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Chemistry During Accretion of the Earth

Chemistry During Accretion of the Earth

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Chemistry During Accretion of the Earth

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  1. Chemistry During Accretion of the Earth Laura Schaefer and Bruce Fegley Planetary Chemistry Laboratory McDonnell Center for the Space Sciences Department of Earth and Planetary Sciences Washington University St. Louis, MO 63130 bfegley@wustl.edu, laura_s@wustl.edu http://solarsystem.wustl.edu

  2. Introduction • During planetary accretion, planetesimals degassed upon impacting the Earth • We want to determine the bulk composition of the atmosphere produced • “Steam” atmosphere (H2O + CO2 ) very popular in literature • e.g. Abe & Matsui 1987, Lange & Ahrens 1982a • At high temperatures, rock-forming elements also enter the atmosphere • Experiments have shown that H and C are devolatilized during impacts • Lange & Ahrens (1982b, 1986) • Speciation of H and C have not been determined for all relevant planetesimal materials • e.g., H2 / H2O, CO2 /CO/CH4 • Only limited determinations for carbonaceous chondrites

  3. What We Did • GOAL: determine composition of degassed volatiles for relevant planetesimal materials • HOW: use thermochemical equilibrium to model impact degassing of planetesimals • Assumed planetesimals were composed of major types of meteoritic material: • Carbonaceous chondrites (CI, CM, CV) • Ordinary chondrites (H, L, LL) • Enstatite chondrites (EH, EL - not shown here) • Elements involved in calculations: • Al, C, Ca, Cl, Co, Cr, F, Fe, H, K, Mg, Mn, N, Na, Ni, O, P, S, Si, Ti • Number of compounds: • Solid and liquid: 229 • Gaseous: 704

  4. “Steam” Atmosphere Composition§ §1500 K, 100 bars. *2(-7) = 2  10-7. †totals may deviate from 100% due to rounding errors.

  5. Gas Composition CI H • Orgueil (CI) chondrite is much more oxidizing • Average H chondrite is a better approximation of Earth’s bulk composition (Schaefer and Fegley, 2007) Gas devolatilized during impact-degassing at 100 bars.

  6. Carbon Gases • Results show that carbonaceous chondrites are significantly more oxidizing than ordinary chondrites • Major C-bearing phase for a C-type chondrite is CO2 • Graphite is stable in CV chondrites to higher T • Major C-bearing phases for O/E-type chondrites are CH4 and CO • Graphite is stable in EL chondrites to high T and converts directly to CO Major carbon gases in a CI (left) and an H chondrite (right). Lines show where phases have equal abundance.

  7. Hydrogen Gases • Carbonaceous chondrites are more oxidized than ordinary chondrites: • Major H-bearing gas for C-type chondrites is H2O • In CV chondrites, H is in hydrous silicates at low temperatures • Major H-bearing gas for O- and E-type chondrites is CH4 at low T, and H2 at high T Major hydrogen gases for a CI (left) and an H (right) chondrite. Lines show where phases have equal abundance.

  8. Nitrogen Gases • Nitrogen is found primarily as N2 in all major chondrite types • NH3 is abundant in a narrow temperature range at higher pressures in O-type chondrites • Related to formation of talc at low T and high P • In E-type chondrites, N is found mostly in Fe4N (s) at low T and high P • At all T and P, N2 is the major N-bearing gas Major nitrogen bearing species for an impact-heated average H chondrite as a function of T and P.

  9. Sulfur • Figure shows the major sulfur-bearing species in the gas phase of a CI chondrite • PT = 100 bars • Sulfur is abundant in the gas at high T for CI (and CM) chondrites • For other chondrites, sulfur remains primarily in sulfides • Major gas species: • CI: H2S (T < 2200 K) : SO2 (T > 2200 K) • CV: H2S (T < 1300 K) : SO2 (T > 1300 K) • H, EH, EL: H2S at all T

  10. Phosphorus • Figure shows the major phosphorus-bearing species in the gas phase of a CI chondrite • PT = 100 bars • P is more volatile in H, EH, and EL chondrites than in carbonaceous chondrites • At T < 1800 K, P is in apatite in all chondrites • minor phosphides in H, EH, and EL chondrites at high T • Major gas species: • CI,CM,CV: PO, PO2 • H, EH, EL: P4O6

  11. Chlorine • Figure shows the major chlorine-bearing species in the gas phase of a CI chondrite • PT = 100 bars • Significant chlorine is found in the gas for T > 1000 K • At T < 1000 K, Cl is found in chlor-apatite, sodalite and some salts • Major gas is HCl for T < 1800 K for all chondrites • At higher T, major gas is: • CI: NaCl • CV, H, EH, EL: KCl

  12. Sodium • Figure shows the major sodium-bearing species in the gas phase of a CI chondrite • PT = 100 bars • Very little Na is in the gas at T < 1500 K • At lower temperatures, sodium is found in feldspar, mica and halite • Major gas is NaCl at most T for all chondrites • CI, CM, H: NaOH + Na gas (T > 2000 K) • EL: Na gas (T > 2300 K) • CV, EH: NaCl at all T

  13. Potassium • Figure shows the major potassium-bearing species in the gas phase of a CI chondrite • PT = 100 bars • At low temperatures (< 1400 K), most potassium is found in feldspar and mica • Potassium is more volatile than sodium in all chondrites • Major gas is KCl at most T for all chondrites • CI, CM, H: KOH + K gas (T > 2000 K) • CV, EH, EL: KCl at all T

  14. Discussion • All chondritic planetesimals produced significant amounts of steam • BUT steam is only the most abundant gas in CI and CM chondritic planetesimals • Meteorite mixing models suggest Earth is primarily composed of H + EH chondritic material • Only minor (<5%) carbonaceous chondritic material in the Earth • Suggests that impact-generated atmosphere may not have been dominated by steam • Solubility of gases in magma ocean will also affect their atmospheric abundances (Abe and Matsui 1985). • H2O is more soluble than other major volatiles such as CO, CO2 and CH4 • Solution of H2O in the magma ocean will reduce its abundance in atmosphere relative to other species

  15. Discussion (cont’d) • Thermal structure of atmosphere is dependent on composition • H2O, CO2, CO, CH4 have different IR spectra • Each produces different amounts of greenhouse warming • More rock-vapor is released at low pressures • Composition of atmosphere is pressure-dependent • Table below gives abundances of major rock-forming vapors at 10-2 bars and 2500 K • Impact plume cools quickly (~30 s for very large impacts, less for smaller) • Rock-vapor will condense as particles in the atmosphere • May catalyze formation of CH4 from CO and H2 (Kress & McKay, 2004; Sekine et al. 2003)

  16. Summary • We calculated the composition of “steam” atmospheres produced by impact-degassing of chondritic planetesimals • Only CI and CM chondritic materials produced atmospheres primarily composed of steam • Major impact-degassed volatiles are H2, CO, H2O, and CO2 • Rock-vapor is also released into the atmosphere. As it cools, it may condense into particles • Particles may catalyze formation of methane in the Earth’s early atmosphere • This work was supported by the NASA Astrobiology and Origins Programs References: Abe and Matsui (1985) JGR, 90(suppl.), C545-C559; (1987) LPSC, 18, 1-2. Kress and McKay (2004) Icarus 168, 475-483. Lange and Ahrens (1982a) Icarus, 51, 96-120; (1982b) JGR, 87(suppl.), A451-A456; (1986) EPSL, 77, 409-418. Schaefer and Fegley (2007) Icarus, 186, 462-483. Sekine, Sugita, and Kadono (2003) JGR 108, doi:10.1029/2002JE002034