1. Characterizing exoplanets’ atmospheres and surfaces
Thérèse Encrenaz
LESIA, Observatoire de Paris
Pathways Toward Habitable Planets
Barcelona, September 2009

2. Outline The planetary zoo Rocky Exoplanets (warm)
Spectral variations with spectral type, RH, abundances
Atmosphere: constraints on resolving power
Surface: mineralogy, Red Vegetation Edge
Icy Exoplanets (cold)
Atmosphere: constraint on R
Surface: ices
Giant Exoplanets (from hot to very cold)
Atmosphere: importance of thermal profile, constraint on R
Conclusions

3. Spectroscopy of an exoplanet
Reflected starlight component (UV, visible, near-IR)
Albedo is about 0.3 for most of solar-system planets
Absorption lines or bands in front of stellar blackbody
Thermal component (IR, submm & mm)
Mostly depends upon the temperature of the emitting region
Emission lines in the stratosphere, absorption lines in the troposphere (function of T(P))
Fluorescence emission (UV, visible, near-IR)
Emission lines in the upper atmospheres (H, H2, N2, radicals)
The IR range is best suited for probing exoplanets’ neutral atmospheres

4. The Solar System: A planetary zoo
Planets with an atmosphere
Rocky planets (warm)
Mars-type (CO2, N2 + H2O) No stratosphere
Earth-type (N2, O2 + H2O) Stratosphere (O3)  
Icy planets (cold)
Titan-type (N2, CH4 + CO) Stratosphere (hydrocarbons, nitriles)
Giant planets (cold to very cold)
Jupiter-type (H2, CH4, NH3 +H2O) Stratosphere (hydrocarbons)
Neptune-type (H2, CH4) « 
Bare planets
Mercury/asteroid-type (refractories)
TNO-type (ices)

5. What kind of exoplanet can we expect? [F*/D2](1-a) = 4  Te4
Te (K)
Stellar distance (AU)
(solar-type star)
Small Exoplanet < ROCKY PLANETS > <ICY PLANETS >
(WARM) (COLD)
( ME) No atmosphere Atmosphere Atmosphere
(Mercury-type) N2, CO2, CO, H2 N2, CH4(+CO)
(Mars-Venus type) hydrocarbons, nitriles
if O2 -> O3 (Earth-type) (Titan-type)
STRATOSPHERE STRATOSPHERE
Giant Exoplanet <PEGASIDES>< GASEOUS GIANTS > <ICY GIANTS>
(HOT) (WARM) (COLD)
( ME) Atmosphere Atmosphere Atmosphere
H2,CO,N2,H2O H2,CH4,NH3,H2O H2,CH4
hydrocarbons hydrocarbons
(Jupiter-type) (Neptune-type)
STRATOSPHERE STRATOSPHERE

6. Variations of asterocentric distances with the stellar type
Te (K)
Stellar type A
(T=10000 K) F
(T=7000 K) G
(T=5700 K) K
(T=4200 K) M
(T=3200 K) HZ

7. However, this is not so simple! Why?
Other parameters are involved:
Albedo -> effect on Te
Rotation period -> effect on Te
Phase-locked planets -> strong day/night contrasts
Possible greenhouse effect -> may increase Ts vs Te
Earth: 15 K; Venus: over 200 K
Obliquity
Atmospheric dynamics -> may change day/night contrasts
Magnetic field -> may prevent atmospheric escape
Migration is possible!

8. Rocky Planets The IR spectrum of Mars (ISO-SWS) Ps = 6 mb
CO2
CO2
H2O CO
Hydrated
silicates
CO2
CO2
Spectral signatures: CO2, H2O, CO (+ traces H2O2, CH4)
Lellouch et al., 2000

9. Variation of a Mars-type spectrum as a function
of the stellar type (D = 1 UA)
Te (K)
476
346
273
174
Stellar
Type A
(10000 K) F
(7000 K) G
(5700 K) K
(4200 K)

10. Variation of a Mars-type spectrum as a function
of the asterocentric distance D
(solar-type star)
D = 0.07, 0.1, 0.3, 1.0 UA
Te = 1000, 863, 496, 273 K
NB: For small D, the reflected component dominates
-> Atmospheric signatures mostly in absorption

11. Variation with atmospheric composition:
H2O-dominated (Earth-like) spectrum
(above clouds)
H2O H2O
H2O
ice
CO2
H2O CO2
Pcl = 10 mb

12. The infrared spectrum of the Earth as seen
by the NIMS instrument aboard Galileo
(Earth flyby, December 1990)
Drossart et al., 1993

13. The thermal spectrum of telluric planets
Venus
Earth
Mars
Hanel et al., 1992

14. Thermal spectra of rocky planets
Earth
R=70,10,5
Earth
Mars
Venus
Resolving power required :
CO2 = 3 mm R = 3
O3  = 1 mm R = 10
CH4  = 0.15mm R = 50

15. Solid signatures in rocky planets
Reflected spectrum: H2O ice 1.25, 1.5, 2.0 m
Silicates 1.0, 2.0 m (broad)
Ferric oxides 1.0 m
Carbonates 2.35, 2.5 m
Hydrated silicates mm (broad)
Thermal spectrum:
Mid-latitudes
Tsurf > Tatm
silicates
Polar cap
Tsurf < Tatm
H2O ice
Silicates: cm-1(broad)
Water ice: cm-1 (broad)
Hanel et al., 1992

16. The Red Vegetation Edge (Earth spectrum)
Seager et al. 2005

17. RVE : Earthshine observations
Problems:
-partial coverage of
the vegetation
-clouds (20-30% of
the disk)
Seager et al. 2005

18. The reflected spectrum of a CH4-dominated planet (icy or giant)
Larson, 1980

19. The atmosphere of an icy planet:
The thermal component
Titan - SWS: CH4, hydrocarbons, nitriles
C2H6
HCN,
C2H2
CH4
Resolving power required: R > 5 ( C2H2-C2H6) ; R > 10 (CH4)

20. Solid signatures on icy planets
H2O ice H2O, CH4, CO, N2
(Ganymede) (Pluto)

21. The atmosphere of two gaseous giants: The thermal component
Jupiter & Saturn - ISO-SWS
Jupiter
Saturn
CH3D, PH NH3
C2H6
PH3
CH4
NB: Jupiter and Saturn are VERY different!

22. Jupiter -SWS The 6-12-mm range: CH4, CH3D, C2H6, NH3, PH3
Resolving power required:
- for NH3 detection: R > 100
- for CH4 detection: R > 150
-for C2H6 detection: R > 20

23. The atmosphere of an icy giant
Neptune - SWS The 2-18-m range: CH4, CH3D, C2H2, C2H6
CH C2H C2H2
Resolving power required: R > 5 ( C2H2-C2H6) ; R > 10 (CH4)

24. In summary…
The diversity in solar-system bodies opens the same possibilities for exoplanets
A resolving power higher than 10 is required for the identification of major gaseous and solid signatures
In the thermal range, hydrocarbons (C2H2, C2H6) are easier to detect than methane
Knowing the thermal structure is essential for interpreting thermal spectra
No stratosphere expected for Rocky Exoplanets (N2, CO2, H2O) except if O2 is present
A stratosphere is expected for Icy Exoplanets (N2, CH4) and Giant Exoplanets (H2, CH4,…)