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

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 Ocean planet: runaway greenhouse when poles cannot radiate all the heat that comes from the tropics Land planets can radiate more efficiently from dry tropics - poles stay habitable energy transport

4. Idealized GCM expts confirm that a generic land planet is more stable at high insolation than an ocean planet relative humidity Precipitable water [m/m 2 ] Planetary Longwave (thermal) Radiation [W/m 2 ]

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

6. stratospheric water vapor The stratosphere of a land planet is extremely dry relative humidity Planetary Longwave (thermal) Radiation [W/m 2 ]

7. Land planets are stable against H escape The dry stratosphere inhibits Hydrogen Escape Planetary Longwave (thermal) Radiation [W/m 2 ]

8. Land planets are stable against H escape The dry stratosphere inhibits Hydrogen Escape Planetary Longwave (thermal) Radiation [W/m 2 ]

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/m 2 ]

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

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

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

18. 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