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

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

CQW Buffer for CO2

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 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

Albite – Scapolite marialite – Andalusite – Quartz

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

Nepheline – Dolomite – Forsterite – Enstatite – Fluor-edenite

Venus

Hot exo-Venus - CO2 buffer

MgCO3 + MgSiO3 = Mg2SiO4 + CO2

Magnesite – Enstatite – Forsterite log10 PCO2 = log10 K =8.85 – 4903 / T

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 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

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

Hot Exo-Venus

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 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

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

Cool Exo-Venus #1

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

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 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

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

Cool exo-Venus #2

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

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

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

Venus