Atmospheres of Hot Terrestrial Exoplanets

1. Atmospheres of Hot Terrestrial Exoplanets Laura Schaefer and Bruce Fegley, Jr. Planetary Chemistry Laboratory Department of Earth and Planetary Sciences Washington University, St. Louis, MO http://solarsystem.wustl.edu

2. What do we expect? • Extreme heating over a planet’s lifetime may lead to loss of volatiles • Venus is in a shorter orbit than the Earth and has lost its water content due to stellar heating and runaway greenhouse effect • Mercury: lost all volatiles (if it formed with any) • Jupiter’s moon Io is dominated by S, and may have lost lighter volatiles such as H, C, and N due to intense tidal heating • Expect a range of planets with variable volatile content • Silicate atmospheres (no volatiles!) • Super-Venus (CO2 atmosphere, trace water)

3. 2600 K 50 K R = 1.67R M = 4.8M ρ ~ 5,600 CoRoT-7b • Tidally-locked transiting planet with a = 0.017 AU (0.85 days) • Dayside temperature is hot enough to melt and vaporize rock • Planet may have a partial magma ocean • Volatiles could have been blown away from the atmosphere or condensed on the cold nightside • Results in a net loss of elements from the magma ocean • Atmosphere may be composed of rock-forming elements

5. Silicate Atmosphere Calculations • MAGMA code calculates composition of silicate atmosphere • vaporization of systems containing Si, Mg, Fe, Ca, Al, Ti, Na, K, and O • As a function of: • temperature (1500 – 3000 K) • mass-loss by isothermal fractional vaporization • Results give composition and total pressure • Fractional vaporization may simulate • removal of material from dayside to nightside on CoRot-7b or • Loss of material from the atmosphere • We also calculate the composition of clouds, which may alter the atmospheric composition

6. Model Planet Compositions

7. Temperature-dependent results • Graph shows initial results for the BSE (Fvap = 0) • Column density (PiNA/μg) is calculated for a planet the size of CoRoT-7b (g ~16.7 m/s2) • Na is the major gas at all temperatures • O2, O, and SiO are also very abundant

8. 2600 K 1800 K 2200 K Fractional Vaporization • Graph shows atmospheric composition as a function of fraction vaporized at constant temperature • Na is lost from system first, then K and Fe • SiO becomes major gas • Mg becomes more abundant than SiO at higher fractions vaporized • O and O2 maintain fairly constant abundance • Elements are lost less quickly as temperature increases

9. Silicate Clouds • We calculated cloud condensation temperatures (Tcond) for the atmosphere generated at 2200 K for the BSE model • Assumes dry adiabat and g ~ 36 m/s2 • Mg, Al, Si, Ca, and Fe may fall back to surface and be reincorporated in the magma ocean • Ti, Na, and K remain in atmosphere to high enough altitudes that they may either be transported to nightside or removed from atmosphere by stellar wind

10. Extended Na cloud • Large clouds of Na exist around Mercury and Jupiter’s moon Io • At Mercury, the Na cloud (~1011 cm-2) extends to ~23RMercury • At Io, the Na cloud (1010 - 1012 cm-2) extends to ~500RJupiter (~19,600RIo) • These clouds are very bright spectral features • An Earth-like exoplanet with a silicate atmosphere may have an extended Na cloud • Na is present in the atmosphere to high altitudes • may interact with stellar wind • A large Na cloud around a transiting planet like CoRoT-7b will occult more of the stellar disk than a closely bound atmosphere • Increases the probability of detection for a super-Earth • Na has already been detected in the atmospheres of several giant exoplanets (HD209458b, HD189733b)

11. Venus-like Exoplanets • Thick CO2 atmosphere, low H2O abundance • Either lost water, or never accreted water • Greenhouse effect = high surface T • Like Venus, may have surface-atmosphere equilibrium • Mineralogy of surface will buffer the abundances of gases in atmosphere • Abundances of several gases would help us narrow down possible mineralogy of the surface • CO2, H2O, HCl, and HF • CO difficult because of photochemistry • Sulfur-gases difficult due to condensation

12. 1. Identify a possible CO2 buffer.

13. Calcite - Quartz - Wollastonite CaCO3 + SiO2 = CaSiO3 + CO2 (g)

14. 2. Identify a possible H2O buffer.

15. 758 K 122 bars Eastonite – Spinel – Enstatite - Kalsilite KMg2Al3Si2O10(OH)2 = MgAl2O4 + MgSiO3 + KAlSiO4 + H2O XH2O = 30 ppm

16. Intersection of CO2 and H2O buffers depends on XH2O. Exact fit gives XH2O = 24 ppm.

17. Albite – Halite – Andalusite – Quartz 2HCl + 2NaAlSi2O6 = 2NaCl + Al2SiO5 + 3SiO2 + H2O 3. Identify possible HCl buffers. XHCl = 0.76 ppm

18. Leucite – Forsterite – Fluor-phlogopite –Enstatite 2 HF + KAlSi2O6 + 2Mg2SiO4 = KMg3AlSi3O10F2 + MgSiO3 + H2O 4. Identify possible HF buffers. XHF = 4.6 ppb

19. Albite Andalusite Calcite Diopside Eastonite Enstatite Fluor-phlogopite Forsterite Halite Kalsilite Leucite Nepheline Quartz Spinel Sodalite Wollastonite

20. To create theoretical exoplanets, find intersections of CO2 and H2O buffers. XH2O = 30 ppm Making a planet hotter than Venus is difficult without increasing XH2O significantly.

21. XH2O = 1000 ppm

22. Albite Anorthite Enstatite Fluor-phlogopite Forsterite Halite Kalsilite Leucite Microcline Magnesite Phlogopite Sodalite Wollastonite Magnesite – Enstatite – Forsterite MgCO3 + MgSiO3 = Mg2SiO3 +CO2 XH2O = 1000 ppm Wollastonite – Sodalite – Halite – Anorthite – Albite 12HCl + 6CaSiO3 + 5Na4[AlSiO4]3Cl = 17NaCl + 6CaAl2Si2O8 + 3NaAlSi3O8 + 6H2O Microcline – Forsterite – Fluor-phlogopite – Enstatite 2HF + KAlSi3O8 + 3Mg2SiO4 = KMg3AlSi3O10F2 + 3MgSiO3 + H2O Phlogopite – Forsterite – Leucite – Kalsilite 2KMg3AlSi3O10(OH)2 = 3Mg2SiO4 + KAlSi2O6 + KAlSiO4 + 2H2O

23. Summary • Planets like CoRoT-7b may have silicate atmospheres • May have extensive Na cloud • Heating over time may deplete planet in more volatile elements • Super-Venus exoplanets may have atmosphere-surface equilibria • Observations of gas abundances would allow an estimate of surface composition • Hot Venus is difficult to make without increasing H2O abundance signficantly • Cold Venus is much easier to make