1. Climate Evolution on Venus and Mars James Kasting Department of Geosciences Penn State University

2. Talk Outline Part 1: Evolution of Venus’ atmosphere: runaway and moist greenhouses Part 2: Evolution of Mars’ atmosphere: the H 2 -CO 2 greenhouse

3. 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?

4. Possible answers: 1)Venus never had any water to begin with -- But this is unlikely for a variety of reasons, including the high D/H ratio 2) Venus’ climate got out of control because of positive feedback loops in the climate system -- The positive feedback loop involving water vapor can lead to a runaway greenhouse  Question: What went wrong with Venus?

5. 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

6. 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

7. 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 

8. 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

9. 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

10. 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 The 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 Catling and Kasting, Atmospheric Evolution, Appendix B, in prep.

11. 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

12. 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

13. 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

14. 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 Venus is at 0.72 AU Moist green- house Runaway greenhouse

15. Update: New albedo calculations using the HITEMP database Goldblatt model * Kasting (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 Goldblatt et al., Nature Geosci. (2013)

16. 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)

17. 3-D modeling of runaway greenhouse atmospheres Fortunately, new studies using 3-D climate models show that the runaway greenhouse threshold is increased by ~10% because the tropical Hadley cells act like radiator fins –This behavior was pointed out 20 years ago by Ray Pierrehumbert (JAS, 1995) in a paper dealing with Earth’s tropics This puts the runaway greenhouse threshold back at 0.95 AU (S eff  1.1) Leconte et al., Nature (2013) Outgoing IR radiation

18. Venus lost its water either during or after accretion because it developed either a runaway or moist greenhouse, allowing hydrogen to escape rapidly to space –Hamano et al. (Nature, 2013) (not discussed) argue that the runaway greenhouse was triggered during accretion, so that water loss occurred early –This model is likely correct, as it more easily explains the loss of Venus’ oxygen Once the water was gone, volcanic CO 2 (and SO 2 ) built up in Venus’ atmosphere, leading to its present, hellish state Summary of Part 1: Evolution of Venus’ atmosphere

19. Part 2: Evolution of Mars’ atmosphere: the H 2 -CO 2 greenhouse

20. Evolution of Mars’ climate and atmosphere Let’s switch now to Mars Present Mars is cold and dry, but early Mars looks like it was wet, and maybe warm

21. MARS PATHFINDER Twin peaks view Mars today is a frozen desert But the surface of Mars is riddled with ancient fluvial features

22. Nirgal Vallis (Viking) From: J. K. Beatty et al., The New Solar System, 4 th ed. 200 km

23. Terrestrial vs. martian valleys Colorado River Canyon Nanedi Vallis 10 km Valleys on Mars look a lot like valleys (or canyons) on Earth Figure credit: C. Harman (unpublished)

24. How did the martian valleys form? Two basic types of models: 1.Warm early Mars model Mars’ early climate was essentially Earth-like. The water needed to carve the valleys came from rainfall (Pollack et al., 1987) 2.Cold early Mars model(s) Mars was cold most of the time, but it warmed up sporadically due to impacts (Segura et al., Science, 2002; Segura et al., JGR, 2008; Segura et al., Icarus, 2012)

25. Nanedi Vallis (from Mars Global Surveyor) ~3 km River channel The Grand Canyon on Earth required ~17 million years to form During this time, roughly 5  10 6 m of rain should have fallen on the Colorado plateau (assuming 30 cm/yr of rainfall) By contrast, those who have argued that the martian valleys could have formed in a cold environment, following large impacts (Segura et al. Science, 2002) assume much lower volumes of precipitated water, 50-500 m The terrestrial analogy suggests that these estimates are too low by a factor of 10 4 -10 5 !

26. How could early Mars have been kept warm? To answer this question, we first need to think about how Earth remained habitable early in its history –I talked about this in the previous lecture Low solar luminosity was a problem for the Earth, as well Earth has stabilizing feedbacks, though, most importantly the one between CO 2 and climate, acting through the carbonate-silicate cycle…

27. The carbonate-silicate cycle This cycle regulates Earth’s atmospheric CO 2 level over long time scales and has acted as a planetary thermostat during much of Earth’s history, because CO 2 builds up as the climate cools This same negative feedback could have operated on early Mars

28. But, there is a problem with early Mars because a CO 2 -H 2 O greenhouse is not capable of warming the planet 

29. J. F. Kasting, Icarus (1991) Martian surface temperature vs. pCO 2 and solar luminosity Previous calculations showed that greenhouse warming by CO 2 (and H 2 O) could not have kept early Mars’ mean surface above freezing S/S 0 = 0.75 at 3.8. b.y. ago, when most of the valleys formed Freezing point of water

30. Why is it difficult to warm early Mars? Two reasons: –Condensation of CO 2 reduces the tropospheric lapse rate, thereby lowering the greenhouse effect –CO 2 is a good Rayleigh scatterer (2.5 times better than air), so as surface pressure increases, the increase in albedo outweighs the increase in the greenhouse effect CO 2 condensation region J. F. Kasting, Icarus (1991)

31. Fortunately, it may be able to supplement the greenhouse effect on early Mars by adding additional greenhouse gases –CH 4 doesn’t work. It’s main absorption band, at 7.7  m, is overlapped by CO 2 when CO 2 levels are high, plus it produces anti-greenhouse cooling from absorption of near- IR solar radiation in the stratosphere –SO 2 doesn’t work. It rains out when the climate is warm, and it forms sulfate aerosols when oxidized that reflect incoming solar radiation –H 2 works, however! H 2 has no permanent electric dipole moment, but collision-induced absorption is able to excite its pure rotational spectrum

32. Science, Jan. 4, 2013 As shown by Wordsworth and Pierrehumbert (2013), H 2 can be an excellent greenhouse gas when present in sufficient concentrations Could H 2 have warmed early Mars?

33. Warming early Mars with H 2 and CO 2 A combination of 1.3-4 bar of CO 2 and 5-20% H 2 by volume is enough to keep early Mars warm This amount of H 2 could have been provided by volcanic outgassing on Mars if the planet was actively recycling volatiles, as Earth does, and if Mars’ mantle was more reduced (which it was, based on evidence from SNC meteorites)

34. Conclusions Venus formed too close to the Sun to hold onto its water, and hence lost that water by a runaway or moist greenhouse –The distance within which this occurs is ~0.95 AU (i.e., S eff  1.1) Mars formed too far from the Sun to be warmed by a CO 2 -H 2 O greenhouse, but it could have been kept warm by the addition of 5-20% H 2 –The distance beyond which CO 2 and H 2 O cannot warm the planet—termed the ‘maximum greenhouse’ limit—is at ~1.7 AU for present solar luminosity (S eff  0.34) These calculations can be used to define the width of the habitable zone, which I will talk about tomorrow

35. Backup slides

36. Sources of H 2 on early Earth H 2 could have come from volcanism On Earth, H 2 O and CO 2 are the main gases emitted by volcanoes The H 2 :H 2 O ratio in volcanic gases is determined by the equilibrium reaction H 2 O  H 2 + ½O 2 pH 2 /pH 2 O = K eq /fO 2 ½ Mantle fO 2 is near QFM (~10  8.5 atm at 1450 K), so pH 2 /pH 2 O  0.024

37. Mantle fO 2 buffers QFM Quartz-fayalite-magnetite SiO 2 -FeSiO 3 -Fe 3 O 4 This is essentially the boundary between Fe +2 and F At 1200 o C, fO 2 is about 10 -8.5 atm IW Iron-wüstite Fe-FeO This is the boundary between Fe +2 and Fe 0 At 1200 o C, fO 2 is about 10 -12.5 atm

38. Oxygen fugacities of SNCs C. K. Shearer et al., Amer. Mineralogist (2006) QFM But, Mars’ mantle appears to be more reduced than Earth’s, based on evidence from SNC meteorites The reason may have to do with Mars being smaller, so that ferrous iron did not disproportionate in its lower mantle

39. Finally, it looks now as if Venus’ runaway greenhouse occurred during accretion and never collapsed 

40. Other work considered what may have happened during planetary accretion Type I planets, like the Earth, develop steam atmospheres during accretion, but then these atmospheres collapse to form oceans within ~5 million years Nature, 2013 Type I planet (Earth-like)

41. Type II planets, like Venus, are too close to the Sun for the steam atmosphere to collapse, so the atmosphere remains in the runaway greenhouse phase until all the water is lost This model can more easily account for the loss of the oxygen left over from H escape, because it can react with the molten Earth as the escape happens Type II planet (Venus-like)

42. Martian ‘Outflow’ Channel (Viking) From: J. K. Beatty et al., The New Solar System, 4 th ed ~ 200 km

43. Physics of H 2 absorption The energy levels of a (homonuclear) rigid rotator are spaced as: Here, I is the moment of inertia of the molecule, R is the intermolecular spacing, and m is the mass of either atom. The spacing between levels is readily shown to be

44. Physics of H 2 absorption (cont.) If we express energies in terms of equivalent wavenumbers (1/ ), then So, remembering that we see that we can absorb at 1000 cm -1 (10  m) at j = 7 for H 2, but that we would need j  250 for N 2. This doesn’t happen because of non-rigid effects (bond stretching and breaking).