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Atmospheric evolution of Venus as a habitable planet

Atmospheric evolution of Venus as a habitable planet. Kevin Zahnle NASA Ames Yutaka Abe Ayoko Abe-Ouchi University of Tokyo Norman H Sleep Stanford. Earth is an ocean planet The inner edge of the Habitable Zone is determined by oceans evaporating at the critical flux.

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Atmospheric evolution of Venus as a habitable planet

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  1. Atmospheric evolution of Venus as a habitable planet Kevin Zahnle NASA Ames Yutaka Abe Ayoko Abe-Ouchi University of Tokyo Norman H Sleep Stanford

  2. Earth is an ocean planet The inner edge of the Habitable Zone is determined by oceans evaporating at the critical flux Mars, Titan*, and Arrakis are Land Planets Land planets can be habitable closer to the Sun *Methane plays the role of water on Titan

  3. Earth: tropics are stabilized by heat transport to the poles energy transport Ocean planet: runaway greenhouse when poles cannot radiate all the heat that comes from the tropics energy transport Land planets can radiate more efficiently from dry tropics - poles stay habitable energy transport

  4. relative humidity Idealized GCM expts confirm that a generic land planet is more stable at high insolation than an ocean planet Precipitable water [m/m2] Planetary Longwave (thermal) Radiation [W/m2]

  5. The Land Planet remains Habitable up to the runaway 0 30 runaway greenhouse 60 30 60 0 Temperature (C) (relative to flux @Earth)

  6. stratospheric water vapor relative humidity The stratosphere of a land planet is extremely dry Planetary Longwave (thermal) Radiation [W/m2]

  7. The dry stratosphere inhibits Hydrogen Escape Land planets are stable against H escape Planetary Longwave (thermal) Radiation [W/m2]

  8. The dry stratosphere inhibits Hydrogen Escape Land planets are stable against H escape Planetary Longwave (thermal) Radiation [W/m2]

  9. As the Sun gets brighter, first Venus then Earth encounter the Runaway Greenhouse Effect Time [Gyrs]

  10. Caladan - a (fictional) ocean planet In this image, Caladan appears to be deep in a moist greenhouse

  11. The Moist Greenhouse Before the runaway, there is a brief time when the planet is habitable and the stratosphere is wet. H escape is fast. This sets Kasting’s inner bound to the Habitable Zone Stratosphere

  12. The Moist Greenhouse: H escape is fast Can the ocean escape before the planet becomes uninhabitable? Planetary Longwave (thermal) Radiation [W/m2]

  13. Earth: Kasting’s (1988) moist greenhouse model with H escape

  14. As the Sun gets brighter, first Venus then Earth encounter the Runaway Greenhouse Effect Time [Gyrs]

  15. The End is Quick 1300 <10 Myr 750 Surface Temperature [K] “Venus” 350 “Dune” Carbonate decrepitates Time

  16. A habitable ocean planet can evolve into a habitable land planet without passing through a runaway greenhouse state • The oceans are lost during moist greenhouse state • The land planet is a kind of afterlife It is possible that this will happen to Earth It is possible that this has already happened to Venus There may be observable consequences

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