Introducing the Moon!

transcript

A primer on lunar formation and evolutionLillian R. Ostrach

5 June 2012

LROC WAC, RGB = 689, 415, 320 nm [NASA/GSFC/ASU]

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

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

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?

Giant Impact

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)

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?

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).

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

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!

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

FeO MgO

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

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

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

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

Fig. 2.5e, Lunar Sourcebook, after Walker, 198330

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

Introducing the Moon!

Documents