views.html
Range of bulk lunar Al2O3 found in variousstudies (diagram from J. Taylor).
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Se
T
C
F S
Mv
M
A
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Highlands (Terrae)• Heavily cratered, thus overturned and mixed to 1
km or more• Ancient ‘flotation’ crust• Magma Ocean (Eu)
– Anorthite floated– Olivines, pyroxenes sunk
• Anorthosites old as 4.5 Ga• Mg-Suite Highlands Rocks until 4.3 Ga (ANT)
– Anorthosite: Anorthite (An)– Troctolite: An + Olivine+ Pyroxene– Norite: An + Pyroxene
• KREEPy highlands rocks 4.35 Ga - end of primary crust formation
• KREEPy basalt 3.85 Ga (Ap 15 samples of Apennine Bench Formation)
• Does the bulk crust composition give insight to bulk lunar composition?
KREEP = dregs of magma ocean
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Lunar samples: Apollo 15
Genesis Rock, coarsely crystalline anorthosite dated at 4.1 derived locally (not 4.4 Ga, but probably formed at that time)
Apollo 15 “Genesis Rock”, #15415
Time to do SCIENCE on the Moon!
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Volcanic PlainsMare Basalts (Dark Areas)
• Formed after crust (bright areas) and most big impacts
• Erupted in vast quantities as a very fluid magma
• Flooded pre-existing topographic lows (craters) forming smooth plains
• Cover about 16% of lunar surface• Very similar to basalts on the Earth
(Deccan traps in India 65 Ma), watch basaltic rocks forming in Hawaii and Iceland
• Ages range from 3.1 to 3.8 Ga, some small fragments as old as 4.3 Ga Basaltic eruption in Hawaii,
mixed pyroclastic and effusive15
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Mare Samples
• Mare basalt samples 3.1 to 3.8 Ga (but perhaps some much younger, 1-2 Ga???)
• 100s to >4000 m thick• Nearly devoid of H2O, very
depleted in other volatiles• Some >10 wt % TiO2
– Ti and O “resources”• Refractory inventory (where is the
Al? Lots of Ti)• Complex mantle! Apollo 11 Basalt
How do the mare fit into magma ocean story? (just wait a bit…) 16
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Volcanic Beads
• Emplaced in explosive eruptions - fire fountains
• Many varieties, giving distinctive colors, some glasses, some crystalline
• Volcanic beads - important for volatile history (some do/did exist!); probably CO2 CO main gases, traces
of Zn, S, Pb...small amounts H2O
• Ap 15 green glass, Ap 17 orange glass...
Shorty Crater Apollo 17 17
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Lunar Mineralogy – The Basics
• Minerals are keys to understanding lunar rocks - compositions and atomic structures reflect formation conditions
• Lunar minerals are (essentially) anhydrous – ~ no water, no hydroxyl, no H!*
• Lunar minerals mostly formed at low pressure
• Lunar minerals formed under low oxygen fugacity (i.e., reducing conditions)
• Iron present as Fe2+ or Fe0
• Ti present as Ti4+ or Ti3+
• Cr present as Cr3+ or Cr2+
Terrestrial volcanics
Lunar Basalts
Slide courtesy of Brad Jolliff18
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Primary silicate minerals – just 3!
• Olivine (Mg,Fe)2SiO4 (ss)
forsterite Mg2SiO4
fayalite Fe2SiO4
• Pyroxene (Mg,Fe,Ca)2SiO6 (lmt’d ss)
orthopyroxene (Mg,Fe)2Si2O6 (enstatite, ferrosilite)
clinopyroxene (Ca,Mg,Fe)2Si2O6 (pigeonite, augite)
• Plagioclase NaAlSi3O8 - CaAl2Si2O8 (ss)
anorthite CaSi2Al2O8
bytownite (Ca1-x,Nax)Al2-xSi2+xO8
Slide courtesy of Brad Jolliff19
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FeO MgO
TiO2
MgTi2O5
MgTiO3
Mg2TiO4
FeTi2O5
FeTiO3
Fe2TiO4
Armalcolite
In rocks w/ilmenite and armalcolite, armalcolite appears to have crystallized early, then reacted to form ilmenite
Textures indicate ulvöspinel also reacts (subsolidus reduction) to form ilmenite.
Rutile: TiO2 Tetrag.
Armalcolite: (Fe,Mg)Ti2O5 Orthorh.
Ilmenite: FeTiO3 Hexag.
Ulvöspinel: Fe2TiO4 Isom. (10 vol.% of some basalts; also contain Cr, Al, Mn, V)
Main Ti-bearing Oxides on the Moon
Slide courtesy of Brad Jolliff20
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KREEP and late-stage lunar minerals
• Potassium (K), Rare Earth Elements (REE), and Phosphorous (P)• Elements that are excluded from the major rock-forming minerals• Late-stage assemblages - rich in phosphates and K-feldspar (indicators!)• Also elements such as Ba, Rb, Cs, Zr, Hf, Nb, Ta, U, and Th• Formed as residue of low-pressure magma crystallization (last stuff)
Incompatible elements: Cations are large and/or highly charged, don’t fit well into crystallographic sites occupied by Fe, Mg, and Ca – cause distortion if allowed
Slide courtesy of Brad Jolliff21
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Quick Peek: Lunar Differentiation & Magma Ocean
• Eu Anomaly (REE see periodic table)• Eu: Same size and charge as Ca so it
substitutes easily in anorthite (feldspar)
• Early lunar crust enriched in Eu, later basalts deficient
Bottom Line: Came from same source -MAGMA OCEAN
We can ‘see’ that basalts are younger22
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Apollo 11 Soil
• Landing site in lunar maria (where?)
• Diverse components Dark: Basalt (volcanic) Light: Plagioclase-rich Breccias (mixed rocks) Glasses:
• Volcanic• Impact
• How did the anorthosite get there?
Fig. 1, Wood et al., 197023
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Explaining Anorthosite Grains at Apollo 11
Wood et al., 197024
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Magma Ocean Theory: The Basics
• As magma ocean cools, anorthosite floats, olivine, ilmenite, and pyroxene sink
• Early crust enriched in Ca and Al, depleted in Fe and Mg
• Secondary crust formation in terms of intrusions and extrusions of denser Fe and Mg rich magmas
• Is the magma ocean, flotation crust, dense minerals sinking, etc., a done deal?
http://www.psrd.hawaii.edu/
Magma ocean, magma seas, or something else?25
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Formation of the Earliest Crust, 1
Fig. 2.5c, Lunar Sourcebook26
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Formation of the Earliest Crust
Slide courtesy of Brad Jolliff
What is going on?
1) Large-scale convection of MO? *overturn!* at least locally (Fe/Ti-rich mins, KREEPy stuff)
2) Partial melting near base of “crust” intrusions
3) KREEPy pockets near base of “crust”, mixed with intrusions
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Time to Solidify an Ocean of Magma
72215: impact melt breccia; high incompatibles – late-stage crystallization
A single zircon in 72215 shows a range of ages: oldest cluster dated at 4.417 Ga
Suggests that MO significantly crystallized by this time
28http://www.psrd.hawaii.edu/Mar09/magmaOceanSolidification.html
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(Some) Problems with the Magma Ocean Hypothesis
Problem: energy source to melt Moon, create global magma oceanSolution: rapid accretion (giant impact); otherwise – no realistic clue
Problem: “layer cake” cumulate pile indicates that mare basalt types are related to depth (but exps show deep source >300 km); similarity in Mg# among VLT-low-high Ti-basaltsSolution: large-scale (global?) overturn resulting from gravitational instabilities in the cumulate pile (more dense rocks overlying less dense) “well-stirred” LMO with mixing (heterogeneous mantle!)
Problem: overlapping ages of FAN and Mg-suite rocks AND different trace elemental abundancesSolution: LMO probably mostly crystallized when Mg-suite rocks formed but last pockets of FAN melt still present; the two rocks not simply related/from the same parent magma
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Fig. 2.5e, Lunar Sourcebook, after Walker, 198330
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Fig. 12, 13 from Jolliff et al., 200031
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