1. Lecture 13: Venus – Plate Tectonics, Runaway Greenhouses, and the Inner Edge of the Habitable Zone Abiol 574

2. Venus 93-bar, CO 2 -rich atmosphere Practically no water (10 -5 times Earth) D/H ratio = 150 times that on Earth What went wrong with it?

3. The Medea and Rare Earth hypotheses Peter Ward Medea hypothesis: Life is harmful to the Earth! Rare Earth hypothesis: Complex life (animals, including humans) is rare in the universe 20092000

4. List of Rare Earth arguments 1.Plate tectonics is rare 2.Exoplanets may lack magnetic fields 3.The animal habitable zone is narrower than the habitable zone 4.The Sun is anomalously metal-rich 5.Evolutionary events like the origin of eukaryotes and the Cambrian explosion are unlikely 6.Nitrogen may not be abundant in a planet’s atmosphere if life is not present (from Lovelock) 7.Large impacts may be more frequent in planetary systems that lack Jupiters 8.A planet’s obliquity may be chaotic if it lacks a large moon

5. Venus and plate tectonics One of Ward and Brownlee’s key “Rare Earth” arguments is that plate tectonics is rare –Argument: There are ~20 rocky planets and large moons in the Solar System. Of these, only Earth has plate tectonics Does Venus have plate tectonics? Image made using synthetic aperture radar (SAR)

6. http://www.kidsgeo.com/geography-for-kids/0012-is-the-earth-round.php Earth topography Earth’s topography shows tectonic features such as midocean ridges

7. http://sos.noaa.gov/download/dataset_table.html Earth topography Linear mountain chains are also observed

8. Venus as seen by Magellan Image made using synthetic aperture radar (SAR) http://www.crystalinks.com/venus703.jpg Venus does not show such features, suggesting that plate tectonics does not operate But, the lack of liquid water on Venus is probably responsible, so this should not be taken as evidence that plate tectonics is rare

9. Interesting observation(s): 1.Craters are located randomly on Venus’ surface (see next slide) 2.There are no craters less than 3 km in diameter What do these observations imply?

10. Equal-area projection showing 842 impact craters Simple cylindrical projection G.G. Schaber et al., JGR 97, 13257 (1992)

11. Possible answers: 1)Venus never had any water to begin with or 2) Venus’ climate got out of control because of positive feedback loops in the climate system Question: What went wrong with Venus?

12. Positive feedback loops (destabilizing) Water vapor feedback Surface temperature Atmospheric H 2 O Greenhouse effect (+) This feedback becomes more and more important as the atmosphere becomes warmer

13. Negative feedback loops (stabilizing) IR flux feedback Surface temperature (-) Outgoing IR flux This feedback can break down when the atmosphere heats up and becomes H 2 O-rich

14. Classical “runaway greenhouse” Goody and Walker, Atmospheres (1972) After Rasool and deBergh, Nature (1970) Assumptions: Start from an airless planet Outgas pure H 2 O or a mixture of H 2 O and CO 2 Solar luminosity remains fixed at present value Calculate greenhouse effect with a gray atmosphere model 1 bar

15. Problems with the classical runaway greenhouse model Gray atmosphere approximation No convection No variation in solar luminosity Planets acquire atmospheres during accretion by impact degassing of incoming planetesimals

16. Alternative runaway greenhouse calculation Imagine a thought experiment in which you push the present Earth closer to the Sun J. F. Kasting, Icarus, 1988 Do this by gradually increasing the surface temperature in one’s climate model 

17. H 2 O surface pressure vs. T s J. F. Kasting, Icarus (1988) Surface pressure approaches the saturation vapor pressure of water at high T s Pressure exerted by a fully vapor- ized ocean is ~270 bars 100 o C Liquid water vanishes here

18. Vertical temperature structure Lower atmosphere temperature structure should be approximately adiabatic Get moist or dry adiabat near the surface, depending on whether liquid water is present Ocean presentNo ocean J. F. Kasting, Icarus (1988)

19. Calculated T and H 2 O profiles TemperatureWater vapor The troposphere expands as the surface temperature rises Water vapor becomes a major constituent of the stratosphere at surface temperatures above ~340 K (Ingersoll, JAS, 1969) Hydrogen can then escape rapidly to space because the diffusion limit is overcome J. F. Kasting, Icarus (1988)

20. Tropopause cold trap Temperature decreases rapidly with height in the troposphere, then levels out (or increases) in the stratosphere The H 2 O vapor pressure decreases with height in the troposphere, then remains constant (or increases) in the stratosphere H 2 O saturation mixing ratio, f sat = P sat /P, must therefore go through a minimum at some height. We call that height the tropopause cold trap Cold trap (= P sat /P)

21. Alternative runaway greenhouse calculation Now, calculate radiative fluxes. Define F IR = net outgoing IR flux F S = net absorbed solar flux for the present solar luminosity Then S EFF = F IR /F s = solar flux (relative to today) needed to sustain that temperature

22. Runaway greenhouse: F IR and F S J. F. Kasting, Icarus (1988) Outgoing IR flux levels out above ~360 K (90 o C) because the atmosphere is now opaque at those wavelengths Present Earth Simpson-Nakajima limit

23. Planetary albedo vs. surface temperature The albedo decreases with increasing T s initially because of increased absorption of solar near-IR radiation by H 2 O At higher T s, the albedo increases because of increased Rayleigh scattering by H 2 O

24. Back to the infrared… The key to understanding the runaway greenhouse is to think about the behavior of the outgoing IR flux, F IR

25. Negative feedback loops (stabilizing) IR flux feedback Surface temperature (-) Outgoing IR flux Above 360 K, the negative feedback loop is broken, so the surface temperature is free to run away

26. J. F. Kasting, Icarus (1988) (S eff ) Recall that S eff = F IR /F S The stratosphere becomes wet (and the oceans are thus lost) at S eff = 1.1. The corresponding orbital distance is 0.95 AU But, stay tuned: these results have just changed!

27. The (liquid water) habitable zone http://www.dlr.de/en/desktopdefault.aspx/tabid-5170/8702_read-15322/8702_page-2/ By using climate models, we can estimate the boundaries of the habitable zone, where liquid water can exist on a planet’s surface The habitable zone is relative wide because of the negative feedback provided by the carbonate-silicate cycle

28. New albedo calculations using the HITEMP database Goldblatt modelKasting (1988) model As first pointed out to us by Colin Goldblatt (U. Victoria), our old climate model may have seriously underestimated absorption of visible/near-IR radiation by H 2 O. New data are available from the HITEMP database

29. Runaway greenhouse thresholds: old and new New model (Kopparapu et al., Ap.J., 2013) Old model (Kasting et al., 1988) Our own calculations using updated absorption coefficients for both H 2 O and CO 2 suggest that the runaway greenhouse threshold is much closer than previously believed (runaway: 0.97 AU, moist greenhouse: 0.99 AU)

30. Revised conventional HZ limits The runaway and moist greenhouse limits on the inner edge of the HZ have recently been revised. They now lie much closer to Earth’s orbit Kasting et al., PNAS, submitted (Figure by Sonny Harman)

31. Revised conventional HZ limits But, these calculations assume fully saturated atmospheres, and they neglect cloud feedback. The real inner edge could be anywhere within the red zone. This calculation needs to be done with 3-D climate models..

32. Revised ZAMS habitable zone in distance coordinates Kasting et al., PNAS, submitted (Figure by Sonny Harman)

33. There could be ways to broaden the habitable zone on both the inner and outer edges Let’s think about the inner edge first. (We’ll get back to the outer edge later.)

34. “Dune” planets Abe et al., Astrobiology (2011) suggested that dry planets with water oases at their poles might remain habitable well inside the inner edge of the conventional HZ –S eff = 1.7, or 0.77 AU Do such planets really exist, though? –In the science fiction novel, much of the planet’s water has reacted with the crust, and they are working hard to recover it

35. Let’s go back to an older diagram and consider some other factors that might affect planetary habitability 

36. Kasting et al., Icarus (1993) ZAMS habitable zone The habitable zone is considered to be reasonably wide as a consequence of stabilizing feedbacks between atmospheric CO 2 and climate Bad things happen, though, to planets around stars much different from the Sun --F and A stars: high stellar UV fluxes, short main sequence lifetimes --Late K and M stars: tidal locking, stellar flares, initial volatile inventories?

37. Gliese 581 is an M3V star, 0.31 M sun, 0.0135 L Sun, so its habitable zone is at roughly 1/10 th the distance of the Sun’s HZ

38. 3-D climate model calculations for M- and K-star planets Clouds dominate the sunny side of tidally locked planets orbiting M and late-K stars, raising their albedos The inner edge of the HZ is therefore pushed way in –S eff  2 for a synchronously rotating planet around a K star (dark blue curves) Yang et al., ApJ Lett (2013)

39. Negative cloud feedback may well have pushed early Venus into the liquid water regime Venus lost its water anyway because the stratosphere became wet, leading to rapid photolysis and escape of H –The loss of water may have happened very early. Hamano et al. (Nature, 2013) argue that a steam atmosphere formed during accretion and never collapsed after that Once the water was gone, volcanic CO 2 (and SO 2 ) built up in Venus’ atmosphere, leading to its present, hellish state Evolution of Venus’ atmosphere

40. Habitable zone inner edge The inner edge of the habitable zone is determined by water loss and/or the runaway greenhouse The actual inner edge for a Sun-like star probably lies somewhere between the orbits of Earth and Venus –Simple 1-D climate models put it close to Earth’s orbit (1 AU) –The ‘recent Venus’ limit at 0.76 AU is a reasonable optimistic estimate for the inner edge –The ‘Dune-planet’ estimate of 0.77 AU agrees well with the recent Venus limit, so both arguments point to this distance representing an optimistic inner edge –Tidally locked planets could be habitable even closer in (S eff = 2 (equivalent to ~0.7 AU for the Sun) because of widespread cloudiness on their sunlit side