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ASTR/GEOL-2040: Search for life in the Universe: Lecture 13

ASTR/GEOL-2040: Search for life in the Universe: Lecture 13. CO 2 on Venus Plate tectonics BIFs and GOE. Axel Brandenburg (Office hours: Mondays 2:30 – 3:30 in X590 and Wednesdays 11-12 in D230). Habitable zone. Sources of CO2 …. …. Decarbonation.

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ASTR/GEOL-2040: Search for life in the Universe: Lecture 13

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  1. ASTR/GEOL-2040: Search for life in the Universe: Lecture 13 • CO2 on Venus • Plate tectonics • BIFs and GOE Axel Brandenburg (Office hours: Mondays 2:30 – 3:30 in X590 and Wednesdays 11-12 in D230)

  2. Habitable zone • Sources of CO2 • …. • …. • Decarbonation

  3. Other factors determining Earth’s temperature • Greenhouse gases, e.g., CO2 • Others? Yes: CH4 and H2O • Sources & sinks of CO2 • Weathering • Volcanoes

  4. The CO2thermostat CO2 high, warm, more rain atmospheric CO2 reduced

  5. The CO2thermostat CO2 low, cool, less rain CO2 high, warm, more rain atmospheric CO2 reduced atmospheric CO2 builds up

  6. On Earth, CO2 is recycled • Sources of CO2 • …. • …. • Decarbonation

  7. Sinksof CO2 • Acid rain: H2O+CO2 =H2CO3 • Contact with rock: weathering • CaSiO3 +H2CO3 CaCO3 +SiO2 • Silicate rock  carbonate rock • carbonate rock = limestone

  8. Chalk:isle of Whight • CO2 from the air locked up in chalk! • Recycled via volcanoes

  9. Earth – Venus - Mars • On Earth: 170,000 times more CO2 in carbonate rocks than in atmosphere • Similarly for Mars • What about Venus? See BS p.139 for details!

  10. Why so much CO2 on Venus? • Venus: 200,000 times more CO2 than Earth • Remember: Earth 170,000 times more CO2 in carbonate rocks • Venus: no return to carbonate rocks • Why?

  11. Why no CO2 return into rockson Venus? • … • …… • ………..

  12. into rocks Why no CO2 return on Venus? • No water, no rain, no feedback • Yet, volcanic activity • Idunn Mons •  infrared • topography enlarged • 0.25 Myr old

  13. Why did Venus lose its water? • Same reason as for Mars • UV light: H2O  H2 + ½ O2  photolysis • H2 lost through thermal escape • Why?

  14. Thermal escape? • Escape velocity? • Apollo 8 (Borman, Lowell, Anders) • ½ mve2=GMm/R • ve=(2GM/R)1/2 = 11.2 km/s • ½ mvH2=kBT • H2 is so light

  15. Why not on Earth? • Water vapor condenses to rain before too much gets lost • Venus: unable to protect itself • Too hot: H2O also greenhouse gas • Hotter  more vapor  hotter still • Runaway greenhouse effect

  16. Runaway greenhouse effect • Water

  17. Earth has far less atmospheric CO2 than Venus because • Earth was born with less gas • CO2 was lost in giant impact • CO2 is locked up in carbonate rocks

  18. Earth has far less atmospheric CO2 than Venus because • Earth was born with less gas • CO2 was lost in giant impact • CO2 is locked up in carbonate rocks

  19. If Earth had more greenhouse gases in its atmosphere, it would • Heat up • Cool off • Accelerate plate tectonics

  20. If Earth had more greenhouse gases in its atmosphere, it would • Heat up • Cool off • Accelerate plate tectonics

  21. Earth’s structure • cc

  22. Earth’s structure • cc

  23. Earth’s structure • cc

  24. Plate tectonics • Upwellings (heat) • Subduction • shear

  25. Also deep in the pacific & atlantic

  26. Rock locations Acasta gneiss • Predictable • Phoenix observed precipitation • But evaporated before reaching ground Isua Shark Bay Barberton Greenstone belt Long, p.211

  27. Next week • Banded iron formation (=rust) • Evidence for early life on Earth • Oceans 4.4 Gyr ago • pp. 56 – 64 in RGS • After that • Significance of 13C isotope • Cambrian explosion of life

  28. Banded iron formations (BIFs) • Best evidence

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