Equilibrium Chemistry of the Atmospheres of Hot Earth-like

Exoplanets

Laura SchaeferKatharina LoddersBruce Fegley, Jr

Introduction• Currently, there are several hundred known exoplanets

– 24 super-Earths (M < 10MEarth)– Number of known exoplanets should go up considerably in

February (Kepler releases data).• The Kepler and CoRot missions are dedicated space

telescopes that are looking for transiting exoplanets• Some of the discovered exoplanets, such as CoRot-7b,

are very hot• Here we report results for an Earth-like planet hot

enough to vaporize its crust• Our results will be useful for spectroscopic studies of the

atmospheres of hot super-Earths

Methods• Chemical equilibrium

calculations with a Gibbs energy minimization code– bulk system has

abundances of the terrestrial continental crust

– 1200 gases, 400 solid compounds included

• T= 500 – 4000 K• P = 10-6 – 10+2.5 bars

– Results here are for 100 bars unless otherwise stated

Continental Crust CompositionWedepohl (1995)

Element Wt% Element Wt%O 47.20 Ti 0.401

Si 28.80 C 0.199

Al 7.96 P 0.076

Fe 4.32 Mn 0.072

Ca 3.85 S 0.070

Na 2.36 F 0.053

Mg 2.20 Cl 0.047

K 2.14 H 0.045

Major Gas ChemistryVolatilesH, C, N, O, S

LithophilesNa, K, Fe, Mg, Si, Ti, Ca, Al

Major Gas Chemistry• Molecular N2 is the major

gas below ~500 K• From 500 – 3400 K, the

major gas is H2O– CO2 is second most

abundant gas

• Molecular O2 is major gas from 3400 – 3900 K

• At higher temperatures, SiO gas is the most abundant gas

Major Gas ChemistryElement Major Gases

H H2O (300 – 3950 K)

OH (3950 – 4000 K)C CO2 (300 – 3950 K)

CO (3950 – 4000 K )N N2 (300 – 2850 K)

NO (2850 – 4000 K)O H2O (300 – 3400 K)

O2 (3400 – 3950 K)

SiO (3950 – 4000 K)S SO2 (300 – 4000 K)

Major Gas Chemistry

• Lithophile gases are dominated by Na and K below ~3600 K– Halides (F,Cl) +

Hydroxides• Above ~3600 K, SiO

and SiO2 are the most abundant lithophile gases

LithophilesNa, K, Fe, Mg, Si, Ti, Ca, Al

Major Gas ChemistryElement Major Gas

Si SiO2 (500-3350 K)

SiO (3350-4000 K)Fe FeCl2 (500 – 1900 K)

Fe(OH)2 (1900 – 3650 K)

FeO (3650 – 4000 K)Na NaCl (500 – 2900 K)

NaOH (2900 – 3700 K)Na (3700 – 4000 K)

Mg Mg(OH)2 (500 – 3650 K)

MgO (3650 – 4000 K)K KCl (500 – 2750 K)

KOH (2750 – 3950 K)K (3950 – 4000 K)

Major Gas ChemistryElement Major Gas

Ti TiO2 (500 – 4000 K)

Al(Xi < 10-3)

FAl(OH)2 (300-2600 K)

Al(OH)3 (2600 – 3200 K)

FAlO (3200 – 3950 K)AlO (3950 – 4000 K)

Ca(Xi < 10-5)

CaF2 (500 – 2200 K)

CaFOH (2200 – 3050 K)Ca(OH)2 (3050 – 4000 K)

Condensates• Graph shows the

gas/condensed phase mole ratio for different total pressures– Higher ratio = more gas

present• At high pressure,

condensed phases persist to very high temperature

% of element in gas at

4000 K, 100 barsNa 11.2%K 49.7%Fe 4.5%Mg 1.3%Al 0.3%Ca ~0%Ti 38.7%Si 9.8%

Condensates• Graph shows the

gas/condensed phase mole ratio for different total pressures– Higher ratio = more gas

present• At high pressure,

condensed phases persist to very high temperature

• At low pressure, complete evaporation occurs at lower temperatures

Temperature of 100% evaporation

P = 10-6 barsNa 1550 KK 1550 KFe 1700 KMg 1850 KAl 2200 KCa 2150 KTi 1800 KSi 2000 K

Summary• Continental crust produces an H2O + CO2

atmosphere over a broad temperature range at 100 bars

• At very high temperatures O2 and SiO gas dominate– Alkali gases (e.g., KOH, KCl, NaOH, NaCl) are very

abundant • In future work, we will explore differences in gas

chemistry for a variety of interesting compositions– Oceanic crust, Bulk Silicate Earth, Moon, meteoritic

compositions, etc.– Any requests?

Major Gas SummaryH2O

CO2

N2 NO

H2O O2

SO2

SiO2 SiO

FAl(OH)2 Al(OH)3 FAlO

FeCl2 Fe(OH)2 FeO

CaF2 CaFOH Ca(OH)2

NaCl NaOH Na

Mg(OH)2 MgO

KCl KOH

TiO2

OH

CO

SiO

AlO

K

Vola

tiles

Lith

ophi

les