chemistry during accretion of the earth
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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 [email protected] , [email protected] - PowerPoint PPT PresentationTRANSCRIPT
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
[email protected], [email protected]://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