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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.

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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

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

mineral buffer reactions
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

venus h 2 o buffer
Venus - H2O buffer

KMg2Al3Si2O10(OH) 2 =

MgAl2O4 + MgSiO3 + KAlSiO4 + H2O

Eastonite – Spinel – Enstatite – Kalsilite

log10 K = −0.782 + 78,856 / T

XH2O = 30 ppm

venus hcl buffer
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

venus hf buffer
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

hot exo venus co 2 buffer
Hot exo-Venus - CO2 buffer

MgCO3 + MgSiO3 = Mg2SiO4 + CO2

Magnesite – Enstatite – Forsterite

log10 PCO2 = log10 K =8.85 – 4903 / T

hot exo venus h 2 o buffer
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

hot exo venus hcl buffer
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

hot exo venus hf buffer
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

cool exo venus 1 h 2 o buffer
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

cool exo venus 1 hcl buffer
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

cool exo venus 1 hf buffer
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

cool exo venus 2 co 2 buffer
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

cool exo venus 2 h 2 o buffer
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

cool exo venus 2 hcl buffer
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

cool exo venus 2 hf buffer
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

h 2 o buffers
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

summary
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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