Chemistry During Accretion of the Earth

Laura Schaefer and Bruce FegleyPlanetary Chemistry Laboratory

McDonnell Center for the Space SciencesDepartment of Earth and Planetary Sciences

Washington UniversitySt. Louis, MO 63130

bfegley@wustl.edu, laura_s@wustl.eduhttp://solarsystem.wustl.edu

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

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

“Steam” Atmosphere Composition§

Vol% H2 H2O CH4 CO2 CO N2 NH3 H2S SO2 other

CI 4.4 69 2(-7)* 19 3.2 0.8 5(-6) 2.5 0.08 0.18

CM 2.7 73 2(-8) 19 1.8 0.6 2(-6) 2.3 0.4 0.17

CV 0.2 18 8(-11) 71 2.5 0.01 8(-9) 0.6 7.4 0.97

H 48 19 0.7 4.0 27 0.4 0.01 0.6 1(-8) 0.29

L 43 17 0.7 5.1 32 0.3 0.01 0.6 1(-8) 0.33

LL 43 24 0.4 5.5 26 0.3 9(-5) 0.7 3(-8) 0.49

EH 44 17 0.7 4.7 31 1.3 0.02 0.5 1(-8) 0.60

EL 15 5.7 0.2 9.9 67 1.8 5(-5) 0.2 1(-8) 0.33

§1500 K, 100 bars. *2(-7) = 2 10-7. †totals may deviate from 100% due to rounding errors.

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.

CICI

HH

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.

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.

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.

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

% Sulfur in gas at 100 bar

T/K CI CV H EH EL

1500 18 4.7 0.3 0.3 0.1

2500 88 4.0 1.5 1.3 0.6

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

% Phosphorus in gas at 100 bar

T/K CI CV H EH EL

2000 0.09 ~0 40 90 65

2500 100 22 100 100 100

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

% Chlorine in gas at 100 bar

T/K CI CV H EH EL

1500 95 10 49 38 34

2500 100 100 100 100 100

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

% Sodium in gas at 100 bar

T/K CI CV H EH EL

2000 4.6 0.8 0.2 1.7 0.7

2500 100 5.0 3.0 3.8 1.8

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

% Potassium in gas at 100 bar

T/K CI CV H EH EL

1500 11 1.8 0.70 4.7 1.5

2500 100 100 70 100 49

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

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)

10-2 bars, 2500 K

Vol % Fe SiO Mg FeO Ni Na MgO

CI 10.5 8.4 5.6 1.5 0.7 0.8 0.9

CV 29.3 8.6 5.6 4.2 6.3 4.1 0.9

H 44.3 13.0 8.5 4.2 3.6 3.5 0.9

EH 40.0 15.3 8.5 3.4 2.4 2.0 0.9

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