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Chemical Models of Terrestrial Exoplanets

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.

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Chemical Models of Terrestrial Exoplanets

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  1. 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

  2. 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

  3. CQW Buffer for CO2

  4. Venus - H2O buffer KMg2Al3Si2O10(OH) 2 = MgAl2O4 + MgSiO3 + KAlSiO4 + H2O Eastonite – Spinel – Enstatite – Kalsilite log10 K = −0.782 + 78,856 / T XH2O = 30 ppm

  5. Venus - HCl buffer 2 HCl + 8 NaAlSi3O8 = 2Na4[AlSi3O8]3Cl + Al2SiO5 + 5 SiO2 + H2O Albite – Scapolite marialite – Andalusite – Quartz log10XHCl = 4.216 - 7,860 / T XHCl = PHCl / PT PT = 92.1 bars XH2O = 30 ppm

  6. Albite – Scapolite marialite – Andalusite – Quartz

  7. Venus - HF buffer 2 HF + NaAlSiO4 + 2 CaMgSi2O6 + Mg2SiO4 + MgSiO3 = NaCa2Mg5Si7AlO22F2 + H2O Nepheline – Diopside – Forsterite – Enstatite – Fluor-edenite log10XHF = 0.2214 - 6,426 / T XHF = PHCl / PT PT = 92.1 bars XH2O = 30 ppm

  8. Nepheline – Dolomite – Forsterite – Enstatite – Fluor-edenite

  9. Venus

  10. Hot exo-Venus - CO2 buffer MgCO3 + MgSiO3 = Mg2SiO4 + CO2 Magnesite – Enstatite – Forsterite log10 PCO2 = log10 K =8.85 – 4903 / T

  11. 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

  12. Hot exo-Venus - HCl buffer 12 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

  13. Hot exo-Venus - HF buffer 2 HF + KAlSi3O8 + 3 Mg2SiO4 = KMg3AlSi3O10F2 + 3 MgSiO3 + H2O Microcline –Forsterite – Fluor-phlogopite – Enstatite log10XHF = 0.2936 – 6,657 / T PT = 439.4 bars XH2O = 1000 ppm

  14. Hot Exo-Venus

  15. 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

  16. Cool exo-Venus #1 - HCl buffer 2 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

  17. Cool exo-Venus #1 - HF buffer 2 HF + NaAlSiO4 + 2 CaMgSi2O6 + 3 MgSiO3 = NaCa2Mg5Si7AlO22F2 + SiO2 + H2O Nepheline – Diopside –Enstatite – Fluor-edenite – Quartz log10XHF = 0.6218 − 6,049 / T PT = 43.29 bars XH2O = 100 ppm

  18. Cool Exo-Venus #1

  19. 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

  20. 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

  21. Cool exo-Venus #2 - HCl buffer 2 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

  22. Cool exo-Venus #2 - HF buffer 2 HF + KAlSi3O8 + 3 Mg2SiO4 = KMg3AlSi3O10F2 + 3 MgSiO3 + H2O Microcline – Forsterite – Fluor-phlogopite – Enstatite log10XHF = 0.3069 – 6,657 / T PT = 43.29 bars XH2O = 100 ppm

  23. Cool exo-Venus #2

  24. 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

  25. 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

  26. Venus

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