Outer Limits of the Habitable Zone

Transcription

1 Outer Limits of the Habitable Zone Raymond T. Pierrehumbert The University of Chicago 1

2 Habitability zones in space 2

3 Inner limit for water ocean planet given by water vapor runaway 3

4 But what determines the outer limit? Gl581d 4

5 A habitable Early Mars? 5

6 581c 5.36 M e 12.9 day 3600 W/m 2 581b M e 5.34 day W/m 2 581d 7.09 M e 66.8 day 367 W/m 2 (vs. 2613W/m 2 Venus, 1367W/m 2 Earth, 589W/m 2 Mars 6

7 Not obvious there should be an outer limit Why not just stuff in enough greenhouse gas until it s habitable? 7

8 Things that can go wrong Rayleigh scattering increases albedo. (Spectrum-dependent!) Shortwave-absorbing greenhouse gas leads to anti-greenhouse effect Depends on stellar spectrum Problem with CH 4 already for G stars For M stars, potential problem even with CO 2 (But this isn t actually a problem) Condensation 8

9 Vertical structure and the greenhouse effect 9

10 Earth and Venus profiles 10 March , 12Z 2.00 S W Pressure (millibars) 100 Troposphere Stratosphere Tropopause p (bars) Venus Pioneer Venus Large Probe, 1978 Magellan Radio Temperature (Kelvin) T 10

11 In the Red 11

12 Noble Lectures. Toronto, April 2010: Outer Limits In the Red 12

13 Normalized Stellar Flux 3000K 6000K 1000 Absorption Coefficient (m 2 /kg) CO Wavenumber (cm -1 ) 13

14 Flux profile 14

15 1 Pure CO2 G star M star 10 p (mb) Downward Stellar Flux (W/m 2 )

16 In the deep and the dark Shortwave Absorption layer 15

17 But actually, we re in the Venus as Glacier regime 10 Pure CO2, M star No Absorption Adiabat With Absorption Adiabat p (mb) T (K) 16

18 Things that can go wrong Rayleigh scattering increases albedo. (Spectrum-dependent!) Shortwave-absorbing greenhouse gas leads to anti-greenhouse effect Depends on stellar spectrum Problem with CH 4 already for G stars For M stars, potential problem even with CO 2 (But this isn t actually a problem; cf AGU2008) Condensation 17

19 Here s what happens Condensation More CO2 Condensation More CO2 Condensation p s = p sat (T s ) CO2 ocean/glacier 18

20 Here s what happens More CO2 More CO2 Condensation CO2 ocean/glacier p s = p sat (T s ) CO2 ocean/glacier p s = p sat (T s ) CO2 ocean/glacier 19

21 0 Tadiabat TGliese581dHot70 Tcond Stellar Absorption T dew = T 1 1+(RT 1 /L) ln(p/p 1 ) Pressure (Pa) Dry Adiabat Temperature 20

22 Gliese 581d with zero albedo TNoCond TadNoCond TCondEq TadCondEq Pressure (Pa) Condensation Layer T 21

23 100 Gliese 581d with 20% albedo TCondEq T1Alb Tadiabat1Alb TeqAlb 1000 Pressure (Pa) TCondEq 22

24 Generalized CO 2 runaway for Gliese 581d W/m 2 Saturated pure CO 2 atmosphere OLR (W/m 2 ) bar surface pressure Surface Temperature (K) 23

25 At the Outer Limits... Runaway is a good thing for habitability (Keeps your atmosphere in the atmosphere) 24

26 100 High gravity makes things worse Surface Pressure (bar) OLR (W/m 2 ) m/s 2 20 m/s 2 10 m/s 2 3 m/s 2 1 m/s Surface Temperature (K) 25

27 Addition of contaminants can help.. (if CO 2 gets you most of the way to habitability) CO 2 ice clouds (Forget and Pierrehumbert), or... 26

28 Halevy, Pierrehumbert and Schrag

29 What this world needs is a less condensable greenhouse gas CH 4 Problems: Haze clouds, shortwave absorption SO 2? NH 3? Photochemical, solubility problems N 2 at high density Jovian-type H 2 -dominated dense atmospheres Collision-induced absorption critical to much of this 28

30 Each of these gases has its own K-I limit. 29

31 Small, rocky Jupiters? 10-5 Jupiter p (bars) T Water-habitable at 5 bars, 2.4g e! 30

32 Small, rocky Jupiters? A habitable Super-Earth with a 5 bar Jovian-type atmosphere getting 9 W/m 2 of illumination? (about 1 au out from M-dwarf) 31

33 But here s the rub Escape energy goes like gr Super-Earth much smaller than Jupiter Unlikely to hold H 2 unless there is a strong interior supply 32

34 And now for something completely different Cold hydrocarbon lake habitability. (Titan analogues) Think Super-Titans about 1au out from Gliese It has been proposed (cf. Lunine) that biochemistry could occur in methane/ethane lakes For such life, the methane/n 2 runaway is fatal 33

35 Below 90K methane freezes. Titan is near outer limit. But mixture with ethane, N 2, etc. depresses freezing point But it s also near inner limit 10 bar vapor pressure at 150K Shallow methane lakes would evaporate High gravity would translate into deeper ocean

36 Maintaining Titanlike conditions involves N 2 CIA runaway N2-N2 continuum at 100mb N2 Absorption Coefficient (m 2 /kg) K 80K 90K 100K 110K 120K Wavenumber (cm -1 ).06 bar at 60K, 1.4 bars at 80K, 10 bars at 100K, 34

37 Conclusions: Surface water habitability Condensation limits on the outer edge of habitability are determined by a generalization of the runaway greenhouse to gases other than water vapor. Early Mars and Gliese 581d are near the threshold where CO 2 could make make the planet water-habitable. Spicing up the atmosphere with clever, exotic stuff can push it over the threshold, but it doesn t take much to hold these planets back, either. For insolation much below Marslike values, CO 2 becomes ineffective at maintaining water-habitability. Most other gases that could step in to replace it have serious problems. Small Rocky Jupiters an interesting possibility, but dubious whether they could hold their H 2. 35

38 Conclusions: Hydrocarbon lakes and Super Titans Outer limit of cold hydrocarbon lake hab. zone given by methane triple point, or depressed triple point for mixtures. Inner limit given by methane/n 2 runaway. Somewhat over 5W/m 2 illumination, but needs more careful mapping out. Need to keep the N 2 in the atmosphere, but keep some methane left in the lakes/oceans. These things would be at 1 A.U. for an M-dwarf (Lunine) 36