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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. CaCO 3 + SiO 2 = CaSiO 3 + CO 2 (gas) Calcite Quartz Wollastonite log 10 P CO2 = log 10 K eq = 7.97 – 4456 / T
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CQW Buffer for CO 2
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Venus - H 2 O buffer KMg 2 Al 3 Si 2 O 10 (OH) 2 = MgAl 2 O 4 + MgSiO 3 + KAlSiO 4 + H 2 O Eastonite – Spinel – Enstatite – Kalsilite log 10 K = −0.782 + 78,856 / T X H 2 O = 30 ppm
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Venus - HCl buffer 2 HCl + 8 NaAlSi 3 O 8 = 2Na 4 [AlSi 3 O 8 ] 3 Cl + Al 2 SiO 5 + 5 SiO 2 + H 2 O Albite – Scapolite marialite – Andalusite – Quartz log 10 X HCl = 4.216 - 7,860 / T X HCl = P HCl / P T P T = 92.1 bars X H 2 O = 30 ppm
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Albite – Scapolite marialite – Andalusite – Quartz
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Venus - HF buffer 2 HF + NaAlSiO 4 + 2 CaMgSi 2 O 6 + Mg 2 SiO 4 + MgSiO 3 = NaCa 2 Mg 5 Si 7 AlO 22 F 2 + H 2 O Nepheline – Diopside – Forsterite – Enstatite – Fluor-edenite log 10 X HF = 0.2214 - 6,426 / T X HF = P HCl / P T P T = 92.1 bars X H 2 O = 30 ppm
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Nepheline – Dolomite – Forsterite – Enstatite – Fluor-edenite
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Venus
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Hot exo-Venus - CO 2 buffer MgCO 3 + MgSiO 3 = Mg 2 SiO 4 + CO 2 Magnesite – Enstatite – Forsterite log 10 P CO2 = log 10 K =8.85 – 4903 / T
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Hot exo-Venus - H 2 O buffer 2 KMg 3 AlSi 3 O 10 (OH) 2 = 3 MgSi 2 O 4 + KAlSi 2 O 6 + KAlSiO 4 + 2H 2 O Phlogopite – Forsterite – Leucite – Kalsilite log 10 P H 2 O = 9.50 – 7,765 / T X H 2 O = 1000 ppm
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Hot exo-Venus - HCl buffer 12 HCl + 6 CaSiO 3 + 5 Na 4 [AlSiO 4 ] 3 Cl = 17 NaCl + 6 CaAl 2 Si 2 O 8 + 3 NaAlSi 3 O 8 + 6 H 2 O Wollastonite – Sodalite – Halite – Anorthite - Albite log 10 X HCl = −1.1406 – 4,115 / T P CO2 = 439.4 bars X H 2 O = 1000 ppm
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Hot exo-Venus - HF buffer 2 HF + KAlSi 3 O 8 + 3 Mg 2 SiO 4 = KMg 3 AlSi 3 O 10 F 2 + 3 MgSiO 3 + H 2 O Microcline –Forsterite – Fluor-phlogopite – Enstatite log 10 X HF = 0.2936 – 6,657 / T P T = 439.4 bars X H 2 O = 1000 ppm
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Hot Exo-Venus
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Cool exo-Venus #1 - H 2 O buffer Ca 2 Mg 5 Si 8 O 22 (OH) 2 = 3 MgSiO 3 + 2 CaMgSi 2 O 6 + SiO 2 + H 2 O Tremolite – Enstatite – Diopsdie – Quartz log 10 P H 2 O = 8.05 – 6,742 / T X H 2 O = 100 ppm
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Cool exo-Venus #1 - HCl buffer 2 HCl + 8 NaAlSi 3 O 8 = 2Na 4 [AlSi 3 O 8 ] 3 Cl + Al 2 SiO 5 + 5 SiO 2 + H 2 O Albite – Scapolite marialite – Andalusite - Quartz log 10 X HCl = 4.6418 − 7,860 / T P CO2 = 43.29 bars X H 2 O = 100 ppm
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Cool exo-Venus #1 - HF buffer 2 HF + NaAlSiO 4 + 2 CaMgSi 2 O 6 + 3 MgSiO 3 = NaCa 2 Mg 5 Si 7 AlO 22 F 2 + SiO 2 + H 2 O Nepheline – Diopside –Enstatite – Fluor-edenite – Quartz log 10 X HF = 0.6218 − 6,049 / T P T = 43.29 bars X H 2 O = 100 ppm
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Cool Exo-Venus #1
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Cool exo-Venus #2 - CO 2 buffer CaMg(CO 3 ) 2 + 4 MgSiO 3 = 2 Mg 2 SiO 4 + CaMgSi 2 O 6 + 2 CO 2 Dolomite – Enstatite – Forsterite – Diopside log 10 P CO2 = log 10 K = 8.52 – 4,511 / T
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Cool exo-Venus #2 - H 2 O buffer 2 KMg 3 AlSi 3 O 10 (OH) 2 = 3 MgSi 2 O 4 + KAlSi 2 O 6 + KAlSiO 4 + 2H 2 O Phlogopite – Forsterite – Leucite – Kalsilite log 10 P H 2 O = 9.50 – 7,765 / T X H 2 O = 100 ppm
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Cool exo-Venus #2 - HCl buffer 2 HCl + 9 NaAlSiO 4 = Al 2 O 3 + NaAlSi 3 O 8 + 2Na 4 [AlSiO 4 ] 3 Cl + H 2 O Albite – Scapolite marialite – Andalusite - Quartz log 10 X HCl = 3.9719 − 8,075 / T P CO2 = 41.33 bars X H 2 O = 100 ppm
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Cool exo-Venus #2 - HF buffer 2 HF + KAlSi 3 O 8 + 3 Mg 2 SiO 4 = KMg 3 AlSi 3 O 10 F 2 + 3 MgSiO 3 + H 2 O Microcline – Forsterite – Fluor-phlogopite – Enstatite log 10 X HF = 0.3069 – 6,657 / T P T = 43.29 bars X H 2 O = 100 ppm
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Cool exo-Venus #2
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H 2 O buffers KMg 2 Al 3 Si 2 O 10 (OH) 2 = MgAl 2 O 4 + MgSiO 3 + KAlSiO 4 + H 2 O Eastonite – Spinel – Enstatite – Kalsilite log 10 P H 2 O = log 10 K = −0.782 + 78,856 / T 2 KMg 3 AlSi 3 O 10 (OH) 2 = 3 MgSi 2 O 4 + KAlSi 2 O 6 + KAlSiO 4 + 2H 2 O Phlogopite – Forsterite – Leucite – Kalsilite log 10 P H 2 O = ½ log 10 K = 9.50 – 7,765 / T Ca 2 Mg 5 Si 8 O 22 (OH) 2 = 3 MgSiO 3 + 2 CaMgSi 2 O 6 + SiO 2 + H 2 O Tremolite – Enstatite – Diopsdie – Quartz log 10 P H 2 O = log 10 K = 8.05 – 6,742 / T
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PlanetP (bars)T (K)Minerals Venus92740 ab, and, ca, di, east, en, f-ed, fo, kls, neph, qtz, sp, sod, wo Hot exo- Venus 439790 ab, an, en, f-phl, fo, ha, kls, leu, mc, mg, phl, sod, wo Cool exo- Venus #1 43647 ab, and, ca, di, do, en, f- ed, fo, neph, qtz, sc-m, trem Cool exo- Venus #2 41653 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
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Summary Spectroscopic observations of CO 2, H 2 O, HCl, HF give information on surface T, P, mineralogy for exoplanets analogous to Venus CO – product of CO 2 photolysis, its abundance does not constrain surface conditions SO 2, H 2 S, OCS, S 1-8 – similar problems due to photochemical gain/loss
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Venus
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