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. CaCO 3 + SiO 2 = CaSiO 3 + CO 2 (gas) Calcite Quartz Wollastonite log 10 P CO2 = log 10 K eq = 7.97 – 4456 / T

3. CQW Buffer for CO 2

4. 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

5. 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

6. Albite – Scapolite marialite – Andalusite – Quartz

7. 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

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

9. Venus

10. 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

11. 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

12. 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

13. 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

14. Hot Exo-Venus

15. 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

16. 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

17. 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

18. Cool Exo-Venus #1

19. 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

20. 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

21. 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

22. 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

23. Cool exo-Venus #2

24. 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

26. 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

27. 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

28. Venus