Planetary Formation Reading this week: White p474-490 OUTLINE Today 1.Accretion 2.Planetary composition 1
11/1/17 Building Earth 2 things are important: Accretion means the assembly of Earth from smaller bits Differentiation means the separation of components within Earth during or after assembly ÞThese probably overlapped Nebula condenses Planets form by condensation of planetesimals Temperatures refer to conditions at initial condensation. 2
Formation in steps Condensation of gas->dust Km-sized bodies form quickly (<10 6 yr), some differentiate Moon & Mars sized bodies may also form quickly and differentiate Orbit crossing limits growth of big bodies: ~ 10 7-10 8 yr Last stages in absence of solar nebula (used/blown away) What got included Planets start forming in a game of billiards Large control from Jupiter/Saturn Earth materials come from many different regions Zonation of composition in terrestrial zone unlikely Volatile depletion in the terrestrial planet forming materials http://www.skyandtelescope.com Results from Chambers, 2003 3
Simplified Earth s formation Fig. 6.4 From (A) a homogeneous, low-density protoplanet to (B) a dense, differentiated planet Importance of giant impacts Mercury head-on, high velocity, collision Total planetary disruption, silicate mantle gone: planet too dense Earth grazing, low velocity, collision Forms very large Moon Global magma oceans on both bodies Mars grazing, low velocity, collision Forms hemispheric dichotomy A baby magma ocean, no large moon 4
Mercury s density? How do we know density? Mercury plots above density-radius relationship For a fully differentiated core and mantle (assuming similar compositions as other planets) Core radius ~75% of the planet Core mass ~60% of the planet Instead expect something between Mars & Moon One explanation is giant impact, blasting silicate mantle into space, and only some of it making it back down Earth and Giant Impacts Simulations: Mars-sized bodies impacted Earth during accretion. Extreme events: Earth will radiate like a lowmass star for 1Ka! A large oblique impact places material in Earth orbit: Origin of the Moon 5
Earth-Moon System Things to explain: Earth-Moon same δ 18 O-δ 17 O line (TFL), not other planets/meteorites Both depleted in volatiles, but Moon more so Moon not exactly in Earth s equatorial plane Have to explain angular momentum Mars s moons probably captured, ours is big(ger) and compositionally similar. Wiechert et al., 2001 Krot et al., 2003 Giant impact Giant Impact Simulations *Earth close to final size *Mars-sized impactor *Both bodies already differentiated *Both bodies formed at ~1 AU *Grazing blow to explain current mass distribution of Earth and Moon (cores, silicate parts) http://sarahtstewart.net/origin-earth-and-moon/ 6
Post-impact Some of material blasted into space returns to Earth, rest accretes into Moon Exact dynamics of impact affect resulting rotation and orbit of both Byproduct of impact If Earth s temperature rises so it can radiate like a low-mass star, what will happen to the accreting spheroid? Think about the elemental budget, what do you expect to happen? Magma Ocean Ron Hartmann 7
Traces of accretion? Overall composition: elemental variations hint at conditions Isotopic compositions allow for timing and fingerprinting (later in the semester) Elemental partitioning = evidence of differentiation (next lecture) Heat flux (accretion +??) Pieces of accretion Meteorites are debris of planetary formation non chondrite meteorites sample... c. metal + silicate (stony/fe) a. silicate (stony) chondrites collide/ metamorphose 1 2 3 accreting planetismals growth/radioactive heating 4 phase segregation: metal blobs + silicate b. metal (Fe) differentiated planetismal As accretion proceeds, different meteorites represent different parts of space & time Chondrites: most primitive material (recall similarity to solar composition) Achondrites: broadly basaltic (igneous textures), early silicate mantle(s) Irons: Fe-Ni alloys, broadly similar to our core Stony/Fe: mixtures of metallic and silicate materials, core-mantle boundary Also have pieces from Moon, Mars, Vesta, 8
Comparing Earth to Chondrites C1 chondrite» Sun (minus few volatiles) Earth s estimated primitive mantle (core removed) vs. C1: ~3 x refractory elements (Al, Ca, U, Th, Si, Ba, rare earths, ) depletions in other elements (including Mg and Si) Intermission some nomenclature Grouped by affinities of elements: 4 types of material thought to exist during accretion: Siderophile: iron- liking elements (liking zero-valent Fe; i.e., metal) Chalcophile: sulfide-liking (S 2- ) Lithophile: silicate-liking ([SiO 4 ] n, also O-loving in practice) Atmophile: gas-phase-liking 9
Close but not quite: CI Chondrite 101 Earth is Volatile-Depleted Where did they go? Bulk Silicate Earth Abundances Normalized by CI Chondrites Earth s Mantle is Siderophile Depleted Why? Bulk Silicate Earth Abundances Normalized by CI Chondrites 101 100 100 10-1 10-1 Mo 10-2 10-2 10-3 10-3 10-4 H Be N Al S K Ti MnNiGaSeRbZrRhCdSb LaNdGdHoYbTaOsAuPb U B O Na Si Cl Ca V FeCuGeBr SrNbPd In TeCsCeSmTb ErLu W Ir Hg Bi Li C F Mg P ScCrCoZnAs YRuAgSn I BaPr EuDyTmHfRe Pt Tl Th Elements in Order of Atomic Mass 10-4 Be Al Ti Ni ZrRh LaNdGdHoYbTaOsAu U O Si CaVFe SrNbPd CeSmTbErLu W Ir Li Mg P ScCrCo As YRu BaPr EuDyTmHfRe Pt Th Elements in Order of Atomic Mass Bulk Earth from McDonough and CI from Palme and O Neill, 2003 A Chondritic Earth Geochemist common assumption: Earth accreted from material with chondritic ratios of the non-volatile elements ÞRefractory isotope systems should look chondritic ÞThis recently became challenged (142Nd next slide) Elements also grouped based on their behavior (=amount vs. C1) (later slides) GG325 L30, F2013 10
Chondrites vs. Earth Short-lived isotope system: 146 Sm -> 142 Nd (T 1/2 = 103 Ma) After ~5xT 1/2 no 146 Sm left Þ 142 Nd differences made in ~ 500 Ma Earth is different from meteorites, leaves 2 options: 1)Major differentiation event on Earth before this time (hides complement) 2)Earth was never perfectly chondritic More and more people accept #2, but differences are small chondritic Earth OK for 1 st order understanding Boyet and Carlson Goldschmidt s Classification and the Geochemical Periodic Chart This classification was proposed in the 1920s by geochemist Goldschmidt. Qualitatively useful for describing the origin of the Earth from materials present in the early solar system Goldschmidt based his groups on the distributions of elements in silicate, metal-rich and gas phases in 1. metal-ore smelter materials 2. meteorites, 3. the modern Earth. 11
Goldschmidt Classification/Geochemical Periodic Chart Elements can be assigned to more than one group depending on the situation, so this scheme provides only generalities. Siderophile iron liking (zero-valent Fe) Chalcophile sulfide liking (S 2- ) Lithophile silicate liking ([SiO 4 ] n, also O loving in practice) Atmophile gas phase liking Where do these groups reside? GG325 L31, F2013 Goldschmidt Classification/Geochemical Periodic Chart The groups have a general relationship to the periodic chart, reflecting an underlying relationship to the electronic configurations of the elements in their common forms. GG325 L31, F2013 12
Major elements in mantle compared to meteorites Primitive Upper Mantle (PUM) composition is determined from intersection of chondritic meteorite array with mantle xenolith array PUM is not equal to any class of meteorites, Mg/Si higher in Earth; Ca/Si, Al/Si show solar system refractory vs. silicate phases Element Relationships: Earth and C1 Chondrites Most important siderophile and lithophile elements: BULK Earth has higher Fe/Si and Mg/Si than the chondrites (Sun) => if bulk earth» CI chondrite: lower mantle / core must host Si, or we got < chondrite 13
Volatiles Earth is variably depleted in volatile elements (e.g., K, Rb, Cs, etc.) relative to chondrites 14