Planetary Accretion Models and the

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1 Planetary Accretion Models and the Mineralogy of Planetary Interiors DanFrost & DaveRubie Bayerisches Geoinstitut John Hernlund Earth Life Science Institute, Institute of TechnologyTokyo Alessandro Morbidelli &Seth Jacobson Observatoire de la Cote d Azur, Nice, France Rebecca Fischer & Andy Campbell bll University of Chicago

Structure and dynamics of Earth-like planets Chemical State of

of mantle Volatile content/h 2 O Model incorporates: Partitioning of

processes Constrain success of the model by comparison with Earth

2 Structure and dynamics of Earth-like planets Chemical State of Mantle/Core Mantle/Core mass ratio FeO content of mantle Mg/Si ratio of mantle Volatile content/h 2 O Model incorporates: Partitioning of siderophile elements Dynamic/ astrophysical constraints on accretion processes Constrain success of the model by comparison with Earth mantle composition Provide robust indicators for ranges of plausible conditions

3 Heterogeneous Formation of Earth and Moon Reduced Volatile free (FeO poor Fe rich) Moon forming Impact of (FeO rich) CI type material Moon Sulphur rich liquid strips HSE to core (Hadean Matte) Late Veneer of 1% chondritic material Complicated can it really be tested? O Neill (1991) GCA 55, 1159.

4 Homogeneous accretion: Metal segregation at the base of a deep magma ocean Constrain a single set of P,T and fo 2 for core formation (Li & Agee, 1996; Righter & Drake 1997)

5 Model of continuous core formation with step-wise increases in fo in fo 2 (Wade & Wood, 2005)

6 Terrestrial Accretion under oxidising conditions Siebert et al. (2013)

7 "N-body simulations" dynamic system of particles, under gravity embryos ( M e ), planetesimals ( M e ) (e.g. O'Brien et al., 2006; Walsh et al., 2011)

8 Location and composition of final terrestrial planets Combine N body simulations with Core/mantle differentiation for each accretion event

9 Earth depleted in volatiles like all chondrites relative to CI Li Na K F Zn Cs Earth s Mantle Palme (2000)

10 Volatile depletion based on models for condensation FeO H 2 O + Fe > FeO + H 2

11 Preferred composition-distance model in meta al Fraction of ele ement Highly reduced Initial oxidation state of primitive bodies Si Fe/(Fe+FeO) Partially oxidized Oxidized or almost fully oxidized Fully oxidized + 20 wt% water ice Rubie et al Heliocentric distance (AU)

equilibrium compositions of co-existing silicate and metal liquids at high P-T: [(FeO) x (NiO) y (SiO 2 ) z (Mg

12 Mass balance approach to core formation modeling 1) bulk composition of accreting material solar system (CI) ratios of non-volatile elements but with oxygen contents varying over a radial gradient 2) Determine equilibrium compositions of co-existing silicate and metal liquids at high P-T: [(FeO) x (NiO) y (SiO 2 ) z (Mg u Al m Ca n )O] + [Fe a Ni b O c Si d ] silicate liquid metal liquid 3) Maintain a mass balance- no assumed oxygen fugacity

$fraction of metal in the$ al., 2011, EPSL) Fraction of

13 Proportion of a target s mantle/magma ocean that equilibrateswith the impactor's core where Ф is the volume fraction of metal in the metalsilicate mixture (Deguen et al., 2011, EPSL) Fraction of equilibrating mantle: % for planetesimal impacts 2-10% for embryo impacts

14 Metal/Silciate partition coefficents at high P/T Laser heated diamond anvil cell Up to 100 GPa Multianvil

15 Multistage Homogeneous Core formation 2Fe + SiO 2 = Si + 2FeO Metal Silicate Metal Silicate

16 Accretion history of Grand Tack simulation 1.0 SA154_767 Ma ass accreted #4 "Mercury" #5 "Venus" #6 "Earth" #30 "Mars" Time (Myr) Each accretion event (collision) results in an episode of core formation Core formation and the evolving mantle and core chemistry are modelled in all the terrestrial planets simultaneously

17 Homogeneous accretion models

18 Evolution of Earth's mantle composition based on two Evolution of Earth's mantle composition based on two homogeneous accretion models homogeneous accretion models Co (ppm) V (ppm) FeO (wt%) HM-1 HM-2 FeO Mass accreted (Me) Co SiO2 (wt%) HM-1 HM V Mass accreted (Me) HM-1 HM Mass accreted (Me) Nb (ppb) Cr (ppm) SiO HM-1 HM Mass accreted (Me) Nb HM-1 HM Cr HM-1 HM-2 Mass accreted (Me) Mass accreted (Me) Ni (ppm) Ta (ppb) Nb/Ta Ni HM-1 HM Mass accreted (Me) Ta HM-1 HM Nb/Ta HM-1 HM-2 Mass accreted (Me) Mass accreted (Me)

20 H 2 O content of the mantle H 2 O + Fe > FeO + H 2 (Deguen et al., 2011, EPSL)

22 Composition of Venus

23 Composition of Martian Mantle

24 Evolution of a layered structure results of 100 N-body simulations entration (wt.%) Fe conce % of core

25 Summary Combining N-body accretion and core formation models enables the evolution of core and mantle compositions to be modelled H 2 O could have been accreted relatively early Using this approach, the cores of Earth and Venus are predicted to contain 7 wt% silicon and 3 wt% oxygen Partitioning of oxygen and silicon into liquid Fe is enhanced by high temperature and batches of core-forming metal contain high concentrations of these elements during late accretion Development of a density stratification is inevitable in Earthmass planets but may be destroyed by a giant impact The lack of a magnetic field on Venus may indicate a stratified core which has survived due to an absence of late giant impacts

28 Evolution of the concentration of light elements in Earth s core Total co ncentratio on of ligh ht elemen nts (wt%) Mass accreted (M e )

32 Metal/Silciate partition coefficents at high P/T K D( P, T) Met 2 2 Met Silciate X Si X FeO Silicate X X Fe SiO2

37 Evolution of Earth's mantle composition based on preferred composition-distance model ) Co (ppm) V (ppm m) FeO (wt%) FeO Mass accreted (Me) Co SiO2 (wt%) Nb (ppb) SiO Mass accreted (Me) Nb Ni (ppm) ) ) Ta (ppb Ni Mass accreted (Me) Ta Mass accreted (Me) Mass accreted (Me) Mass accreted (Me) V Cr (ppm m) Mass accreted (Me) Mass accreted (Me) Mass accreted (Me) Cr H2O (w wt%) H 2 O Final core composition: 82 wt% Fe, 5 wt% Ni, 9 wt% Si, 3 wt% O, 48 ppm H

40 Evolution of mantle compositions of Mercury and Mars based on preferred composition-distance model Mars Mercury 0.15 Mars Mercury Fe eo (wt%) 10 5 H2 2O (wt%) Mass accreted (Me) Mass accreted (Me)

41 Constraints on core-formation modeling Earth-mantle concentrations of the non-volatile siderophile elements: Fe, Si, Ni, Co, Nb, Ta, V and Cr + Nb/Ta (FeO contents of mantles of Mars & Mercury) 2-5least-squares fitting parameters: 1-4 parameters that define the composition of primitive bodies as a function of their heliocentric distances of origin Metal-silicate equilibration pressure as a fraction of a proto-planets's t CMB pressure (typically ). 06)

42 Cause of oxidation Oxygen fugacities of a solar gas are orders of magnitude more reducing than the intrinsic oxygen fugacities at which the terrestrial planets formed dbut are consistent it twith the region of highly reduced hl d compositions at <1.3 AU postulated here. Thus oxidation is required. Due to the inward net flow of material in the solar nebula, ice- covered dust moves inwards from beyond the snow line. Inside the snow line, water ice sublimes, adding H 2 O to the vapor phase. As temperatures continue to rise and material continues to move inward, H 2 O-rich vapour reacts with Fe-bearing dust, resulting in oxidation. Inward still, vapour is H 2 O poor because the products of sublimed water ice have not mixed all the way to the inner most solar system. Here Fe remains largely free of oxidation.

43 Evolution of the concentration of light elements in Earth s core Total co ncentratio on of ligh ht elemen nts (wt%) Mass accreted (M e )

44 Tested composition-distance models for primitive bodies (a) Fract tion of Fe in metal 1.0 Homogeneous accretion models 0.8 HM HM Reduced chi squared = 72 Fract tion of Fe in metal (b) "Step model" HT-1 Highly reduced Reduced chi squared = 80 Partially oxidized Heliocentric distance (AU) Heliocentric distance (AU) Fract tion of elemen nt in metal (c) Fe Highly reduced Si "Step model" HT-2 Reduced chi squared (d) Fr raction of elem ment in metal 1.0 "Gradient model" HT = Reduced chi squared Partially oxidized Si Fe = Heliocentric distance (AU) (1) (2) Heliocentric distance (AU)

45 Lower mantle seismic observations Karason & van der Hilst 2000

46 A peridotitic lower mantle BSE e.g McDonough & Sun mole % (Mg,Fe)(Al,Si)O 3 Bridgmanite 22 mole % (Mg,Fe)O Ferropericlase 6 mole %CaSiO 3 Perovskite Pressure (GPa) Murakami et al. 2012

47 Brillouin scattering Laser light interacts with phonons and is scattered with Doppler shifted frequency V i = / 2n*sin ( /2) Laser Fabry-Perot interferometer Incident Laser q Scattered Light

48 Diamond anvil cell and Brillouin scattering

$diffraction$

49 Brillouin scattering X ray diffraction lab Brillouin optics

50 MgSiO 3 Bridgmanite- room temperature

51 (Mg 0.96 Fe 0.04 )SiO 3 Bridgmanite- room temperature MgSiO 3 (Mg 0.96 Fe 0.04 )SiO 3 MgSiO 3 (Mg 0.96 Fe 0.04 )SiO 3

52 Ultrasonic measurements at high pressure and temperature Chantel et al. (2012) GRL 39,L19307

53 Thermo-elastic properties of mantle end-members Thermo-elastic paramaters of the mantle phasesused for calculating the sound wave velocities and densities as a function of pressure and temperature in the transition zone and lower mantle. V Phase Formula 0 K T0 (cm 3 K' G /mol) (GPa) T0 q 0 o G (GPa) 0 ' Wadsleyite Mg 2 SiO Wadsleyite Fe 2 SiO Ringwoodite Mg 2 SiO Ringwoodite Fe 2 SiO Ca- Perovskite CaSiO Stishovite SiO Perovskite MgSiO Perovskite FeSiO Perovskite FeAlO Perovskite AlAlO Periclase MgO Wüstite FeO values in italics are taken from Stixrude and Lithgow-Bertelloni (2011); values for perovskite are from Boffa Ballaran et al. (2012); Chantel et al. (2012); Kurnosov et al. (in preparation) s0

54 A peridotitic lower mantle 72 mole % (Mg,Fe)(Al,Si)O 3 Bridgemanite 22 mole % (Mg,Fe)O Ferropericlase 6 mole %CaSiO 3 Perovskite Irifune et al. 2010

55 Fe and Mg exchchange between Bridgmanite and ferropericlase Irifune et al. 2010

56 Mineral-physics model for a BSE deep mantle

57 Influence of Fe-Mg exchange Murakami et al. 2012

58 High ferric iron in Bridgmanite Fe3+/TFe Irifune et al. 2010

59 Fe 3+ content of Bridgmanite in equilibrium with Fe metal

60 FeO Disproportionation constant BSE bulk oxygen 3FeO = Fe 0 + Fe 2 O 3 <3 %Fe 3+ /ΣFe ~40 % Fe 3+ /ΣFe

61 Mineral physics model with constant BSE bulk oxygen

62 Summary - This model does not include Al in perovskite- however previous ultrasonic measurments indicate a limited influence which is currently being tested with Brillouin measurments. - Vs and Vp estimated t for a BSE composition mineral assemblage in the top 1000 km of the lower mantle match seimsic observations. - Seismic properties of LLSVPs do not match Fe enrichment in a perovskite dominated mineral assemblage.

63 Conclusions Combining N-body accretion and core formation modeling provides new constraints on both processes. Accretion of Earth and Venus was heterogeneous For Earth, Mars and Venus, excellent results are obtained when embryos and planetesimals that form close to the Sun, at <1 1.5 AU, are highly reduced and bodies that form further from the Sun are partially to fully oxidized. Other composition distance models fail badly. H 2 O bearing bodies in the SA154_767 model originate at >8.5 AU and result in ~1000 ppm H 2 O in the mantle Results are based on impactor cores equilibrating completely, but with a very limited fraction of a proto planet s mantle, at average pressures of 50 60% of CMB pressures.

67 Accretion, heating & metal delivery by impacts Core formation and accretion are multistage processes that cannot be separated

68 Grand Tack accretion model Classical models (e.g. O'Brien et al 2006) consistently result in a model Mars that is too massive The Grand Tack model (Walsh et al., 2011) is based on the inward and then outward migration of Jupiter and Saturn that truncates the planetesimal disk at ~1 AU and results in a realistically ll small mass for Mars.

69 Modeling gplanetary accretion Late stage accretion of terrestrial planets is modeled using "N-body simulations". The "Grand Tack" model (Walsh et al., 2011) has been especially successful in reproducing the small size of Mars. Start with ~40 embryos (0.07 M e ) and ~1500 planetesimals l ( M e ), initially dispersed between 0.7 AU and 13 AU, and collide/accrete to form larger bodies. Combine core mantle differentiation with N body accretion models

70 Accretion history of Grand Tack simulation SA154_ SA154_767 Mass s accreted #4 "Mercury" #5 "Venus" #6 "Earth" #30 "Mars" Ti (M ) Time (Myr) Here we consider that each accretion event (collision) results in an episode of core formation and we thus model core formation and evolving mantle and core chemistry in all the terrestrial planets simultaneously

71 Model for the influence of a spin transition in ferropericlase Wu et al. 2013

Chemical Composition of the Lower Mantle: Constraints from Elasticity. Motohiko Murakami Tohoku University

Chemical Composition of the Lower Mantle: Constraints from Elasticity Motohiko Murakami Tohoku University Acknowledgements Jay D. Bass (University of Illinois) Stanislav V. Sinogeikin (University of Illinois)