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 (H 2 O + CO 2 ) 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., H 2 / H 2 O, CO 2 / CO / CH 4 Only limited determinations for carbonaceous chondrites

3. What We Did GOALGOAL: determine composition of degassed volatiles for relevant planetesimal materials HOWHOW: 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 § Vol%H2H2 H2OH2OCH 4 CO 2 CON2N2 NH 3 H2SH2SSO 2 other CI 4.4692(-7)*193.20.85(-6)2.50.080.18 CM 2.7732(-8)191.80.62(-6)2.30.40.17 CV 0.2188(-11)712.50.018(-9)0.67.40.97 H 48190.74.0270.40.010.61(-8)0.29 L 43170.75.1320.30.010.61(-8)0.33 LL 43240.45.5260.39(-5)0.73(-8)0.49 EH 44170.74.7311.30.020.51(-8)0.60 EL 155.70.29.9671.85(-5)0.21(-8)0.33 § 1500 K, 100 bars. *2(-7) = 2  10 -7. † totals may deviate from 100% due to rounding errors.

5. Gas Composition 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. CI H

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 CO 2 Graphite is stable in CV chondrites to higher T –Major C-bearing phases for O/E-type chondrites are CH 4 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 H 2 O In CV chondrites, H is in hydrous silicates at low temperatures –Major H-bearing gas for O- and E-type chondrites is CH 4 at low T, and H 2 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 N 2 in all major chondrite types NH 3 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 Fe 4 N (s) at low T and high P –At all T and P, N 2 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 –P T = 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: H 2 S (T < 2200 K) : SO 2 (T > 2200 K) –CV: H 2 S (T < 1300 K) : SO 2 (T > 1300 K) –H, EH, EL: H 2 S at all T % Sulfur in gas at 100 bar T/KCICVHEHEL 1500184.70.3 0.1 2500884.01.51.30.6

10. Phosphorus Figure shows the major phosphorus-bearing species in the gas phase of a CI chondrite –P T = 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, PO 2 –H, EH, EL: P 4 O 6 % Phosphorus in gas at 100 bar T/KCICVHEHEL 20000.09~0~0409065 250010022100

11. Chlorine Figure shows the major chlorine-bearing species in the gas phase of a CI chondrite –P T = 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 % Chlorine in gas at 100 bar T/KCICVHEHEL 15009510493834 2500100

12. Sodium Figure shows the major sodium-bearing species in the gas phase of a CI chondrite –P T = 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 % Sodium in gas at 100 bar T/KCICVHEHEL 20004.60.80.21.70.7 25001005.03.03.81.8

13. Potassium Figure shows the major potassium-bearing species in the gas phase of a CI chondrite –P T = 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 % Potassium in gas at 100 bar T/KCICVHEHEL 1500111.80.704.71.5 2500100 7010049

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). –H 2 O is more soluble than other major volatiles such as CO, CO 2 and CH 4 Solution of H 2 O 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 –H 2 O, CO 2, CO, CH 4 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 CH 4 from CO and H 2 (Kress & McKay, 2004; Sekine et al. 2003) 10 -2 bars, 2500 K Vol %FeSiOMgFeONiNaMgO CI10.58.45.61.50.70.80.9 CV29.38.65.64.26.34.10.9 H44.313.08.54.23.63.50.9 EH40.015.38.53.42.42.00.9

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 H 2, CO, H 2 O, and CO 2 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