Chemical Models of Terrestrial Exoplanets

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Chemical Models of Terrestrial Exoplanets

• Bruce Fegley, Jr. and Laura Schaefer
• Planetary Chemistry Laboratory
• Department of Earth and Planetary Sciences
• McDonnell Center for the Space Sciences
• Washington University
• St. 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 buffer

• 2 HCl 8 NaAlSi3O8 2Na4AlSi3O83Cl 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 buffer

• 2 HF NaAlSiO4 2 CaMgSi2O6 Mg2SiO4 MgSiO3
  NaCa2Mg5Si7AlO22F2 H2O
• Nepheline Diopside Forsterite Enstatite
  Fluor-edenite
• log10 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 buffer

• 12 HCl 6 CaSiO3 5 Na4AlSiO43Cl
• 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 buffer

• 2 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 buffer

• 2 HCl 8 NaAlSi3O8 2Na4AlSi3O83Cl 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 buffer

• 2 HF NaAlSiO4 2 CaMgSi2O6
• 3 MgSiO3 NaCa2Mg5Si7AlO22F2
• SiO2 H2O
• Nepheline Diopside Enstatite
• Fluor-edenite Quartz
• log10 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 Diopside
• log10 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 buffer

• 2 HCl 9 NaAlSiO4 Al2O3 NaAlSi3O8
  2Na4AlSiO43Cl 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 buffer

• 2 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 buffers

• KMg2Al3Si2O10(OH) 2 MgAl2O4 MgSiO3 KAlSiO4
  H2O
• Eastonite 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 92 740 ab, and, ca, di, east, en, f-ed, fo, kls, neph, qtz, sp, sod, wo
Hot exo-Venus 439 790 ab, an, en, f-phl, fo, ha, kls, leu, mc, mg, phl, sod, wo
Cool exo-Venus 1 43 647 ab, and, ca, di, do, en, f-ed, fo, neph, qtz, sc-m, trem
Cool exo-Venus 2 41 653 ab, 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 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 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 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