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Abundances of the Elements – Solar, Meteoritic, and Outside the Solar System

Abundances of the Elements – Solar, Meteoritic, and Outside the Solar System. Why are we interested in the abundances of the elements? Constitution of (baryonic) matter, quantities of stable elements/isotopes Formation of the solar system

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Abundances of the Elements – Solar, Meteoritic, and Outside the Solar System

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  1. Abundances of the Elements – Solar, Meteoritic, and Outside the Solar System Why are we interested in the abundances of the elements? Constitution of (baryonic) matter, quantities of stable elements/isotopes Formation of the solar system Composition of the solar system, planetary compositions Origin of the elements Abundance distributions are critical tests for nucleosynthesis models Katharina Lodders, Washington University Saint Louis, USA

  2. Abundances of the Elements – Solar, Meteoritic, and Outside the Solar System Solar abundances: present-day observable composition of Sun, mainly photosphere, but also sun spots, solar flares, solar wind Solar system abundances: composition at time of solar system formation ISM/ molecular cloud composition 4.6 Ga ago proto-solar abundances Li, D, short-lived radioactive nuclides (26Al, 129I), long-lived (=still present) radioactive nuclides (87Rb, 235U, 238U, 232Th) Cosmic abundances: this term should be avoided many stars similar in composition as Sun, amount of elements heavier than He (=metallicity) changes with time there is no “generic” cosmic composition abundances vary across Milky Way Galaxy, other galaxies

  3. Concentration and Abundance Concentration: Quantifies amount of mass or number of particles per unit mass or unit volume Concentration by mass (or weight): g of element per 100 g sample = percent g of element per ton (106 g) = ppm = parts per million 10,000 ppm = 1 mass% e.g., CI chondrite analysis gives 18.28 g Fe per 100 g sample Fe concentration is 18.28% by mass Concentration by number: number of particles per unit volume e.g., number of O2 molecules in air 20.95% of air molecules are molecular O2 at room T and P, there are ~2.5×1019 particles/cm3 so there are 2.5×1019×20.95/100 = 5.2×1018 (molecules O2)/cm3

  4. Abundance is a relative quantity Abundance Scales and Notations Most commonly used: Atoms by number, N Normalized to a reference element Astronomical abundance scale: normalized to H, the most-abundant element in universe eH = 1012 atoms converted to log scale: A(H) = log eH = 12 abundances are measured relative to H, e.g., N(Fe)/N(H) log eFe = log { N(Fe)/N(H) } + 12 used for H-rich systems: stars, ISM Cosmochemical abundance scale: normalized to Si, the most abundant cation in rock, N(Si) = 106 atoms used for planetary modeling, meteorites Coupling the scales:

  5. Derivation of elemental and isotopic abundances Earth crust Meteorites Other solar system objects, gas-giant planets, comets, meteors Solar photospheric spectrum, SW, SEP Spectra of other dwarf stars (B stars) Interstellar medium H II regions Planetary Nebulae (PN) Galactic Cosmic Rays (GCR) Also presolar grains

  6. Start with Earth’s crust & meteorites in 1847 (Elie de Beaumont) ~59 out of 83 elements known at the time spectral analysis invented later (1860s) Mendeleev’s periodic table later (1869) There are 16 abundant elements common to different crustal rocks, mineral & ocean waters, organic matter, and meteorites: H, Na, K, Mg, Ca, Al, C, Si, N, P, O, S, F, Cl, Mn, Fe Light elements with atomic masses up to Fe are abundant, heavier elements than Fe are rare Notable exceptions Li, B, Be Gluconium =Be, Didymium=Nd+Pr, Columbium=Nb

  7. What controls the abundances of the elements? Abundances of the elements as a function of atomic number Earth crust, igneous rocks Clarke 1898, Harkins 1917 Quantitative analyses limited to abundant elements in rocks no discernable trends of abundance with atomic number or atomic weight  Abundances are modified from solar during Earth’s formation and differentiation

  8. Derivation of elemental abundances • Earth’s crust: • - available material that can be analyzed in the lab • but not representative for composition of entire Earth • Earth is also not representative for solar system composition • Chemical and physical fractionations during • (1) formation of planetesimals from molecular cloud material processed in the protoplanetary accretion disk (solar nebula) • (2) differentiation of Earth materials into core, silicate mantle and crust • elements follow geochemical character • siderophile elements: metallic elements into core • Fe, Ni, Co, Au, Ir, Pt-group elements, … • lithophile elements: oxide and silicate rock-forming elements • go into silicate mantle and crust • Mg, Si, Al, Ca, Ti, REE… • Crust preferentially contains elements with large ionic radii that enter silicate melts during differentiation: Si, Al, Ca, K, Na, REE, … • (incompatible in silicate mantle minerals olivine, pyroxene)

  9. Molecular cloud composition 4.6 billion years ago = solar system composition Earth crust today

  10. Extend search for chemical elements to other celestial objects (Kleiber 1885; 68 elements out of 83 are known) Earth’s crust meteorites Comets Meteors Sun Other stars Composition of celestial objects is not random Helium found in 1868 but not plotted by Kleiber, no other noble gases known at the time

  11. Abundances vs. atomic number 1917: Harkins 1917 Use average abundances from meteorites, no photospheric abundances yet Even-numbered elements are more abundant than their odd-numbered neighbors Li, B, Be low in abundances C low because of volatility, but still more abundant than odd numbered neighbors B or N Fe peak Abundances of elements heavier than Fe (Z > 26) are quite low Harkins’ discovery graph of the odd-even effect in elemental abundances

  12. Odd-Even distribution of the elements as a function of atomic number is also seen for heavy elements Good example: rare earth elements (REE) Similar geochemical behavior Odd-even abundance effect not (completely) erased during re-distribution of REE among minerals in rocks Figure updated from Goldschmidt 1937 Normalized to Y= 100 atoms (Y not shown) Russell

  13. Meteorites Chondrites: most common types of meteorites Mixtures of silicates, metal and sulfides that may have been processed subsequently on their parent asteroids Achondrites and iron meteorites: experienced melting and/or extensive re-crystallization on their parent asteroids

  14. Kodaikanal meteorite Found 1898, 15.9 kg, IIE iron, fine octahedrite with silicate inclusions Photo Christian Anger 2006 Specimen shown from Natural History Museum Vienna A 3.5 kg sample is at the Geological Survey India, Calcutta

  15. Chondritic Meteorites Chondrites: contain mineral phases that most closely resemble the original solids that were present in the solar nebula Many types of chondrites contain round silicate spheres called chondrules Chondrite groups: Ordinary chondrites: H, L, LL Enstatite chondrites: EH, EL Carbonaceous chondrites: CI, CM, CV, CO, CK, CR, CH Bjurboele L/LL3 chondrite Chondrules in the Tieschitz ordinary chondrite

  16. Photo: Le Muséum National d'Histoire Naturelle, Paris Orgueil meteorite Abundances of CI chondrites give the best match to elemental abundances in the Sun (except for volatile elements H, C, N, O, noble gases) “CI” for carbonaceous chondrite of Ivuna type 5 observed CI chondrite falls: Alais 1806 (6 kg), Ivuna 1938 (0.7 kg), Orgueil 1864 (14 kg), Revelstoke 1965 (1 g), Tonk 1911 (10 g)

  17. CV chondrite: • CI chondrites • do not contain chondrules • their minerals are aqueously altered • hydrous silicates, salts • - metal and sulfide are oxidized • Secondary mineral alterations did not change overall elemental composition. • CI chondrites contain the highest amount of volatile elements when compared to other chondrites Ordinary chondrite:

  18. Independent of geochemical character

  19. Independent of geochemical character

  20. Independent of geochemical character

  21. Abundances in the photosphere, CI chondrites, meteors, and cometary samples Comet Halley data: Schultze et al, Jessberger et al; Comet Wild 2 data from Flynn et al. 2006

  22. Sun holds more than 99% of the solar system’s mass Composition of Sun should be good approximation for solar system as a whole

  23. Element determinations in the Sun ~66 elements out of 83 naturally occurring elements identified in photosphere all stable elements up to atomic number 83 (Bi) plus radioactive Th and U ~30 – 35 elements well determined in photosphere Determined in photosphere with larger uncertainties: > 0.10 dex: (factor 1.3) Li, Be, B, N, Sc, Cr, Ni, Zn, Ga, Rh, Cd, In, Nd, Tb, Ho, Tm, Yb, Lu, Os, Pt > 0.05 dex: (factor 1.12) Mg, Al, Si, Ti, Fe, Co, Nb, Ru, Ba, Ce, Pr, Dy, Er, Hf, Pb Difficult to determine (line blends, low abundance) Ag, In, Sn, Sb, W, Au, Th, U As, Se, Br, Te I, Cs, Ta, Re, Hg, Bi Detected but difficult to quantify from spectra: He, Ne, Ar, Kr, Xe found in solar wind Determined from Sun-spot spectra, relatively uncertain: F, Cl, Tl

  24. Challenges for photospheric determinations: Line list for neutral atoms, ions, excited state transitions Fe several thousand lines, other elements only 1 line accessible Line blending Ni and Fe lines interfere; e.g., determinations of O, Th Transition probabilities and lifetimes of atomic states recent re-analysis of transition metals and REE Atmospheric models continuum modeling (e.g., UV for Be) LTE vs. non LTE, is radiation in local equilibrium with matter 2D, 3D granulation, convection

  25. He abundance Determined from helioseismic models and solar evolution models that must match the current luminosity and radius of the Sun One input to models is ratio of mass fraction of H ( = X) to mass fraction of heavy elements (= Z) Mass fraction of He ( = Y) follows from X + Y + Z = 1 Z is the sum of the mass fraction of all elements heavier than He (the “metals”) Z is dominated by C, N, O, Ne (CNO ~66%, CNONe ~75%) Ratio of Z/X is uniquely defined, convert mass fraction of He to atomic abundance

  26. Problem: New lower C, N, O photospheric abundances (Allende Prieto et al., Asplund) give a lower mass fraction of metals Lower C, N, O abundances reduce opacity which is needed to obtain agreement of solar evolution models with observations • Possible solution: Increase the Ne abundance • to increase opacity in sun’s interior (Bahcall) • But solar Ne abundance is also derived indirectly, e.g., from Ne/O ratio in SW • Lower O  also lower Ne “Solar” Ne abundance needs to be worked out

  27. Solar Ar abundance

  28. Comparison of solar and CI chondritic abundances (both scales normalized to Si=106): Good correlation for many heavy elements (1:1 line) Meteorites depleted in elements that form volatile compounds Noble gases, CO, CH4, N2, H2O Photosphere depleted in Li Abundances of “missing” rock-forming elements in photosphere can be derived from CI-chondrites

  29. Photospheric abundances are not equal to protosolar abundances He and heavy elements settled from the solar photosphere over Sun’s lifetime Protosolar Photospheric A(He) 10.984 10.899 drop by ~18% Heavy elements relative to H drop by ~16% A(El) protosolar = A(El) photospheric + 0.074 Add 0.074 dex to the photospheric abundance to get protosolar (solar system) abundances for heavy elements On the cosmochemical scale relative to Si=106, only the H and He abundances change

  30. Elemental abundances vs. atomic number Solar abundances of the elements are controlled by nuclear properties, not chemical (electron shell) properties

  31. Elemental abundances as function of atomic number (= proton number=Z) show “peaks” e.g., O (N), Fe (Ni), Ba (I), Os (Ir) Do not follow electron shell stabilities (noble gases are not the most abundant heavy elements) 280 naturally occurring stable (266) and long-lived nuclides (14) Nuclides belonging to the same element (same proton number Z) are isotopes “isos + topos” = in the same place in the periodic table Separate for even and odd-numbered elements Fe Sn-Ba Os-Pt Mn I-Cs Ir Look at nuclide distributions to decipher what controls elemental abundances

  32. Abundances of the nuclides versus mass numbers Abundances peak at mass numbers for closed proton and neutron shells “magic numbers” 2, 8, 20, 28, 50, 82, 126 Doubly-magic nuclei; e.g. 4He Z = N = 2 16O Z = N = 8 40Ca Z = N = 20 • 266 stable nuclei • Z even, N even: 159 nuclides • Z even, N odd: 53 nuclides • Z odd, N even: 50 nuclides • Z odd, N odd: 4 nuclides (2H, 6Li, 10B, 14N) • Lower number of odd-Z numbered isotopes • lower abundances of odd-Z numbered elements

  33. S, R, P –process contributions to the elemental abundances Example Zr: 5 isotopes, relative contribution of each isotope to total element is known (usually a terrestrial std)  Abundance of each isotope rel. Si=106 atoms For each isotope: amount from main S-process from low & intermediate AGB stars from nucleosynthetic network calculations; Arlandini et al. 1999, Winckler 2006 Amount from weak S-process (Raiteri et al. 1992, Travaglio et al. 2004) R process contribution by difference to isotope abundance (“R residuals”), Or R process network calculations (Kaeppeler, Karlsruhe group) Sum of R and S abundances of each isotope gives contributions of S and R process to elemental abundance of Zr: main S = 65%, weak S = 2%, R = 33%

  34. Contributions of different nucleosynthesis processes to the abundance of each element Pb Ba Main S process: evolved low & intermediate mass (AGB) stars Weak S and P and R processes: massive stars

  35. The solar system abundances of the elements are reasonably well known, but large uncertainties remain for several elements Re-analyses for several elements are needed Are differences between solar photosphere and CI-chondrite data real or the result of analytical difficulties? Photosphere: e.g., Sc, Ti, Mn, Ca, Ga, Ge, Sn, Rb, Ag, In, Au, W, Tm, Lu, Th, U, Hg Sun-spot data: F, Cl, Tl But also in CI-chondrites: e.g., Be, Hg Isotopic compositions of the elements are mainly taken from terrestrial rocks and meteorites – are these the same in the Sun?

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