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When did Earth become habitable ? (which does not imply it was inhabited) Formation of solar system by contraction of the solar nebula Contraction volume decrease Proto-sun . . . .. . . 12x106 K ignition T-Tauri stage . . .. . . T T Evolution of solar nebula ~ 4.6 Ga More details Elephant trunk nebula (Spitzer telescope) = stellar nursery with forming stars glowing. As the solar nebula most likely did, it contains Si-dust, H, He and complex PAH’s Planetary formation from planetary embryos to the proto-Earth Time frame 10 to 100 Myr With Tº decreasing away from the Sun, the refractory material (metal, silicates) accumulate near the center (rocky planets) w h i l e vo l a t i l e s a n d i c e s accumulate in the outer rings (giant gas planets) Fine dust particles floating in gas collide into km-size planetesimals that keep on growing through continuous collisions. Open questions: physics of collisions should be disruptive ?, radial migration into the sun by gas drag on meter-sized particles ? unless multi-km objects form < 1000 years. Gradually proto-Earth develops, gravity re-shapes it into a sphere, and immediately differentiation into core, mantle crust starts Planetary formation from planetary embryos to the proto-Earth Time frame 10 to 100 Myr With Tº decreasing away from the Sun, the refractory material (metal, silicates) accumulate near the center (rocky planets) w h i l e vo l a t i l e s a n d i c e s accumulate in the outer rings (giant gas planets) Fine dust particles floating in gas collide into km-size planetesimals that keep on growing through continuous collisions. Open questions: physics of collisions should be disruptive ?, radial migration into the sun by gas drag on meter-sized particles ? unless multi-km objects form < 1000 years. Gradually proto-Earth develops, gravity re-shapes it into a sphere, and immediately differentiation into core, mantle crust starts Solar system formation 1) The first million year: the stellar era: formation of the sun by accretion/contraction of material circumstellar disk 2) The first 10 million years: the disk era: evolution of circumstellar disk to give birth to planets; oldest objects in solar system formed and are still preserved in primitive meteorites CAI & chondrules in CC first solid formed at Tº > 1800 K. U/Pb age of CAI = 4567.2±0.6 Ma, time zero t0. Based on short-lived nuclide 26Al, chondrules are either 2-3 Myr younger or CAI condensed closer to sun (reading) 3) The first 100 million years: the telluric era, the rocky planets formed and differentiated (layered structure, atmosphere, ocean, crust etc) Meteorite classification a clue to the early Solar System Almost 10 tons of meteorite fall on Earth / year (> cm size) coming from asteroid belt between Mars and Jupiter, link to asteroid spectral classification and composition. Chondrite composition Except for volatile elements chondrite have the same composition as the Sun Non-differentiated meteorites almost no planetary evolution since the origin of solar system Differentiated meteorites Underwent planetary evolution SiAlCaNa K SiAlNaK Silicates Metal + silicates SiMgFe (CaAl) FeNi FeNi Fe-Ni Chronology Formation of first bodies 1st mineral phases Preserved in primitive nondifferentiated meteorites Allende CAI Allende CC Chondrule Matrix Age of 1st phases to condendate = age of Earth Considered as T0 for evolution of the Earth CAI Efremovka meteorite 4.567 Ga (Amelin et al. 2002) Absolute dating using radioactive decay Parent isotope (P) to daughter isotope (D) Law of radioactivity: dP/dt = -λ x P P = P0 x e(-λ x t) P0 - P = F - F0 Chronometers Isotopes Half life 40K - 40Ar 1.250 Ga 147Sm - 143Nd 1.060 Ga 176Lu - 176Hf 3.50 Ga 232Th - 208Pb 14.010 Ga 235U - 207Pb 0.703 Ga 238U - 206Pb 4.46 Ga 14C - 14N 5370 y Mass spectrometry for isotopic measurements Example 87Rb to 87Sr (λ = 1.42 x 10-11 years-1) or half life of 48.8 x 109 years Relative dating using extinct radioactivity P is completely gone, very short half life only daughter isotope remains P0 - P = F - F0 Chronometers Isotopes Half life 41Ca - 40Ar 0.1 Ma 60Fe - 60Co 1.5 Ma 10Be - 10B 1.5 Ma 135Cs - 135Ba 2.3 Ma 53Mn - 53Cr 3.7 Ma 107Pd - 107Rh 6.5 Ma 182Hf - 182W 9.0 Ma 129I - 129 Xe 15.9 Ma Example 26Al to 26Mg (λ = 9.1 x 10-7 years-1) or half life of 0.7 x 106 years Timing of the differentiation •Recent data (182Hf to 182W, T1/2=9Myr chronometer) indicate bulk metal-silicate segregation in < 30 Myr after begin of Solar System formation •Core formed more rapidly in smaller bodies (Vesta, Moon, Earth) (reading) •Moon forming impact took place as this differentiation was already going on within the proto-Earth •182Hf-182W of lunar basalt indicates a differentiation at 45 ± 5 Myr, which could mark the end of the Lunar magma ocean •Isotopic composition of Earth’s atmosphere differs from solar nebula, primitive meteorites and comets, composition of the primary atmosphere is unknown •Earth’ atmosphere was subject to several episodes of loss to space (large impacts or with isotope fractionation such as thermal loss or pick up ion loss) during tens of Myr before closure. •Earth’s atmosphere formed from several volatile-rich components •High dynamic state of the mantle during all of the Hadean (~ 0.5 Gyr) Chronology of early planetary processes 4567 All complex planetary processes happen within first 100 Myr or less... What is Earth made off ? Bulk composition CI = solar system (except H, He), other meteorites lost volatiles (H2O, K, Cl, S etc.) during T-tauri phase and by subsequent evolution of parent body Accretion is fast, gas planets formed within few Ma, rocky ones a few 10 Ma. Lots of material exchange in Solar System, as planetary embryos destabilized by Jupiter. Primordial material does not remain in orbit around Earth more than 100 Ma Earth’s ∂17O values differs from other planetary bodies. Earth-Moon fractionation line (slope 0.5) indicate origin from similar orbits Earth originally made of Enstatite chondrite-like material Origin of the Earth - Moon system Moon: a very unusual satellite formed by mega-impact on the young Earth The detailed Moon forming-scenario (Canup 2004) A m o n g a l l s c e n a r i o ’s proposed only the megaimpact explains all properties of the Earth-Moon system Such mega-collision is not unusual during planetary formation Film illustrating the mega-impact and formation of the Earth-Moon system Considered as the end of planetary accretion After the mega-impact scenario reformation and start of the geo-evolution of Earth: Hadean Origin of water Moceans= 1.4 x 1024 g M⊕ = 5.97 x 1027 g Moceans= 250 ppm⊕ H2Omantle = 5-10 x Moceans MH2O = 350-500 ppm⊕ Earth is relatively dry Mwater-today = 1.6 x 1021 g (MARSIS) Mearly ocean = 1.7 x 1022 g (volcanism) Mearly ocean = 7.3 x 1022 g (MOLA) M∅ = 6.42 x 1026 g Moceans = 2.5-113 ppm∅ … just like the other rocky planets Primitive meteorites carbonaceous chondrites H2O = 17-22 wt% (CI; Orgueil) H2O = 3-11 wt% (CM2; Murchinson) H2O < 2 wt% (CV; Allende) in comparison to other planetary bodies Micromteorites from Antarctica (CM2): H2O = 2-8 wt% Comets H2O ≤ 50 wt% Hale-Bopp Heliocentric distribution of water Short & long period comets H2O ≤ 50 wt% Ordinary chondrites Enstatite chondrites H2O ≤ 1 wt% or anhydrous Carbonaceous chondrites 2-22 wt% IDP-AMM 1-8 wt% 1 ua 2-5 ua (asteroid belt & Troyans) 30-50 ua (Kuiper belt) How did water get to Earth ? Heliocentric distribution of water D/H Sun D/H = 25 x 10-6 Comets (Oort) D/H = 300-330 x 10-6 Earth D/H = 153.7 x 10-6 Carbonaceous chondrites D/H = 130-170 x 10-6 Interstellar clouds HCN - D/H = 2000 x 10-6 Cold cores - D/H = 1260 x 10-6 Hot cores - D/H = 110 x 10-6 Temperature favors D loss and makes water lighter HDOice + H → D + H2Ogas Chondritic origin of water Ocean water has an D/H isotopic signature that is in the range of that of chondritic bodies (CO, CI, CM). Currently, these bodies occur in the main asteroid belt between Mars and Jupiter or among the Troyans (Jupiter orbit) Micrometeorites that have a chemistr y and mineralogy similar to CM2 could be another transport agent to Earth Yokochi & Marty (2007); Robert (2003); Aléon et al., (2005); Engrand et al. (1999)) Another (less likely) possibility 0,012 CI 0,011 Solar Earth CM 0,010 R = 0.001 0,009 0,008 13 C/ 12 C Ocean water could result from mixing heavy cometary water with light H from proto-solar nebula, that was later oxidized in H2O 0,007 However C & N isotopes do not agree with this hypothesis 0,006 R = 1000 Cometary CN 0,005 0,004 0,003 0,0E+00 R = (H/12C)solar/(H/12C)cometary 1,0E-04 2,0E-04 3,0E-04 4,0E-04 D/H Yokochi & Marty (2007); Hashizume et al. (2000). Figure Yokochi & Marty (2007) modifiée. A chondritic Earth 1,E+00 1,E-02 Mantle+atm+hydr Mantle Terrestrial abundance, mol/g 1,E-04 H2O 1,E-06 1,E-08 C N Terrestrial=chondritic A chondritic input between 0.3 to 2% of the Earth mass explains the whole volatile budget on Earth Terrestrial=0.02 x chondritic 1,E-10 Terrestrial=0.003 x chondritic 1,E-12 1,E-14 22 Ne 1,E-16 36 Ar 84 Kr 130 Xe 1,E-18 1,E-18 1,E-16 1,E-14 1,E-12 1,E-10 1,E-08 1,E-06 1,E-04 1,E-02 1,E+00 Chondritic abundance, mol/g This agrees with the 0.5 to 1% mass necessary to explain high siderophile elements after core formation Late Veneer hypothesis sprinkel material on early Earth When did water arrive on Earth? 100 Initial upper limit = 50 x oceans RASSC = 109today FAMM = 5x1010 g/yr; H2O = 8 wt% 10 RASSC = 109today FAMM = 1010 g/yr; H2O = 2 wt% MH2O 1-10x Moceans 1 AMM could produce oceans if flux 109 x higher than today (10-50 x 109 g/y) Model for a lower flux (1000x) only 4% of the water 0,1 RASSC = 106today FAMM = 5x1010 g/yr; H2O = 8 wt% 0,01 RASSC = 106today FAMM = 1010 g/yr; H2O = 2 wt% 0,001 Zircons : a cool early earth Moon formation Earth accretion 0,0001 0 40 80 Time since ASSC, Ma 120 160 If water comes with CI-CM bodies assume 10x current mass of asteroid belt + small late cometary contribution (~%) First cooling of magma ocean Alteration of basalt to produce serpentinite crust ~ as today on seafloor 146Sm → 142 Nd (T1/2= 103 Ma) silicate/silicate fractionation before total decay of 146Sm < 150 Ma, done early Hadean Habitable ocean: Abe, 1993. Habitable ocean: Propiano model Asteroidal water delivery AMM water delivery LHB Zircons: evolved crust Primordial crust Moon-forming impact Core formation Earth accretion 4600 4400 4200 4000 Time before present (Ma) 3800 Early ocean Warm based on O & Si isotopes in chert Only 2 x more saline than today, fluid inclusions in chert Na-Ca-Cl chemistry Major hydrothermal input, trace elements & noble gas More acid and anoxic Chondritic origin Early (during) after end of accretion, direct after Moon impact The Hadean: Building an habitable planet There is no real rock record of the first 700 Myr (4.5 to 3.8 Ga) because of resurfacing of Earth Analogy with other planets that preserved their oldest terranes (Moon, Mars), basaltic meteorites (Eucrites) Jack Hills zircons, core at 4.4 Ga, recycled in younger sequences Oldest rock Acasta Gneiss (Canada) at 4.1 Ga metamorphosed Isua oldest rock sequence (Greenland) at 3.8 Ga