THE OXYGEN ISOTOPE COMPOSITION OF THE SUN

IMPLICATIONS FOR OXYGEN PROCESSING IN MOLECULAR CLOUDS,

STAR-FORMING REGIONS, AND THE SOLAR NEBULA.

THE OXYGEN ISOTOPE COMPOSITION OF THE SUN

IMPLICATIONS FOR OXYGEN PROCESSING IN MOLECULAR CLOUDS,

STAR-FORMING REGIONS, AND THE SOLAR NEBULA.

Trevor R. IrelandPlanetary Science Institute and

Research School of Earth SciencesThe Australian National University

Canberra, Australia

Trevor R. IrelandPlanetary Science Institute and

Research School of Earth SciencesThe Australian National University

Canberra, Australia

IntroductionIntroduction

Nuclear AstrophysicsH-R Diagram

Nuclear compositions sensitive to origin

Isotope CosmochemistrySolar System abundances mixtures of processes

Presolar materialPresolar material

Premisepresolar material will differ from solar system in terms of its isotopic composition

SS is an average

any discrete component will likely deviate from average in one or more isotopic systems

presolar grains or presolar memory?

Chemical MemoryChemical Memory

Refractory Inclusions - earliest Hi-T objects

6 104210.80.6solar

-0.6 -0.4 0.0 0.2 0.4 0.6 0.8 1.0-0.2

C/O

log C/O

800

1000

1200

1400

1600

1800

2000

2200

Condensation Temperature (K)

TiC

Graphite

SiCCorundum

PerovskiteGehleniteSpinelFe

FeAlNCaS

Condensation Sequence(Lodders and Fegley, 1993)

Ptot = 10-5 bars

6 104210.80.6solar

-0.6 -0.4 0.0 0.2 0.4 0.6 0.8 1.0-0.2

C/O

log C/O

800

1000

1200

1400

1600

1800

2000

2200

Condensation Temperature (K)

TiC

Graphite

SiCCorundum

PerovskiteGehleniteSpinelFe

FeAlNCaS

Condensation Sequence(Lodders and Fegley, 1993)

Ptot = 10-5 bars

Formed in solar system

Isotopic AnomaliesIsotopic Anomalies

Prior to 1973Isotopic anomalies only in noble gases, H

Hot homogeneous solar nebula

1973O isotopic anomalies

Apparent enrichment in 16O by up to 4 %

nucleosynthetic component

hot-cold heterogeneous nebula

Fe-group isotope anomaliesFe-group isotope anomalies

Anomalies in n-rich isotopescommonly at 0.1 %

FUN inclusions at 1 %

hibonites at 10%

Solar NebulaSolar Nebula

Nucleosynthetic addition of 16O

Correlated with 50Ti (n-rich isotope) 26Al (halflife = 0.7 Ma)

Supernova injection Trigger for collapse of molecular cloud into solar nebula

Hibonite InclusionsHibonite Inclusions

O isotopic anomalies do not scale with Ti isotopic anomalies

∂50Ti: -70 to +270 ‰

excess 16O: +40 to +60 ‰

no sign of presolar 16O carriers

Presolar SilicatePresolar Silicate

Imaging of IDP slices

TEM

Messenger et al. (2002)0.25 µm forsterite

16O/17O = 44017O: 5 times solar!

16O 17O/16O

Source of Oxygen Isotope AnomaliesSource of Oxygen Isotope Anomalies

Nucleosynthetic (?)

Chemical FractionationMass independent fractionation

Stratospheric oxygen-ozone

Airfall nitrates

Mechanism in solar nebula

AlO + O•, SiO + O•

CO - Predissociation

17O/16O

18O/16O

ozone

oxygen

air

Photochemical FractionationPhotochemical Fractionation

Predissociation of CO in molecular cloud by UV CO + hv ->C• + O•

Outer cloudsufficient photons to dissociate C16O C17O C18O

but progressive self shielding of C16O (99.8

Inner cloud not enough photons to dissociate C16Oenrichment of dissociated 17O• and 18O• 17O• and 18O• react with H (H

2O ice)

residual CO gas 16O enriched

17O/16O

18O/16OdustCO

Originally, and outer cloud

17O/16O

18O/16O

H2O ice

dust

CO Inner cloud

Solar Nebula FractionationSolar Nebula Fractionation

R. N. Clayton (2002)Early solar UV causes predissociation

Dust becomes enriched in 17O, 18O

Dust recycled through disk

• Nebula temperature incompatible with

quantized absorption (?)

Molecular Cloud InheritanceMolecular Cloud Inheritance

Yurimoto and Kuramoto (2004)Oxygen isotope fractionations inherited from the molecular cloud17O and 18O react with dust

altered dust

unreacted dust preserves original composition

refractory dust

Mixing of altered dust and refractory dust

CAI mixing line

The solar oxygen isotope composition

The solar oxygen isotope composition

Key to understanding solar system evolutionPhotochemical predissociation (nebula or MC) predicts Sun should be most 16O enriched

Mass Independent Fractionation suggests solar close to terrestrial

Directly sample solar wind

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Solar Wind on the MoonSolar Wind on the Moon

Solar windsputtering by H, He (98% of solar wind)

isotopic mass fractionation in soil (Si, Ca, K, O...)

implantation

H, He, C, N, O, ...

Lunar soil predominantly silicates and oxidesLook for target mineral with low intrinsic oxygen

metal grains

Oxygen isotopes in metalOxygen isotopes in metal

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A natural Genesis Experimentlittle intrinsic oxygen

surface oxidation?

implanted solar wind O

Target is 50 µm, 70 ng spheruleanalyze 1 ng of Fe

less than 10 pg of oxygen

with variable composition

SHRIMPSHRIMP

Sensitive High Resolution Ion MicroProbe

high surface concentration of oxygenmass fractionated

contamination

exotic oxygen at depth (20 nm +)16O fractionated

SW Oxygen isotopes?SW Oxygen isotopes?

Δ17O = +27 ‰

Δ16O = -53 (±5, 2σ) ‰

Solar or Local Origin?Solar or Local Origin?

IS solar windsolar wind is representative of Sun

primordial nebula composition

solar wind is not representative of Sun or nebula

late stage infall to Sun (nebula water)

IS NOT solar windlocal lunar phenomena (?)

mass independent fractionation or another source

If Solar Composition...If Solar Composition...

What happened?Sun appears inconsistent with any model

Some indications for low 16O in

cometary water

high dust/gas enrichments (Wiens et al. 1999)

Need to explain why Sun is heaviest composition observed

Molecular Cloud Inheritance

Solar Nebula Processing

Molecular Cloud InheritanceMolecular Cloud InheritanceModel of Yurimoto & Kuramoto (2004)

Sun is lightest composition

Gas - Dust Fractionationremove C16O

increases average isotopic weight

but, requires removal of >80% of C16O gas

17O/16O

18O/16O

H2O ice

dust

average

17O/16O

18O/16O

H2O ice

dust

CO

average

Solar Nebula ProcessSolar Nebula Process

Nebula Photochemistryrequires 16O enrichment of most refractory solids

Mass independent fractionationc.f stratospheric oxygen

production of isotopically light reactant

reaction scheme?

Outer solar system watere.g. cometary

but where’s the solar wind?

Earth +5%

Sun

CAI +11%

ConsequencesConsequencesRefractory Inclusions can not be solar condensates

how do (REE) volatility fractionations occur?

consistent with preservation of isotopic anomalies in residues

Planets highly fractionated from Solar Composition

terrestrial planets (very small component of SS)

Cometary sourcewhere’s the solar wind?

Local lunar phenomenonmass independent fractionation occurring in nebula

Synopsis - more Q than ASynopsis - more Q than A

Measurement of solar wind in lunar grains reveals a new oxygen isotopic component in the solar system

The solar composition is apparently the heaviest (most 16O depleted) component yet measured

Difficult to reconcile with any current models of solar system formation-evolution

Analysis of Genesis, Stardust, Hayabusa samples needed to resolve nature of solar wind