Introducing the Moon!

A primer on lunar formation and evolutionLillian R. Ostrach

5 June 2012

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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

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

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

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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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

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

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

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

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

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

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

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

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

Explaining Anorthosite Grains at Apollo 11

Wood et al., 197024

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

Formation of the Earliest Crust, 1

Fig. 2.5c, Lunar Sourcebook26

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

(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

Fig. 12, 13 from Jolliff et al., 200031

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