Key Atmospheric Chemistry in Rocky Planet Atmospheres

Transcription

1 Key Atmospheric Chemistry in Rocky Planet Atmospheres John Lee Grenfell Department of Exoplanets and Atmospheres (EPA) German Aerospace Centre (DLR) Berlin

2 Overview Processes affecting Atmospheric Chemistry on Rocky Planets Atmospheric Photochemistry Some Basics Atmospheric Photochemistry of Venus-Earth-Mars compared Earth-like Biosignatures (Hot) Super-Earths and Mini-Neptunes Conclusions

3 Processes affecting Atmospheric Chemistry on Rocky Planets Photons Protection Delivery Escape Clouds Gas-phase reactions Gas-aerosol reactions Surface OCEAN Biology Volcanism

4 Processes affecting Atmospheric Chemistry on Rocky Planets Photons Protection Delivery Escape Clouds Gas-phase reactions Gas-aerosol reactions Surface OCEAN Biology Volcanism

5 Atmospheric Photochemistry Some Basics AB+M A+B+M Thermal decomposition Comment e.g. N 2 O 5 (1.0 ev) ~300K CO (11.1 ev) >1000K AB+hv Photolysis Rate= F(λ)σ(Τ,λ)φ dλ A+B C+ D Bimolecular Rate=k[A][B] [k=aexp(-e a /kt) Arrhenius Equation] (A=collision frequency, E a =activation energy) A+B+M AB+ C Termolecular Rate=k[A][B][M] k=f(t,p,bathgas) e.g. O 2 +hv O+O Most σ s known <400K (in JPL Evaluation 17) About half of (JPL) Earth-stratosphere (H-N-O) k-values not (well) known >800K k=f([m])at p<p lower k saturates at p>p upper k depends on bathgas (k s for e.g. CO 2, H 2 bathgases not well known)

6 Photochemistry of Venus Earth Mars compared

7 Discovery Timeline of Gases on Venus-Earth-Mars 1932 CO 2 (Adams) 1968 CO (Connes) 1970s N 2 (Pioneer) 2011 O 3 (Montmessin) 1756 CO 2 (Black) 1770s O 2 (Priestley, Scheele) N 2 (Rutherford) N 2 O (Priestley) CH 4 (Volta) 1839 O 3 (Schönbein) 1894 Noble gases (Rutherford) 1952 CO 2 (Kuiper) 1969 CO (Kaplan) 1971 O 3 (Barth) 1972 O 2 (Barker) 1977 N 2 (Owen)

8 Venus-Earth-Mars Photochemical Composition (vmr) 1.E+00 1.E-01 1.E-02 1.E-03 1.E-04 1.E-05 1.E-06 1.E-07 1.E-08 1.E-09 Venus Earth Mars 1.E-10 CO2 O2 O3 N2 H2O 2 CO O 2 O 3 N 2 H 2 O SOx HCl D/H References: Earth, CO@1mb (Minschwaner et al., 2013); H 2 O@1mb (Randall, 1998); HCl@29km (May and Webster, 1989); D/H; Mackwell, 2013; Venus, O (Montmessin et al., 2011); CO@90km (Svedhem et al., 2007); Mars, SAMS (Rover) 2013; Owen (1977); Barth (1974); SO 2 (lower limit) Encrenaz et al. (2011); HCl (lower limit) Hartogh et al. (2013)

9 Summary: Venus Earth Mars Atmospheres No oceans Thick CO 2 High chlorine High sulphur Oceans CO 2 in minerals Chlorine in ocean Sulpur in ocean No Oceans Thin CO 2

10 Photochemistry of CO 2 Atmospheres Venus Cycles too slow -missing reactions? -wrong rates? (Mills and Allen 2007) CO 2 Stability Problem: CO 2 + hv CO + O Question: What regenerates CO 2? Answer: (Numerous) Catalytic Cycles Example Cycle: CO+OH CO 2 +H H+O 2 +M HO 2 +M O+HO 2 O 2 +OH Net: CO+O CO 2 (e.g. Parkinson and Hunten, 1972) Mars Cycles too fast -missing het. chem? (Atreya and Gu, 1994)

11 Results: Catalytic Cycles in Mars Atmosphere Method: Chemical Column model Pathway Analysis Program Cycles (Stock et al., 2012; Icarus )

12 Results: Catalytic Cycles in Mars Atmosphere Method: Chemical Column model Pathway Analysis Program Cycles (Stock et al., 2012; Icarus )

13 Atmospheric Chemistry of Biosignatures

14 Remote Atmospheric Biosignatures Species Biotic Source Abiotic Source Oxygen Ozone Nitrous Oxide Cyanobacteria (De)nitrifying Bacteria CO 2 +hv H 2 O+hv photochemistry Methane Methanogens geology Chloromethane Seaweed photochemistry CFCs Humans none

15 Atm. O 2 and O 3 Biosignatures and Caveats O 2 produced by life and O 3 layer produced by O 2 Caveats 1) Abiotic Sources e.g. Segura; Wordsworth; Goldman: (CO 2 +hv, H 2 O +hv) - produce O 2, O 3 2) Triple Signature Selsis et al. (2002): search for [O 3 -CO 2 -H 2 O] - more robust than O 3 alone Simoncini et al. (2013): Chemical disequil. (CH 4 -O 2 ) needs ~0.7 TW (Earth) 3) Spectral effects von Paris et al. (2011): O 3 band overlapping effects Kitzmann et al. (2011): Cloud effects von Paris et al. (2013): Spectral Retrieval degeneracies

16 Methane* Sources and Sinks (Earth) Hydrocarbons Soils *~95% Emissions (Earth) are biogenic (IPCC) adapted from Grenfell et al. PSS 55 (2007)

17 Nitrous Oxide Sources and Sinks (Earth) N 2 +O 1 D ( ) adapted from Grenfell et al. PSS 55 (2007)

18 Ozone Sources and Sinks (Earth) O 3 See-Saw Sinks Reservoirs adapted from Grenfell et al. PSS 55 (2007)

19 Atmospheric Chemistry Modelling Studies Earth Moving the Earth Changing the star Effect of Cosmic Rays Kasting Group Rauer Group Kaltenegger Group Seager Group and others GJ1214b (Kreidberg et al., 2014) (Valencia et al., 2013) Fortney et al. (2013) Miller-Ricci et al. (2012) Super-Earth 3g 10Me Atmospheres Interiors Biology Geology

20 Moving the Earth over the Complex Life HZ (T surf 0-30 o C)* *Dole (1964) dotted=outer, dashed=central, solid=inner Grenfell et al. PSS (2007)

21 Moving the Earth over the Complex Life HZ (T surf 0-30 o C)* *Dole (1964) dotted=outer, solid=inner Grenfell et al. PSS (2007)

22 Moving the Earth over the Complex Life HZ (T surf 0-30 o C)* *Dole (1964) dotted=outer, solid=inner Grenfell et al. PSS (2007)

23 Moving the Earth over the Complex Life HZ (T surf 0-30 o C)* *Dole (1964) dotted=outer, solid=inner Grenfell et al. PSS (2007)

24 Moving the Earth over the Complex Life HZ (T surf 0-30 o C)* *Dole (1964) dotted=outer, solid=inner Grenfell et al. PSS (2007)

25 Changing the Star of the Earth Chemical Formation of Atmospheric Ozone Earth around the Sun Earth in HZ of M5 Dwarf 60km 0km Chapman Ozone (from O 2 photolysis) Smog Ozone (from hydrocarbons, NOx and weak UV) 0km Smog Ozone rate (molecule cm -3 s -1 ) rate (molecule cm -3 s -1 ) Grenfell et al. Astrobiology (2013)

26 Effect of varying Stellar UV upon Planetary Biosignatures M7 blackbody Grenfell et al. PSS (2014) Nitrous Oxide (N 2 O)

27 Effect of Cosmic Rays in Earth-like (N 2 -O 2 ) Atmospheres

28 Modelling Chemical Effects of Cosmic Rays (likely important for Earth-like planets in HZ of M-stars) Top-of-Atmosphere CR Proton Spectrum Cosmic Ray Air Shower Secondary electrons destroy N 2, O 2 NOx (HOx) Atmospheric Climate Chemical 1D Model T,p,c Theoretical spectra Grenfell et al. (2007); Grenfell et al. (2012); Tabataba-Vakili et al., (2014)

29 Effect of Cosmic Rays (CRs) on Atmospheric Composition for Earth-like Planet in HZ of M-Dwarf NO and N produced Tabataba-Vakili et al., in prep. (2014) NO only produced Observations Grenfell et al., (2012) More penetration (higher E)

30 Atmospheric Spectrum inactive star flaring star O 3 (9.6µm) band is destroyed for the flaring case because cosmic rays lead to high NOx which removes O 3 Grenfell et al., Astrobiology, 12, (2012) (but see Tabataba-Vakili et al. in prep)

31 Possible Super-Earth (mini Neptune) Atmospheres Hu et al. ApJ 784 (2014)

32 C 2 H 2 as precursor for hazes on GJ1214b? Flux Eddy mixing: Solid=10 8 cm 2 s -1 Dashed=10 10 cm 2 s -1 Metallicity: x50 Fortney et al. ApJ (2013)

33 A Cosmic Ray Ion Pathway to C 2 H 2 hence Hazes on mini Neptunes? P (bar) Free floating planet with solar metallicity. Chemistry network <800K; Harada et al CRs Rimmer, Helling and Bilger (2012)

34 Three (Hot) Super-Earth Atmosphere Compositions 100s of bar H 2 O Early Earth before ocean formation Catalytic Cycles? jh 2 O, k H2O bathgas? 100s of bar H 2 Primary Envelope Lammer et al. (2014) Chemistry? Catalytic Cycles? k H2 bathgas? 100s of bar CO 2 Venus like Stable? (HOx, NOx) k CO2 bathgas?

35 Effect of (550<T<800K) region on rate coefficients. PS6.1-EGU Example: k O3 for reaction: [O+O 2 +M O 3 +M] for three bath gases k O3 (cm 6 s- 1 ) 2E E-34 He Ar N2 1.6E E E-34 1E-34 8E-35 6E-35 4E-35 2E-35 k O3 is bath-gas dependent T (K) Hippler et al. (1990)

36 Conclusions Atmospheric chemistry interacts with numerous processes in biology, geology, interior etc. in terrestrial atmospheres Comparing Venus-Earth-Mars suggest that losing water could favour CO 2 -dominated atmospheres with potentially complex photochemistry Earth-like modeling parameter studies: e.g. move the Earth, change the star then investigate e.g. effect upon biosignatures Understanding (Hot) Super-Earth atmospheres requires improved photochemical data (sigmas, rate coefficients) and comparison of databases e.g. from Earth Science, Combustion Science, Astronomy.