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What makes a planet habitable? Helmut Lammer [ISSI Team evolution and characterization of habitable planets & research stimulated within Europlanet DWGs & IWF related bilateral projects] Austrian Academy of Sciences, Space Research Institute (IWF) Schmiedlstr. 6, A-8042 Graz, Austria (email: [email protected]) IWF/ÖAW GRAZ 1 A classification for habitats [Lammer et al., to be submitted to Astron. Astrophys. Rev., 2008] Water-rich bodies at the beginning Class. I E M V Venus -like Icy moons IWF/ÖAW GRAZ Evolutionary time line Classical habitable zone Class. II Inner and outer edge of the habitable zone or habitable zones of low mass stars Class. III Beyond the ice-line Earth-like Mars -like Habitats suitable for the evolution of higher life forms on the surface Microbial life may have evolved and habitats in subsurface, ice/H2O, may have remained Life forms may have evolved Europalike and populate subsurface H2O oceans 2 A classification for habitats [Lammer et al., to be submitted to Astron. Astrophys. Rev., 2008] Water-rich bodies at the beginning Class. I E M V Evolutionary time line Venus -like Icy moons Classical habitable zone Class. II Inner and outer edge of the habitable zone or habitable zones of low mass stars Class. III Beyond the ice-line Earth-like Mars -like Habitats suitable for the evolution of higher life forms on the surface Microbial life may have evolved and habitats in subsurface, ice/H2O, may have remained Life forms may have evolved Europalike and populate subsurface H2O oceans Migrating “super-Europa’s”, Class. IV IWF/ÖAW GRAZ Ice-rich exoplanets which migrate inside a habitable zone or “hot ice giants”, “Ocean planets” Lower or/and higher life may evolve but 2 Earth: A geodynamic active planet/habitat IWF/ÖAW GRAZ 3 Geophysical relevance of water: Convecting mantle Efficient cooling Large amount of ⇒ H2O in the mantle is Regassing important ! (Oceans) Volcanism Dynamo Action Subduction Crust Degassing Magnetosphere Shielding Hydros- + Cryosphere Biosphere Atmosphere From D. Breuer, DLR, Berlin IWF/ÖAW GRAZ 4 The magnetic protection of Earth´s IWF/ÖAW GRAZ 5 One plate planets (present Venus and Convecting mantle Very hot (dry) or frozen planets (inner and outer Inefficient cooling boundary of the Volcanism Dynamo Action classical habitable zone) Crust Degassing Magnetosphere Shielding Hydros- + Cryosphere Atmosphere Biosphere ? Space From D. Breuer, DLR, Berlin IWF/ÖAW GRAZ 6 One plate planets (present Venus and Convecting mantle Very hot (dry) or frozen planets (inner and outer Inefficient cooling boundary of the Volcanism Dynamo Action classical habitable zone) Crust Degassing Magnetosphere Shielding Hydros- + Cryosphere Atmosphere Biosphere ? Space From D. Breuer, DLR, Berlin IWF/ÖAW GRAZ 6 One plate planets (present Venus and Convecting mantle Very hot (dry) or frozen planets (inner and outer Inefficient cooling boundary of the Volcanism Dynamo Action classical habitable zone) Crust Degassing Magnetosphere Shielding Hydros- + Cryosphere Atmosphere Biosphere ? Space From D. Breuer, DLR, Berlin IWF/ÖAW GRAZ 6 One plate planets (present Venus and Convecting mantle Very hot (dry) or frozen planets (inner and outer Inefficient cooling boundary of the Volcanism Dynamo Action classical habitable zone) Crust Degassing Magnetosphere Shielding Hydros- + Cryosphere Atmosphere Biosphere ? Erosion by solar/stellar plasma flow Space From D. Breuer, DLR, Berlin IWF/ÖAW GRAZ 6 No magnetic atmosphere protection IWF/ÖAW GRAZ 7 Solar/stellar-planetary relations and their IWF/ÖAW GRAZ 8 The young Sun I (G stars): → total luminosity IWF/ÖAW GRAZ [Guinan and Ribas: ASP, 269, 85 – 107, 2002] 9 The young Sun/stars II: → solar/stellar winds [Ribas et al.: ApJ 622, 680 – 694, 2005] IWF/ÖAW GRAZ 10 The young Sun/stars II: → solar/stellar winds → Maximum expected → Average expected → Minimum expected ®Sudden cut-off ? [Ribas et al.: ApJ 622, 680 – 694, 2005] [Wood et al.: ApJ 574, 412 –425, 2002; Kulikov et al.: PSS, 54, 1425, 2006] IWF/ÖAW GRAZ 10 The young Sun/stars II: → solar/stellar winds → Maximum expected → Average expected → Minimum expected ®Sudden cut-off ? [Ribas et al.: ApJ 622, 680 – 694, 2005] [Newkirk, Jr.: Geochi. Cosmochi. Acta Suppl., 13, 293–301; Kulikov et al.: PSS, 54, 1325, [Wood et al.: 2006] ApJ 574, 412 –425, 2002; Kulikov et al.: PSS, 54, 1425, 2006] IWF/ÖAW GRAZ 10 The young Sun (G stars) III: → X-ray and EUV EK Dra (100 Myr) p1 UMa (300 Myr) k1 Cet (650 Myr) Sun (4.56 Gyr) [Ribas et al.: ApJ, 622, 680 – 694, 2005] IWF/ÖAW GRAZ 11 The upper atmosphere (Thermosphere, exosphe exobase IWF/ÖAW GRAZ 12 Thermospheric heating and cooling processes The most important heating and cooling processes in the upper atmosphere of Earth can be summarized as follows [e..g., Dickinson, 1972; Chandra and Sinha, 1974; Gordiets et al., 1978; Gordiets et al., 1981; Gordiets et al., 1982; Dickinson et al., 1987] heating due to O2, N2, and O photoionization by solar XUV radiation ( λ ≤ 1027 Å), heating due to O2 and O3 photodissociation by solar UV radiation (1250 ≤ λ ≤ 3500 Å), chemical heating in exothermic binary and 3-body reactions, neutral gas molecular heat conduction, IWF/ÖAW GRAZ 13 Time evolution of the exobase temperature based on Earth's present atmospheric composition ? Hydrostatic equilibrium is assumed →no hydrodynamic flow and adiabatic cooling 5000 K (H atoms) [Kulikov et al., SpSciRev., 2007] The blow-off temperature for atomic hydrogen of about 5000 K would be exceded during the first Gyr For XUV fluxes more than 10 times the present flux (> 3.8 Gyr ago) one would expect extremely high exospheric temperatures IWF/ÖAW GRAZ 14 Time evolution of the exobase temperature based on Earth's present atmospheric composition ? Hydrostatic equilibrium is assumed →no hydrodynamic flow and adiabatic cooling CO ? 2 5000 K (H atoms) [Kulikov et al., SpSciRev., 2007] The blow-off temperature for atomic hydrogen of about 5000 K would be exceded during the first Gyr For XUV fluxes more than 10 times the present flux (> 3.8 Gyr ago) one would expect extremely high exospheric temperatures IWF/ÖAW GRAZ 14 Expected scenarios of atmosphere responses present Earth composition (Earth) [Lammer et al., Space Sci. Rev., in press, 2008; Tian et al., JGR, in press, 2008] [Kulikov et al., Planet. Space Sci., 54, 1425 – 1444, 2006] 96 % CO2 atmosphere (Venus) IWF/ÖAW GRAZ 15 M stars and radiation activity IWF/ÖAW GRAZ 16 X-ray/EUV activity of low mass stars Early Venus, Earth, Mars, Titan, gas giants, comets 0.1 Gyr IWF/ÖAW GRAZ Exoplanets 0.3 Gyr 1.0 Gyr [Scalo et al., Astrobiology, 7, 85, 2007] 3.16 Gyr 10 Gyr 17 Soft X-ray and EUV induced expansion of the upper atmospheres can lead to high non-thermal loss rates present Venus, Mars present Earth [Lammer et al., Astrobiology, 7, 185, 2007] IWF/ÖAW GRAZ Early Earth ? terrestrial 18 Its not so simple! →No analogy for habitable zones of lower mass stars (K and M-types) Atmospheric effects and habitability of Earth-like exoplanets within close-in habitable zones Enhanced EUV and X-rays Neutron fluxes Coronal mass ejections (CMEs) Intense solar proton/electron fluxes (e.g., SPEs) Solar – stellar analogy Data from Sun + Stars Space and ground-based data Correlated analysis of events IWF/ÖAW GRAZ Establishing an extreme event data-base 19 Plasma environment within close-in habitable zones 0.05 AU 0.1 AU 0.2 AU 1 AU nmin (d) = 4.88 d/d0 -2.31 nmax(d) = 7.10 d/d0 -2.99 v mod CME = 1 AU = 450d0km/s [Khodachenko et al., Astrobiology, 7,167, 2007] White light [Vourlidas, et al., ESA SP-506, 1, 91, 2002] Radio [Gopalswamy and Kundu, Solar Phys., 143, 327, 1993] UV [Ciaravella, et al., ApJ., 597, 1118, 2003] IWF/ÖAW GRAZ similar values at 3-5 RSun: nCME ~106 cm –3 20 Plasma environment within close-in habitable zones IWF/ÖAW GRAZ 21 Hot and background O atoms as function of IP O2 + e → O + O + ΔE + * * IP [Kulikov et al., Planet. Space Sci., 54, 1425, 2006] →Extreme plasma interaction with extended atmosphere IWF/ÖAW GRAZ 22 3D MHD simulation of Venus solar wind interaction with present & extreme conditions IWF/ÖAW GRAZ 23 3D MHD simulation of Venus solar wind interaction with present & extreme conditions IWF/ÖAW GRAZ 23 O+ loss rates of present Venus at 0.7 AU Venus Express 24 O+ loss rates of present Venus at 0.7 AU Venus Express 24 O+ loss rates of Venus 4.25 Gyr ago; 30 XUV; nsw=1000 cm-3 (60 × pr.) or M-star Exo-Venus at 0.3 AU Total O+ ion loss rate ~ 2 bar → 150 Myr 25 O+ loss rates of Venus 4.5 Gyr ago 100 XUV; nsw=1000 cm-3 or (active M-star) Exo-Venus at 0.3 Total O+ ion loss rate ~ 20 bar → 150 Myr 26 3D MHD simulation of a Venus-like planet under extreme stellar plasma conditions → 0.05 AU (100 XUV) Total O+ ion loss rate ~ 500 bar → 150 Myr 27 Magnetospheres and model input Rotational Effect on Magnetic Dynamo →Expected Stronger Magnetic Field 3.6 – 4 Gyr ago [Dehant et al., Space Sci. Rev., 2007] Plasma parameters 7 EUV: nsw ~ 90 cm-3; vsw ~780 km/s RMagnetopause ~ 5.2 REarth → Rexo ~ 2.5REarth RMagnetopause ~ 6.1 REarth → Rexo ~2.5REarth 10 EUV: nsw ~ 200 cm-3; vsw ~ 900 km/s RMagnetopause ~ 5.2 REarth → Rexo ~ 4.8REarth 20 EUV: nsw ~ 270 cm-3; vsw ~1010 km/s RMagnetopause ~ 4.7 REarth → Rexo ~ 12.7REarth IWF/ÖAW GRAZ Estimated magnetic moment of early Earth, relative to its present-day value, as a function of possible early planetary rotation periods in hours. The dotted vertical line denotes the present-day sidereal M star planets may have weak rotation rate of the Earth magentic moments → slow rotation Ionization processes in the extended exosphere(s) Solar Solar EUV: wind hν charge +N→ exchange: N + + ep+ + N → N+ + HENA Electron impact ionization: e- + N → N+ + 2e- Cummings et al. ApJ 578:194, [2002] NIFS data base: 28 http://dpc.nifs.ac.jp/aladin/ Test-particle model results of N+ pick up ion 7EUV (3.8 Gyr ago) 20EUV (4.13 Gyr ago) Exosphere density above the exobase is calcuated from Chamberlain`s theory Parometric law modified with particion functions [Chamberlain, Test-particle results based on a Spreiter-Stahara model: 1963] [e.g., Lichtenegger et al. JGR, 2001; Lammer et al., Icarus, 2003; Kulikov et al. Planet. Space Sci., 2006; Lamme et al., Planet. Space Sci., 2006; Lammer et al. Astrobiology, 2007] 7EUV (3.6 Gyr ago), subsolar obstacle distance 6.0REarth: N+ion pick up loss rate ~ 5 ×1028 s-1 10EUV (3.8 Gyr ago): subsolar obstacle distance 5.0REarth:N+ion pick up loss rate ~5 × 1029 s-1 20EUV (4.13 Gyr ago): subsolar obstacle distance 12.7REarth N+ion pick up loss rate ~2 ×1030 s-1 Total loss of nitrogen would result in an equivalent amount of ≤ 20 bar during ~ 50 Myr Simulations indicate that the atmosphere should have been protected more efficiently most likely due to higher carbon-dioxide mixing ratios during the first 500 Myr after the young Sun IWF/ÖAW arrived at the ZAMS GRAZ 29 H O inventories and atmospheres are strongly 2 Class I planets may evolve to class II habitat types at M stars IWF/ÖAW GRAZ 30 Expected evolution of Earth’s atmosphere Lower mass stars K, M stars Atmosphere evolution of Earth-like planets will be different (low mass K and M stars) Sun G stars Earth (G star Earth-like planets, F star Earth’s?) IWF/ÖAW GRAZ and cools the upper atmosphere so that expansion and loss rates should be reduced 31 Where are they? Star-types and expected preferred habitats Class I Earth-like habitable planets may preferably be found in orbits of Sun-like G-type and some K-type stars, F-type where the originally defined habitable zone definition is valid → see Earth! Class II, III and IV habitats should also populate G-type and F, K, and M-type stars Lower mass stars should have less class I habitable planets but class II, class III and class IV habitability-types may be common like on G-stars. Many planets which start in the habitability class I domain at its origin may evolve to class II-types IWF/ÖAW GRAZ 32