Chemical Models of Terrestrial Exoplanets
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Chemical Models of Terrestrial Exoplanets
Bruce Fegley, Jr. and Laura SchaeferPlanetary Chemistry LaboratoryDepartment of Earth and Planetary Sciences
McDonnell Center for the Space SciencesWashington UniversitySt. Louis, MO 63130
USA
We use thermodynamic calculations to model atmospheric chemistry on terrestrial exoplanets that are hot enough for chemical equilibria between the atmosphere and lithosphere, as on Venus. The results of the calculations place constraints on abundances of spectroscopically observable gases, the surface temperature and pressure, and the mineralogy of the planetary surface
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Mineral Buffer Reactions• Co-existing minerals control (buffer) gas
partial pressures – single unique gas pressure at each temperature, e.g.
CaCO3 + SiO2 = CaSiO3 + CO2 (gas)
Calcite Quartz Wollastonite
log10 PCO2 = log10 Keq = 7.97 – 4456 / T
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CQW Buffer for CO2
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Venus - H2O buffer
KMg2Al3Si2O10(OH) 2 =
MgAl2O4 + MgSiO3 + KAlSiO4 + H2O
Eastonite – Spinel – Enstatite – Kalsilite log10 K = −0.782 + 78,856 / T
XH2O = 30 ppm
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Venus - HCl buffer2 HCl + 8 NaAlSi3O8 = 2Na4[AlSi3O8]3Cl +
Al2SiO5 + 5 SiO2 + H2O
Albite – Scapolite marialite – Andalusite – Quartz
log10 XHCl = 4.216 - 7,860 / T
XHCl = PHCl / PT
PT = 92.1 bars
XH2O = 30 ppm
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Albite – Scapolite marialite – Andalusite – Quartz
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Venus - HF buffer2 HF + NaAlSiO4 + 2 CaMgSi2O6 + Mg2SiO4
+ MgSiO3 = NaCa2Mg5Si7AlO22F2 + H2O
Nepheline – Diopside – Forsterite – Enstatite – Fluor-edenitelog10 XHF = 0.2214 - 6,426 / T
XHF = PHCl / PT
PT = 92.1 bars
XH2O = 30 ppm
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Nepheline – Dolomite – Forsterite – Enstatite – Fluor-edenite
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Venus
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Hot exo-Venus - CO2 buffer
MgCO3 + MgSiO3 = Mg2SiO4 + CO2
Magnesite – Enstatite – Forsterite log10 PCO2 = log10 K =8.85 – 4903 / T
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Hot exo-Venus - H2O buffer
2 KMg3AlSi3O10(OH) 2 =
3 MgSi2O4 + KAlSi2O6 + KAlSiO4 + 2H2O
Phlogopite – Forsterite – Leucite – Kalsilite log10 PH2O = 9.50 – 7,765 / T
XH2O = 1000 ppm
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Hot exo-Venus - HCl buffer12 HCl + 6 CaSiO3 + 5 Na4[AlSiO4]3Cl =
17 NaCl + 6 CaAl2Si2O8 + 3 NaAlSi3O8
+ 6 H2O
Wollastonite – Sodalite – Halite – Anorthite - Albite
log10 XHCl = −1.1406 – 4,115 / T
PCO2 = 439.4 bars
XH2O = 1000 ppm
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Hot exo-Venus - HF buffer2 HF + KAlSi3O8 + 3 Mg2SiO4 =
KMg3AlSi3O10F2 + 3 MgSiO3 + H2O
Microcline –Forsterite – Fluor-phlogopite – Enstatite
log10 XHF = 0.2936 – 6,657 / T
PT = 439.4 bars
XH2O = 1000 ppm
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Hot Exo-Venus
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Cool exo-Venus #1 - H2O buffer
Ca2Mg5Si8O22(OH) 2 =
3 MgSiO3 + 2 CaMgSi2O6 + SiO2 + H2O
Tremolite – Enstatite – Diopsdie – Quartz log10 PH2O = 8.05 – 6,742 / T
XH2O = 100 ppm
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Cool exo-Venus #1 - HCl buffer2 HCl + 8 NaAlSi3O8 = 2Na4[AlSi3O8]3Cl +
Al2SiO5 + 5 SiO2 + H2O
Albite – Scapolite marialite – Andalusite - Quartz
log10 XHCl = 4.6418 − 7,860 / T
PCO2 = 43.29 bars
XH2O = 100 ppm
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Cool exo-Venus #1 - HF buffer2 HF + NaAlSiO4 + 2 CaMgSi2O6 +
3 MgSiO3 = NaCa2Mg5Si7AlO22F2 +
SiO2 + H2O
Nepheline – Diopside –Enstatite –
Fluor-edenite – Quartzlog10 XHF = 0.6218 − 6,049 / T
PT = 43.29 bars
XH2O = 100 ppm
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Cool Exo-Venus #1
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Cool exo-Venus #2 - CO2 buffer
CaMg(CO3)2 + 4 MgSiO3 = 2 Mg2SiO4 + CaMgSi2O6 + 2 CO2
Dolomite – Enstatite – Forsterite – Diopsidelog10 PCO2 = log10 K = 8.52 – 4,511 / T
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Cool exo-Venus #2 - H2O buffer
2 KMg3AlSi3O10(OH) 2 =
3 MgSi2O4 + KAlSi2O6 + KAlSiO4 + 2H2O
Phlogopite – Forsterite – Leucite – Kalsilite log10 PH2O = 9.50 – 7,765 / T
XH2O = 100 ppm
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Cool exo-Venus #2 - HCl buffer2 HCl + 9 NaAlSiO4 = Al2O3 + NaAlSi3O8 +
2Na4[AlSiO4]3Cl + H2O
Albite – Scapolite marialite – Andalusite - Quartz
log10 XHCl = 3.9719 − 8,075 / T
PCO2 = 41.33 bars
XH2O = 100 ppm
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Cool exo-Venus #2 - HF buffer2 HF + KAlSi3O8 + 3 Mg2SiO4 =
KMg3AlSi3O10F2 + 3 MgSiO3 + H2O
Microcline – Forsterite – Fluor-phlogopite – Enstatite
log10 XHF = 0.3069 – 6,657 / T
PT = 43.29 bars
XH2O = 100 ppm
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Cool exo-Venus #2
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H2O buffersKMg2Al3Si2O10(OH) 2 = MgAl2O4 + MgSiO3 + KAlSiO4
+ H2OEastonite – Spinel – Enstatite – Kalsilite
log10 PH2O = log10 K = −0.782 + 78,856 / T
2 KMg3AlSi3O10(OH) 2 = 3 MgSi2O4 + KAlSi2O6 + KAlSiO4 + 2H2O
Phlogopite – Forsterite – Leucite – Kalsilite log10 PH2O = ½ log10 K = 9.50 – 7,765 / T
Ca2Mg5Si8O22(OH) 2 = 3 MgSiO3 + 2 CaMgSi2O6 + SiO2 + H2O
Tremolite – Enstatite – Diopsdie – Quartz log10 PH2O = log10 K = 8.05 – 6,742 / T
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Planet P (bars) T (K) Minerals
Venus 92 740ab, and, ca, di, east, en, f-ed, fo, kls, neph, qtz, sp, sod, wo
Hot exo-Venus
439 790ab, an, en, f-phl, fo, ha, kls, leu, mc, mg, phl, sod, wo
Cool exo-Venus #1
43 647ab, and, ca, di, do, en, f-ed, fo, neph, qtz, sc-m, trem
Cool exo-Venus #2
41 653ab, co, di, do, en, f-phl fo, kls, leu, mc, neph, phl, sod
Ab-albite, an-anorthite, and-andalusite, ca-calcite, co-corundum, di-diopside, do-dolomite, east-eastonite, en-enstatite, f-ed-fluor-edenite, f-phl-fluor-phlogopite, fo-forsterite, ha-halite, kls-kalsilite, leu-leucite, mc-microcline, mg-magnesite, neph-nepheline, phl-phlogopite, qtz-quartz, sc-m-scapolite marialite, sod-sodalite, sp-spinel, trem-tremolite, wo-wollastonite
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Summary
• Spectroscopic observations of CO2, H2O, HCl, HF give information on surface T, P, mineralogy for exoplanets analogous to Venus
• CO – product of CO2 photolysis, its abundance does not constrain surface conditions
• SO2, H2S, OCS, S1-8 – similar problems due to photochemical gain/loss
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Venus