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Disparate Evolutions - Caitlin Griffith


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Brave new worlds
May 29-June 03, 2016 – Lake Como School of Advanced Studies

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Disparate Evolutions - Caitlin Griffith

  1. 1. Venus, Earth and Titan
  2. 2. —  Venus’ lack of water —  The Runaway Greenhouse —  Earth’s gain of O2 —  Evolution according to the geological record —  Titan’s dwindling CH4 —  What’s happening here.
  3. 3. —  Venus formed closer to the Sun, where H2O was depleted compared to the building blocks of Earth. —  Venus was warm enough to catastrophically loose all it’s water in a Runaway Greenhouse event.* * This further suggests that there is a tipping point for which atmospheres can irreversibly reach an entirely different state.
  4. 4. Convection involves the rise of hot air, which is more buoyant than the overlying air. This occurs fast enough that little heat is exchanged and the process is adiabatic.
  5. 5. Assume ideal gas: (1) (2) Differentiate (1): Include (2): (3) First law of thermodynamics: For adiabatic motion (dq=0): Considering (3) And hydrostatic equilibrium: Where
  6. 6. Liquid Saturated vapor Trc Stratosphere (radiative equilibrium) Optical depth due to vapor alone Convective region Dry lapse rate Flux not conserved Radiative Equilibrium
  7. 7. Assume hydrostatic equilibrium: Write condensable mass density: Assume grey atmosphere: Then: Let: Then: H is the humidity, Ps the saturation pressure, Pv is the partial pressure of the vapor , k the absorption coefficient, mv and m the mean molecular weights of the vapor and the atmosphere, and g the gravity Optical depth at Trc
  8. 8. RT equation: Hydrostatic Equilibrium: Optical Depth: Outgoing Flux:
  9. 9. For T small: F ~ T4 solutions increase with T For T large: F ~ T4/Ps solutions decrease with T -> Solutions exist only for values below a maximum F -> This critical value depends on Ps and K
  10. 10. Cold Trap
  11. 11. Cold Trap VENUS Only H2O Photons
  12. 12. Consider: K= 0.1 cm2/g (appropriate for 8-20 um window H = 100% m = 29 g (N2) and mv = 18 g (H2O) g = 10 m/s Po = 8 mm Hg Fmax = 0.63 cal cm-2 min-1 T = 260 K Average incident sunlight on Earth is 0.5 cal cm-2 min-1 Average incident sunlight on Earth is 0.9 cal cm-2 min-1 But the albedos of Earth is 0.3 and for Venus is 0.78 So… both Venus and Earth absorb 0.3 They are subcritical. But what if Venus was not as cloudy as in the past Note: runaway greenhouses grow from below
  13. 13. —  Atmosphere is transparent at short wavelengths —  This prohibits vast cloud cover above the Trc level —  Surface has a large condensable reservoir —  When vaporized the condensable is optically thick at long wavelengths. However, this study indicates that an atmosphere can reach a tipping point, beyond which a planet irreversibly ends up in a radically different state.
  14. 14. BIF
  15. 15. Namibia
  16. 16. Titan Ganymede Callisto R=2575 km R=2631 km R=2410 km d=1.88 gm/cm3 d=1.93 gm/cm3 d=1.83 gm/cm3 Why different? Formed from different ices, perhaps with more carbon? Acquired an atmosphere but lost it through impacts. (Griffith & Zahnle 1995)
  17. 17. Image from UCL Planets group
  18. 18. N2 CH4 H2O A mess. Main Players: C2H6 (l) C2H2 (s) HCN (s) Haze (s)
  19. 19. From Cassini
  20. 20. Horst et al. 2012
  21. 21. Griffith 2009
  22. 22. Particles scatter most efficiently at wavelengths equal to or smaller than the particle size.
  23. 23. From Lorenz 2000
  24. 24. How can a molecule or atom transition between 2 states? LTE: the occupation is set by collisions and follows a Boltzmann distribution Non-LTE: spontaneous and stimulated emission can depopulate states Spontaneous emission Stimulated emission Absorption Collision excitation Collision deexcitation
  25. 25. Lunine, Stevenson and Yung 1982, Sagan & Dermott 1982
  26. 26. Equivalent to 1 meter of CH4
  27. 27. The atmosphere has the equivalent of 5 m of methane. It is close to saturation.
  28. 28. —  Just running out of methane after billions of years —  Idea: CH4 outgassed early on, heated e.g. by accretion —  Consequence: the surface has 0.5 km of organics* —  Has a recent bout of geological activity & outgassing —  Idea: Titan’s interior is freezing now, which circularizes the orbit —  Advantage: Explains Titan’s non-circular orbit —  Consequence: there are not a lot of organics coating the surface —  Is in some sort of equilibrium with subsurface CH4 —  Idea: Subsurface aquifers or methanogens balance CH4 loss —  Consequence: There’s more than meets the eye. * The byproducts of CH4 & N2 photolysis
  29. 29. Tobie et al. 2006
  30. 30. Griffith et al. Nature, 486, 238, 2012
  31. 31. PCA analysis of Cassini Data Griffith, in preparation
  32. 32. —  Hot Jupiters —  Composition of abundant elements likely established by thermochemical equilibrium because hot and “surfaceless” —  Secondary molecules affected by photochemistry —  Complications: magnetospheric effects on atmospheric structure and chemistry, disentangling host star & planet signals, and effects of clouds on measurements of gas abundances and temporature structure. —  Rocky Planets —  If hot, potentially somewhat in thermochemical equilibrium, but surface interaction/effects are inevitable. —  Complicated non-equilibrium effects inevitable and perhaps most interesting
  33. 33. —  Planets —  In situ measurements of V, E, M, J, A, T —  Surface, Atmosphere, Magnetosphere, Ionosphere, —  Spatially resolved observations (e.g. 1 meter by HiRISE) —  Detailed information of the Sun’s attributes too —  Exoplanets —  More examples —  Greater breadth of conditions to test processes —  E.g. The C/O ratio in giant planets —  Future Prospects —  More photons (TMT) —  More spectral coverage (JWST) —  But not spatially resolved, nor in situ… or not… A = Asteroid (Bennu, a carbonaceous asteroid) Mission: OSIRIS-Rex (University of Arizona)
  34. 34. The StarChip can be mass-produced at the cost of an iPhone and be sent on missions in large numbers to provide redundancy and coverage Light beam to propel gram-scale ‘nanocrafts’ to 20% speed of light could reach Alpha Centauri (4.37 light years) within about 20 years of its launch.
  35. 35. Components: StarChip: cameras, photon thrusters, power supply, navigation and communication equipment, and constituting a fully functional space probe. Lightsail: Advances in nanotechnology are producing increasingly thin and light- weight metamaterials, promising to enable the fabrication of meter-scale sails no more than a few hundred atoms thick and at gram-scale mass. 2. Light Beamer The rising power and falling cost of lasers, consistent with Moore’s law, lead to significant advances in light beaming technology. Meanwhile, phased arrays of lasers (the ‘light beamer’) could potentially be scaled up to the 100 gigawatt level. To be lead by Pete Worden, the former director of NASA AMES Research Center