Habitable Planets. Much of it stolen from. Yutaka ABE University of Tokyo

Transcription

1 Habitable Planets Much of it stolen from Yutaka ABE University of Tokyo

2 1. Habitability and Water Why water?

3 Importance of Liquid Gas: highly mobile, but low material density. Solid: high density but very low mobility. Liquid: high density and high mobility Habitable environments likely use a liquid. H 2 O is the most abundant condensible substance in the cosmos

4 2. Water (Ocean) planets One kind of Habitable planet i.e., Generalized Earth

5 Caladan An ocean planet where Paul Atreides was born This image is from a video game In this image, Caladan appears to be deep in a moist greenhouse

6 Water (Ocean) planets In zero dimensions = globally averaged consideration Bounded thermally by complete vaporization = runaway greenhouse global freezing

7 The Traditional Habitable Zone Plot (Kasting Whitmire Reynolds 1993 Runaway Greenhouse Limit Freezing Limit

8 Runaway Greenhouse Modern and Snazzy! Freezing Limit

9 2.1 Runaway greenhouse A moist atmosphere (saturated by water vapor) has an upper limit of the outgoing infrared flux that can be emitted: If insolation exceeds this limit, temperature increases until complete evaporation of liquid water from the surface. The runaway greenhouse limit is also called the critical flux

10 radiative equilibrium There are two runaway greenhouse effects in the literature (Nakajima et al 1992) Altitude stratosphere troposphere convective equilibrium Komabayashi-Ingersoll limit applies to F IR through radiative-equilibrium stratosphere that is saturated at its base Simpson s paradox applies to F IR from convective, water-saturated troposphere Temperature tropopause

11 Ingersoll-Komabayashi limit (1) (2) (1) Radiative equilibrium in the stratosphere (2) Condensation at the tropopause (3) Optical depth at the tropopause Gray Approximation: σt 4 t = 1 2 F 3 IR 2 τ +1 t τ t = 2 4 2σT t 3 F IR p * ( T) = p * L RT 0 e 1 (3) τ t = κ ν h p * ( T) m v gm τ t = κ ν h p 0 * m v gm e L RT t F IR = planetary thermal radiation T t =Tropopause Temperature L = latent heat condensation h = relative humidity κ v = absorption [cm 2 /g] m v,m = mol weights of vapor, atm

12 IR Optical Depth at Cold Trap stable solution unstable solution Ingersoll-Komabayashi limit 30% albedo 100% relative humidity τ t = κ h p * ν 0m v gm τ t = 2 4 2σT t 3 F IR Radiative F IR = 250 W/m Cold Trap Temperature [K] e L RT t 1

13 IR Optical Depth at Cold Trap Runaway Ingersoll-Komabayashi limit 30% albedo 100% relative humidity F IR = 400 W/m Cold Trap Temperature [K]

14 IR Optical Depth at Cold Trap runaway Ingersoll-Komabayashi limit 30% albedo τ t = κ ν h p 0 * m v gm 100% e L RT t 40% rel humidity F IR =480 W/m Cold Trap Temperature [K] 40%

15 Simpson s Paradox (runaway in radiative-convective equilibrium) Saturation vapor pressure p* relates the optical depth τ to the radiating temperature T of the moist troposphere. This is illustrated for a gray atmosphere: ( ) σt 4 e 3τ 2 3 F IR τ=0 τ T z 0 ( ) dz ( ) h p * T( z) 2 dτ ( 310 W/m 2 for Earth) where optical depth depends on the relative humidity h and the saturated column of water vapor The critical flux depends weakly on g (roughly as g 0.25 )

16 radiative equilibrium Altitude stratosphere Simpson s paradox applies to F IR from convective, water-saturated troposphere troposphere convective tropopause Temperature Contribution Function (where thermal IR comes from)

17 radiative equilibrium Altitude stratosphere troposphere When the moist atmosphere becomes optically thick, the surface no longer contributes to F IR Simpson s limit is lower (stronger) than the Ingersoll-Komabayashi limit) tropopause convective Temperature Contribution Function (where thermal IR comes from)

18 450 Simpson s (tropospheric) limit for 100% rel humid Surface Temperature [K] bar N2 100% rel. humid. 10 bar N2 1 bar N2 I-K (stratospheric) limit for 100% rel. humid Net Insolation [W/m 2 ] 1-D follows Nakajima et al J. Atmos. Sci. (1992)

19 Surface Temperature [K] Insolation vs. Earth (constant albedo = 0.3) ocean planet 100% rel. hum. ocean planet 60% rel. hum. ocean planet GCM, stable ocean planet GCM, unstable Outgoing Thermal Radiation [W/m 2 ] 1-D follows Nakajima et al J. Atmos. Sci. (1992) GCM expts by Ishiwatari et al J. Atmos. Sci. (2002)

20 2.2 freezing limit A simple global energy balance model with ice albedo feedback. πr 2 S( 1 A) = 4πr 2 F F = 2σT 4 s 3 2 τ s + 2 ( ) ( ) A = 0.3 T s T s < 273 πr 2 4πr 2

21 Multiple equilibria - snowball Earth τ s = snowball Earths

22 Multiple equilibria τ s = an ice planet can be metastable at very high levels of insolation critical flux for the runway greenhouse

23 2.3 H 2 O-CO 2 Planets The HZ in 2D! Earth is an example of an H 2 O-CO 2 Planet, to first approximation

24 Stability regime of liquid water CO2 amount [bars] Planetary Radiation [W/m 2 ] Yutaka Abe (1993)

25 Earth-sized Venus,Earth orbit CO2 amount [bars] CO2 amount [kg] Planetary Radiation [W/m 2 ] Yutaka Abe (1993)

26 Evolution of the central star CO2 amount [bars] Planetary Radiation [W/m 2 ] Yutaka Abe (1993)

27 Effect of carbonate formation CO2 amount [bars] Planetary Radiation [W/m 2 ] Yutaka Abe (1993)

28 3. Land Planets Dune (Herbert 1965) is a fictional example Titan is an example in our Solar System

29 Facts of Dune Dune has powerful dust storms There are some evaporite deposits Nitrogen-oxygen atmosphere It is sparsely habitable near the poles Dune is dry! the tropics are extremely dry small polar ice caps extensive polar aquifers there are polar dews

30 Ocean planet and Land planet Water Planet (ocean planet) Precipitation and evaporation are not in balance Precipitation and evaporation are in balance

31 Example of A Land Planet Surface Temperature Precipitation Zonal Wind (sigma=0.81) Meridional Wind (sigma=0.81)

32 energy transport Earth: tropics are stabilized by heat transport to the poles energy transport Ocean planet: runs away when poles cannot radiate all the heat that comes from the tropics energy transport Land planet can radiate more efficiently from dry tropics - poles stay habitable Abe & Abe-Ouchi 2005

33 Some idealized GCM experiments by Professor Abe (345 ppm) Ocean Planet:

34 runaway greenhouse Temperature (C)

35 Precipitable water [m/m2] Simpson Runaway, labeled by humidity Land planet is more stable at high insolation than ocean planet Abe & Abe-Ouchi 2005

36 Precipitable water [m/m2] Simpson Runaway, labeled by humidity no wet solutions for 415 < I < 450 The discontinuity from 415 W/ m 2 to 450 W/m 2 is caused by an albedo jump: clouds and polar caps disappear Abe & Abe-Ouchi 2005

37 Simpson Runaway, labeled by humidity Hysteresis Effect: To condense the water requires bringing the planet to the same critical flux as an ocean planet Abe & Abe-Ouchi 2005

38 relative humidity stratospheric water vapor The stratosphere of a land planet is extremely dry Abe & Abe-Ouchi 2005

39 Ocean Lifetime [Ga] Ocean 10-3 Ocean Energy-limited escape 10X Energy-limited land planet ocean planet Abe 1988 diffusion-limited escape Hydrogen Escape is inhibited Land planets are stable against H escape 0.01 ocean planet Kasting 1988 diffusion-limited escape Emitted Thermal IR Radiation [W/m -2 ]

40 Insolation (d) energy transport Planetary Radiation Planetary Radiation Critical Flux Sub-solar Pole Dry Surface Terminator Wet Surface Anti-solar Pole A dry synchronous M-dwarf planet is another form of land planet: the dry hemisphere radiates away most of the sunlight

41 Water Planet 3.2 Different Freezing limits, too Land Planet Land planets are harder to freeze than water planets.

42 3.3 Land Planet vs Water Planet Stability region of Liquid water Stability region of Liquid water

43 The Land Planet HZ Runaway Greenhouse Limit Freezing Limit

44 4. Is there really an Outer Limit?? Runaway Greenhouse Limit Freezing Limit?

45 Titan Temperature profile

46 It takes merely the will to do it Temperature Atitude [km] -100 dt dz = g c p p T γ γ 1 ( ) (-140 km, 270 K, 60 bars)

47 5. The Evolving Sun after Sackmann et al 1993 red giant Luminosity We are here Time (Gyr)

48 1.5 habitable zones 1.5 Distance from the Sun (AU) the outer limits hydrogen escapes desert world Time (Gyr) 1

49 1.5 habitable zones 1.5 Distance from the Sun (AU) the outer limits hydrogen escapes desert world Time (Gyr) 1

50 The finger of God or a parochial view? 10 1 Kasting et al 1993 Time Ga Europa 1 10 Current Distance [AU] How long a planet at a given current distance from the Sun remains "habitable."

51 The finger of God or a parochial view? 10 1 Kasting et al 1993 Abe 1993, 2010 Time Ga Europa 1 10 Current Distance [AU] How long a planet at a given current distance from the Sun remains "habitable."

52 Europa under the HB Sun (butterfly metaphor)

53 6. Creation myths: how a Dune might evolve

54 Kasting s (1988) moist greenhouse model (I ve added the H escape) Ocean Volume (Earth Oceans) Albedo Ocean Volume Kasting 1988 albedo Surface Temperature Surface Temperature NOW Time (Gyr)

55 Surface T is ~400 K when the ocean evaporates... Works better with a thin atmosphere Ocean Volume (Earth Oceans) Albedo Ocean Volume Kasting 1988 albedo Surface Temperature Surface Temperature NOW Time (Gyr)

56 1.2 "Venus" 500 Ocean Volume (Earth Oceans) T s constant albedo=0.32 Ocean Volume T s constant albedo=0.35 T s constant albedo = Surface Temperature Time (Gyr)

57 Some Words A habitable land planet appears to be more stable against the runaway greenhouse effect than an ocean planet like Caladan or Earth The inner edge of the HZ for land planets was inside of Venus s orbit when the Sun was young Land planets are also more stable against ice-albedo runaway There is no obvious outer limit to the HZ other greenhouse gases can contribute soluble greenhouse gases work better for land planets land planets will be among the first extrasolar habitable planets to be discovered in transit tidally-locked planets of M dwarfs have excellent prospects for providing habitable, Dune-like conditions

58 Some Words A habitable land planet appears to be more stable against the runaway greenhouse effect than an ocean planet like Caladan or Earth The inner edge of the HZ for land planets was inside of Venus s orbit when the Sun was young Land planets are also more stable against ice-albedo runaway There is no obvious outer limit to the HZ other greenhouse gases can contribute soluble greenhouse gases work better for land planets land planets will be among the first extrasolar habitable planets to be discovered in transit tidally-locked planets of M dwarfs have excellent prospects for providing habitable, Dune-like conditions