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.

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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



Venus
Venus Fluor-edenite


Hot exo venus co 2 buffer
Hot exo-Venus - CO Fluor-edenite2 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 - H Fluor-edenite2O 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 Fluor-edenite

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 Fluor-edenite

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


Hot exo venus
Hot Exo-Venus Fluor-edenite


Cool exo venus 1 h 2 o buffer
Cool exo-Venus #1 - H Fluor-edenite2O 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 Fluor-edenite

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 Fluor-edenite

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 1
Cool Exo-Venus #1 Fluor-edenite


Cool exo venus 2 co 2 buffer
Cool exo-Venus #2 - CO Fluor-edenite2 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 - H Fluor-edenite2O 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 Fluor-edenite

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 Fluor-edenite

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


Cool exo venus 2
Cool exo-Venus #2 Fluor-edenite


H 2 o buffers
H Fluor-edenite2O 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 Fluor-edenite

  • 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


Venus1
Venus Fluor-edenite


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