Planetary accretion and core segregation: Processes and chromnology Asteroid belt Earth-Moon Venus Mercury Mars

Solar system: general zoning from metal and silicates to gas and ice Terrestrial planets: metal and rock Gas giants: H, He Ice giants: ice + gas Kuiper belt: ice Compression effect

Composition of stellar and planetary materials Solar abundance of the elements Earth and planetary redox conditions

Terrestrial planets Mercury, Venus, Earth, Moon, Mars, Vesta Mantle / core -ratio ~ O/Fe-ratio of accretion zone Cosmic O/Fe-ratio: Insufficient to oxidize all Fe Simple, first-order structure Mantle: Rock (silicates and oxides) Core: Fe-Ni-metal

Terrestrial planets: sizes, pressures, core fractions

Planetary accretion Variable f O2 and f H2 give variable FeO/Fe-ratios, i.e. variable core fraction Intermediate f O2 Large f O2 Low f O2 High FeO mantle 21% in Vesta Interm. FeO mantle 8% in Earth Low FeO mantle 3% in Mercury 30 wt%65 wt% 16 wt%

Mantle FeO (wt%) Mercury: 3 Venus: 7 Earth: 8 Moon: 12 (Theia: >12 ?) Mars: 16 Vesta: 18 Melting of planetary mantles  basalt volcanism FeO mantle / FeO basalt ≈ 1 How do we know FeO in the mantle(s) ?

FeO mantle - core fraction

Oxidation state recorded by core-mantle ”bulk equilibrium” O Fe = 2 FeO (I ron - W ustite buffer reaction) in core in mantle K = a FeO 2 / (f O2 * a Fe 2 ) = 1 (at IW) f O2 = a FeO 2 / a Fe 2 = (a FeO / a Fe ) 2 log f O2 (IW) = 2 * log (a FeO /a Fe ) X FeO log f O2 mantle IW 0.90 – 0.85 e.g Observations: - Solar nebula zoning with increasing f O2 outwards? - Mercury’s large core: caused mostly by low f O2 ? - Moon’s tiny core – Moon created mainly from mantles of Theia and Earth under oxidizing conditions (Mantle FeO : Moon 12%, Earth: 8%) Mercury: 0.03  2.9 Venus: 0.07  2.2 Earth: 0.08  2.1 Moon: 0.12  1.7 Mars: 0.16  1.5 Vesta: 0.18  1.4

Exsolution of tiny amounts of kamacite Garrick-Bethel and Weiss (2010, EPSL) McSween (1999, Cambridge Univ. Press) Large amounts of kamacite – small amounts of taenite

drop of troilite (FeS) Pallasite (metal + olivine): piece of CMB-material

Asteroid belt: main source of meteorites Mars Ceres Vesta Pallas Beatty et al. (1999, Cambridge Univ. Press)

Asteroid belt: mass distruibution Asteroid belt, total mass: kg very small mass relative to the planets, only 4% of Moon, 0.05% of Earth 47% of the total mass in the 3 largest asteroids Ceres: 31% of asteroid belt Vesta: 9% Pallas: 7 % Ceres Pallas Vesta Kirkwood gaps, Jupiter resonances

7 Iris 3 Juno 6 Hebe 8 Flora 15 Eunomia532 Herculina Carbonaceous chond.: C + BFGD - outer belt Ordinary chond.: S - inner belt HED: Vesta, Vestoids: V +R Metal, Enstatite: M +E 1 Ceres 10 Hygiea 2 Pallas (B) 250 Bettina 44 Nysa (E) Asteroid spectral groups: link to meteorite types Mars 4 Vesta 349 Dembowska (R) 16 Psyche 433 Eros

7 Iris Juno Hebe: H, IIE Flora family: L, 480 Ma Eunomia Herculina Ceres Hygiea Pallas (B) Bettina Nysa Mars Dembowska (R) Psyche Eros Vesta family: HED Baptistina family: CM, 160 Ma Hungaria family (3103 Eger): aubrites Carbonaceous chond.: C-type (+ B F G D) - outer belt Ordinary chond.: S-type - inner belt HED: Vesta, Vestoids: V-type (+ R) Metal, Enstatite: M-type (+ E) Asteroid spectral groups: link to meteorite types

Primitive solar syst. material IDP (interplanetary dust part.) chondrites (esp. carbonaceous chondr.) Material samples Differentiated samples Meteorites Vesta (HED - howardites, eucrites, diogenites) Other asteroids (Fe-met., angrites, aubrites, etc.) Mars (SNC-met.) Moon (basalt, anorthosite, breccia) Planetary samples Earth Moon (Apollo/Luna missions)

Differentiated meteorites Lunar breccia Shergottite (basalt) Fe-meteorite Diogenite – Vesta (HED-group)

evaporation and condensation of dust  CAI flash melting and solidification  chondrules Refractory inclusions and chondrules Microscope images of chondrules Strong magnetic fields from early Sun (T-Tauri phase)

Infrared image, dust and gas disk around  -pictoris Planetary accretion (theoretical modelling) Dust accretion by settling and sticking: < years Runaway growth (gravitational): < 0.5 Ma Late stage, giant collisions: 0.5 – 100 Ma Planetesimals, km Planets Planetary embryos km

Walsh et al. 2011, Nature Saturn 0 Ma 0.3 Ma 0.07 Ma 0.1 Ma 0.6 Ma 0.5 Ma Jupiter Uranus 0.5 Ma 0.1 Ma (Heliocentric distance) Earth The ”grand tack” mod el (the Nice model) - very early formation of gas giants, Jupiter and Saturn, - inward migration of first Jupiter and then Saturn (gas drag effect) - then outward migration of both Jupiter and Saturn Inner terr. planets

Cumulative result of 8 terrestrial planet simulations Mercury Venus Earth Mars Walsh et al. 2011, Nature Start: 80 embryos of M Ea Start: 40 embryos of M Ea Horizontal error bars: perihelion–aphelion excursion of each planet along their orbits

Giant collision stage: large energy -production 1) Short-lived radioactivity: 26 Al, 60 Fe 2) Collisional heat 3) Core segregation (also exothermic process) 4) Tidal friction (important for the early Earth-Moon system)

Moon formation: giant impact strongly favoured - Moon’s tiny core - Moon’s volatile depletion - Earth-Moon chemical features - Earth-Moon dynamic coupling Details by Diego Gonzales, on Wednesday Results from the Apollo-program mapping and sampling Recognition of silicate magma ocean crystallization for early planetary differentiation

Crystallization of the lunar magma ocean Crust - upper mantle stratigrahy

Geochemical signs of cumulate formation in the lunar magma ocean Mare Imbrium Oceanius Procellarium M. Serenitatis M. Tranquilitatis Mare basalt volcanism was most intense at 3.8  3.6 Ga Caused by gravitational instabilities and turn-over in the magma ocean cumulates (?) High-density cpx-ilm cumulates over more common peridotites

Source regions for mare basalts KREEP-basalts - low Hf/W High-Ti basalts - high Hf/W Low-Ti basalts - intermediate Hf/W Crust – upper mantle stratigrahy McSween (1999, Cambridge U. Press) High-Ti Low-Ti Anorthosites, pos. Eu-anomalies Sufficiently low f O 2 for Eu 2+. Eu 2+ is similar to Sr 2+ and both fits nicely into the Ca-position of plagioclase

Important heat source: the first 2-3 Ma

Chronology – radiometric U-Pb isotope systematics 235 U → 207 Pb, T h : 0.7 Ga 238 U → 206 Pb, T h : 4.5 Ga 206 Pb / 206 Pb 207 Pb / 204 Pb Earth’s initial composition  = U/Pb  =8  =9  =10 1 Ga 2 Ga 3 Ga Geochron (0 Ga)

The concordia diagram (Pb*: radiogenic lead) - suitable for dating minerals that crystallize without Pb, but much U (e.g. zircon) Partial Pb-loss when zircon is 2.5 Ga old Further 1 Ga Pb-growth, until zircons are 3.5 Ga old

Satellite-image (175 km width) covering parts of the Pilbara craton, NW Australia. Such cratons typically contain granitic domes (here: Ga) emplaced between dark greenstone belts comprising altered basaltic og komatiitic lavas (here: Ga) SEM-image of zircon crystals from a quartsite at Jack Hills, Pilbara craton, Western Australia. These are the oldest zircons found on Earth (ages in Ma) Oldest zircon crystals on Earth, dated by U/Pb-geochronology

evaporation and condensation of dust  CAI flash melting and solidification  chondrules Refractory inclusions and chondrules Microscope images of chondrules Strong magnetic fields from early Sun (T-Tauri phase)

Amelin & Ireland (2013, Elements)

Age of meteorites and the Earth Mostly established already in 1956 !! Correct age, within the stated uncertainty

Amelin et al. 2002, Science Dating refractory inclusions and chondrules t 0 = 4567 Ma Chondrules: t = t Ma CV chondrite CR chondrite

Bouvier et al. (2010, Nature Geoscience) Assumed constant (solar) 238 U/ 235 U-ratio Northwest Africa 2364 CV3 chondrite

Connelly et al. (2012, Science): Used measured (variable) 238 U/ 235 U-ratio Amelin and Ireland (2013, Elements) CAIs: ± 0.16 Ma Chondrules: 4567  4564 Ma

Short-lived isotope decay Important early heat producers: 26 Al → 26 Mg, T h : 0.7 Ma 60 Fe → 60 Ni, T h : 1.5 Ma Others with low concentration or low initial ratios: 41 Ca → 41 K, T h : 0.1 Ma 10 Be → 10 B, T h : 1.5 Ma 182 Hf → 182 W, T h : 8.9 Ma 146 Sm → 142 Nd, T h : 103 Ma  26 Mg = [( 26 Mg/ 24 Mg) sample/stand -1] * 1000‰  182W = [( 182 W/ 184 W) sample/stand -1] * 10 4  26 Mg-excess in Al-rich minerals (plagioclase) formed early  f: lithophile (”silicate-loving”) W: siderophile (”iron-loving”) W Hf Mantle: high Hf/W  high  W mantle Core: low Hf/W  low  W core ǀ  W ǀ (the  W -deviation from zero): inversely proportional to the age of core-segregation Hf/W-ratio must be known or estimated for a good age determination

Giant collisions: largely molten planetary embryos and planets with segregated cores Extent of core-mante re-equilibration Two extremes: Minimal re-equil.: Impactor core merges directly into big core Extensive re-equil.: Impactor core emulsifies in silicate magma ocean

The Mars case (Dauphas and Pourmand, 2011, Nature) Mars mantle:  W Ma-ma ≈ 0.4 (at least) Hf/W-ratio of SNC-meteorites is disturbed by partial melting and fractional crystallization processes However, the Hf/W-ratio can be estimated from: (Hf/W) Ma-ma = (Th/W) Ma-ma / (Th/Hf) Ma-ma (Th/W) shergottites ≈ (Th/W) naklites = 0.75 ± 0.09 (independent of mineral proportions) i.e. insignificant fractionation of Th/W (Th/Hf) Ma-ma ≈ (Th/Hf) CHUR because both Th and Hf are refractory lithophile elements proxy for Lu/Hf: (Th/Hf) CHUR

Growth curve, Mars 95% conf. interval, Monte Carlo simulation of uncertainties Model simulations of embryo growth at 1 and 3.2 AU (Mars: 1.5 AU) M(3 Ma) / M final = 80% Mars can be considered a planetary embryo! Growth curve, Mars

182 Hf W chronology (T h : 9 Ma) - measuring small metal grains in mare basalts  W = ( 182/184 W sample / 182/184 W stand -1) * 10 4 Original study of Kleine et al. (2005, Science): Indication of  t ≈ Ma age for the Lunar Magma Ocean (LMO) Later study: Touboul et al. (2007, LPCS): No  W -variation (KREEP, high-Ti, low-Ti) → LMO:  t > 60 Ma

Pb-model ( 206/204 Pb - 207/204 Pb) Earth: 60 – 80 Ma W-model ( 182 Hf- 182 W) Vesta, Fe-meteor.: 1  3 Ma Mars: 1-3 Ma Earth, Moon: > 60 Ma Core segregation models

Measuring initial 26 Al/ 24 Al at t 0  26 Mg = [( 26 Mg/ 24 Mg) sample/stand -1] * 1000% Amelin et al. 2002, ScienceBouvier & Wadhwa 2010, Nature Geoscience CAI, Northwest Africa 2364 CV3 chondrite CAI, Efremovka

Model for 26 Al-heating of chondritic 60 km (diameter) object Walter & Trønnes (2004, EPSL) Canonical CAI-based 26 Al/ 27 Al initial ratio The canonical, CAI-based 26 Al/ 27 Al-ratio results in a very extensive and long heating period for planetesimals The presence of undifferentiated planetesimals then requires either: - a long period of planetesimal accretion or - a delay for the initial accretion relative to t 0 in contradiction with most dynamic models

Investigation of the 26 Al/ 27 Al ratio in the angrite parent body Schiller et al. (2015, EPSL)

Early planetesimal formation and heating becomes feasible with the lower 26 Al/ 27 Al-ratio Schilling et al (2015, EPSL)

Halliday (2008, Phil.Trans.R.Soc.): Agreement between Rb-Sr- and U-Pb-model ages for the Moon: 4.47 ± 0.02 Ga (i.e.  t: 100 Ma) based on Rb- and Pb-loss from Moon by volatilization and the oldest date of lunar anorthositic crust: 4.46 ± 0.04 Ga (Norman et al. 2003) Earth-Theia collision and Moon formation:  t ≈ 100 Ma  Additional time constraints on the Moon

Science, April 17, 2015 Model based on Ar-Ar and U-Pb impact ages High frequency of meteoritic impact ages following the perceived giant, Moon-forming impact (GI). Dynamical evolution of GI ejecta: colour contours are collision velocities at about 2.5 AU (Vesta-position)

Few ages of Ma, but many at Ma (LHB) Only impact heating ages Only impact heating ages Mixed cryst, metam, and impact ages Model supports giant Moon-forming impact at Ma

Chronological summary Hf-W systematics of core segregation: Fe-meteorite parent bodies: < 1 Ma Vesta:  t = 1-2 Ma Mars:  t ≈ 3 Ma Earth, Moon:  t > 60 Ma Important implication: Chondrite parent bodies accreted rel. LATE:  t > 1-2 Ma after most of the 26 Al (primary heat source) had decayed CAI: t 0 : ± 0.2 Ma Chondrules:  t ≈ ± 0-3 Ma Small planetesimals, including Fe-meteorite PB :  t < 0.5 Ma (using the low ( 26 Al/ 27 Al) 0 ratio from the angrites) Chondrites:  t > 1-2 Ma (after most of the 26 Al-decay)

Hf-W-models for Earth accretion and core segregation Exponentially decreasing accretion growth models Giant impact Equilibrium model 60% core mixing during accretion Equilibrium model

Likely explanation: Fe-rich meteorites are formed before injection of 60 Fe from a late supernova ? (within 1 Ma after t 0 ) Main Group Pallasites Angrite  60 Ni* = ( 60/58 Ni sample / 60/58 Ni stand -1) * 10 4 Bizzarro et al. (2007, Sci 316, 1178): Fe-rich meteorites have variable Fe/Ni, but constant and low 60/58 Ni Correlation between stable, neutron- rich isotopes 62 Ni and 54 Cr (formed by common supernova processes) Earth, Mars, chondrites: highest 62/58 Ni and 54/52 Cr Carb C Mars Irons Ord C Enst C Pallasites Earth Ureilites Angrites