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Complicated Terrestrial Planets- Caitlin Griffith


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

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Complicated Terrestrial Planets- Caitlin Griffith

  1. 1. Main culprits: Surfaces, Escape and Disequilibrium* * On a ridiculous scale
  2. 2. 1 amagat = 2.686 x 1025 m-3
  3. 3. —  Main attributes of each atmosphere —  Composition —  Clouds —  Origin of elements and their molecular forms —  A little more RT —  Atmospheric structure (greenhouse effect)
  4. 4. —  A thin CO2 atmosphere (0.006 bars) —  20—30 % of the atmosphere condenses out in the winter
  5. 5. High Resolution Imaging Science Experiment (HiRISE) camera (Mars Reconnaissance Orbiter) University of Arizona, PI Alfred McEwen
  6. 6. north south The two landers were at different altitudes. Smith 2008 CO2 + H2O H2O
  7. 7. Reull Vallis observed by ESA’s Mars Express
  8. 8. Current Pressure: 0.006 bar
  9. 9. Ar-36/Ar-38
  10. 10. Ground-based Measurements of H2O & HDO Results: Variable D/H values On average, D/H enriched by a factor of 7 relative to Earth’s oceans. This indicates water loss, and suggests a water layer 137 m deep 4.5 Gyrs ago Villanueva et al. 2015
  11. 11. Hu et al. 2015
  12. 12. “Three views of an escaping atmosphere” The Mars Atmosphere and Volatile EvolutioN (MAVEN)
  13. 13. Hu et al. 2015 C13/C12 indicate past atmosphere with P < 1 bar The current P=0.003 bar can be explained by Present pressure explained by sputtering, moderate carbonate deposits, and photochemical escape
  14. 14. Global climate simulations of the early Martian assuming a CO2 atmosphere with surface pressure between 0.1 and 7 bars. Explore a wide range of possible climates, using various values of obliquities, orbital parameters, cloud microphysics parameters, atmospheric dust loading, and surface properties. Forget et al. 2013 Volcanism and Impacts instead explain fluvial geology? “no combination of parameters yields surface temperatures consistent with the melting and flow of liquid water”
  15. 15. Thick atmosphere could have existed. But the resulting greenhouse would not surface temperatures above freezing and Water flow. CO by itself is a lousy greenhouse gas Maybe a bigger exoMars could do this?
  16. 16. Venus absorbs 66% of the solar energy absorbed by the Earth 160 W/m2 vs 243 W m2 Teq is 20K cooler than Earth
  17. 17. Past infrared observations indicate a flux of a 230—280 K. Here 9.4 and 3.15 cm observations indicate a 560—620 K black body. Also buried in the text the paper states says that the IR observations could be the upper atmosphere (consistent with equilibrium temperature) But it could be non-thermal emission from the ionosphere… UP Surface 600 K 250 K (Emission to space)
  18. 18. http:// authors/ author_resources/ how_write.html
  19. 19. Rough idea: T4 g = (N+1) Teff
  20. 20. Derive the thermal profile for an atmosphere that is: 1)  Transparent to sunlight 2)  Opaque at longer wavelengths 3)  Emits according to LTE 4)  Does not scatter 5)  Is grey Atmosphere heated by surface Use 2-Stream
  21. 21. θ µ = cos (θ)
  22. 22. RT equation for LTE emission: if (Note: altitude defined to increase in opposite direction to tau) Definition of mean intensity: Net flux across area parallel to surface: Integrate RT equation over a sphere:
  23. 23. Assume 2-stream: Simple expressions for J and F: Multiply RT by u & integrate: Simplifies to: Constant Flux: RT equation: 0
  24. 24. Combine these two equations: Substitute for Jv in the left equation:
  25. 25. Integrate over all frequencies: This gives us: Where: Assume the condition of radiative equilibrium: Then from We get: or
  26. 26. Derive the thermal profile for an atmosphere that is: 1)  Transparent to sunlight 2)  Opaque at longer wavelengths 3)  Emits according to LTE 4)  Does not scatter 5)  Is grey Atmosphere heated by surface First, get rid of J: + Use 2-Stream
  27. 27. 1)  Write down the equation of radiative transfer 2)  Establish the boundary conditions 3)  Solve for the temperature
  28. 28. Atmosphere heated by surface Use: RT Eq. : Intensities:
  29. 29. Atmosphere heated by surface At Ground: where T1 is the temperature of the atmosphere above the surface No downward intensity at the top of the atmosphere: Note: The air above the surface is cooler than the surface
  30. 30. Boundary Conditions:Flux at top of atmosphere: Upward intensity at top of atmosphere: Since F is constant: Thus from the boundary condition The top of the atmosphere (τ=0) radiates as T0 The upward flux is characteristic of that at τ= 2/3 RT Equation
  31. 31. Temperature profile: Where: Mean emission temp (equivalent T): i.e. Assume uniform temperature heated by Sun: For Earth: A = 0.29 & Te = 255 K To = 215 K & T(τ=1) = 277 K Derived T-P Profile: Upper boundary Condition 277 K 215 K τ=1
  32. 32. Boundary Conditions: Ground temperature discontinuity: Then: Since: Since: Then: Note that the ground temperature can be high for an atmosphere with a high optical depth That is the greenhouse effect
  33. 33. Planet Te Ts τg Tran Venus 264 750 85 0 Earth 255 288 0.9 42% Mars 213 223 0.3 77%
  34. 34. Missing scattering, convection. and real boundary conditions, wavelength dependence… Note: CO2 is a mediocre greenhouse gas
  35. 35. VENUS gas Abundance EARTH gas Abundance CO2 96.4 % N2 78.1 % N2 3.5 % O2 21.0 % SO2 0.015 % H2O 0.01-4 % Ar 0.007 % Ar 0.93 % H2O 0.002 % CO2 0.40 %
  36. 36. Ninety-nine percent of the gas molecules emitted during a volcanic eruption at Kilauea volcano are water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2). The remaining one percent is made up of hydrogen sulfide (H2S), carbon monoxide (CO), hydrogen chloride (HCl), hydrogen fluoride (HF) VENUS gas Abundance CO2 96.4 % N2 3.5 % SO2 0.015 % Ar 0.007 % H2O 0.002 %
  37. 37. Photochemistry: SO2 à H2SO4 (sulfuric acid) Global sulfuric acid clouds On a timescale of ~1.9 Myr thermochemical reactions with CaO-bearing minerals on Venus’ surface convert SO2 into anhydrite (CaSO4). SO2 and H2SO4 are thus lost to the atmosphere. Sulfur must be supplied to the atmosphere by volcanism
  38. 38. Lost by photochemistry and H escape, and oxidation of ferrous iron minerals over a timescale of 108—109 years Supplied by volcanism, possibly comet impact H2O plays a critical role in Venus’ greenhouse Equivalent to a 1 cm global layer of liquid SO2 (however liquid is unstable on Venus surface…)
  39. 39. Venus CO2: equivalent to 0.88 km of calcium carbonate (CaCO3) This is 2 times the calcium carbonate in Earth’s crust. Origin of CO2 is volcanism. Earth CO2: 25% is anthropogenic and 75% biological. Volcanism: minor source Most of the carbon is in calcium carbonate, mainly skeletal fragments of marine organisms such as coral, forams and molluscs
  40. 40. Fegley 2004
  41. 41. We’ll consider two cycles
  42. 42. e.g. limestone Plate tectonics Covers process from the atmosphere to the mantle, with the largest reservoirs remnants of ancient life.
  43. 43. N2 fixation: N2 + 8H+ + 8e- è 2NH3 + H2 Through enzymes (nitrogenase) produced by prokaryotes (both bacteria and archaea) Nitrification Bacteria convert NH4+ to NO2 − then (with other bacteria) to NO3 − Ammonification Bacteria & Fungi produce NH4+ from plant & animal remains Denatrification Bacteria convert NO3 − to N2
  44. 44. Measurements over a range of IR wavelengths
  45. 45. On Earth: 21 % of atmosphere On Venus 0.3 ppmv (1/10,000 times Earth) Spectrum of Venus’ O2 nightglow at 1.27 μm; nine emission lines are seen (Krasnopolsky, 2010) Abiotic oxygen produced by the photocatalytic reaction of titanium oxide
  46. 46. —  Earth, Venus & Mars require surface interaction to explain the atmospheres —  Cache elements (rust and oxygen on Mars) —  Supply elements such as sulfur (volcanism of S02) —  Participate in the molecular changes (N cycle) —  Small planets are susceptible to volatile loss. —  Mars’ lack of volatiles —  Earth’s Moon-forming collision Earth’s biosphere is totally dependent on life.