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Modeling Atmospheres of Planets and Exoplanets

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Presentation on theme: "Modeling Atmospheres of Planets and Exoplanets"— Presentation transcript:

1 Modeling Atmospheres of Planets and Exoplanets
James Kasting Department of Geosciences Penn State University

2 Book Advertisement If you want to learn a lot about modeling planetary atmospheres, pick up a copy of our new book Just out in May, 2017 15 chapters, 3 appendices Lots of equations 579 action-packed pages Cambridge Univ. Press, 2017

3 Department of Geosciences
Habitable Zones Around Stars and Climate Limit Cycling on Planets Like Early Mars James Kasting Department of Geosciences Penn State University

4 Talk Outline Part 1—What are the requirements for life?
Part 2—Definition and boundaries of the habitable zone Part 3—Climate limit cycling Part 4—Application to early Mars

5 Part 1—What are the requirements for life?

6 First requirement for life: a liquid or solid surface
It is difficult, or impossible, to imagine how life could get started on a gas giant planet Need a liquid or solid surface to provide a stable P/T environment This requirement is arguably universal

7 Second requirement for life: carbon
Carbon is unique among the elements in forming long, complex chains Something like 95% of known chemical compounds are composed of organic carbon Silicon, which is located right beneath carbon in the Periodic Table, forms strong bonds with oxygen, creating rocks, not life Proteins

8 Third requirement for life (as we know it) : Liquid water
Life on Earth requires liquid water during at least part of its life cycle So, our first choice is to look for other planets like Earth Subsurface water is not relevant for remote life detection because it is unlikely that a subsurface biota could modify a planetary atmosphere in a way that could be observed (at modest spectral resolution)

9 Part 2—Definition and boundaries of the habitable zone

10 Definitions (from Michael Hart, Icarus, 1978)
Habitable zone (HZ) -- the region around a star in which an Earth-like planet could maintain liquid water on its surface at some instant in time Continuously habitable zone (CHZ) -- the region in which a planet could remain habitable for some specified period of time (e.g., 4.6 billion years)

11 The carbonate-silicate cycle
The habitable zone is relatively wide because of negative feedbacks in the carbonate-silicate cycle: Atmospheric CO2 should build up as the planet cools Higher CO2 is also at least part of the problem to the faint young Sun problem on Earth

12 The carbonate-silicate cycle
Caveat: For planets with low volcanic outgassing rates and with low stellar insolation, it may not be possible to maintain warm climates all the time. Instead, one may get limit cycling behavior, in which the climate alternates between warm and globally glaciated states (see, e.g., Kadoya and Tajika, 2014; Menou, 2015; Haqq-Misra et al., 2016)

13 The ZAMS habitable zone
With this in mind, one gets a habitable zone that is fairly wide compared to the mean planetary spacing Figure applies to zero-age-main-sequence stars; the HZ moves outward with time because all main sequence stars brighten as they age

14 The ZAMS habitable zone
Note that Earth is comfortably within the HZ at this time in this particular calculation

15 But then Colin Goldblatt and colleagues came along and showed that H2O absorbs weakly all the way through the visible, using the HITEMP database instead of HITRAN (Nature Geoscience, 2013) This moved the inner edge of the Sun’s present HZ perilously close to 1 AU Visible H2O absorption

16 3-D modeling of habitable zone boundaries
Fortunately, new studies using 3-D climate models predict that the runaway greenhouse threshold is increased by ~10% because the tropical Hadley cells act like radiator fins This was pointed out 20 years ago by Ray Pierrehumbert (JAS, 1995) in a paper dealing with Earth’s tropics We have adjusted our (1-D) HZ inner edge back inward to 0.95 AU to account for this behavior Outgoing IR radiation Leconte et al., Nature (2013)

17 Updated habitable zone (Kopparapu et al., 2013, 2014)
Note the change in the x-axis from distance units to stellar flux units. This makes it easier to compare where different objects lie Credit: Sonny Harman

18 Updated habitable zone (Kopparapu et al., 2013, 2014)
Conservative HZ We should define the HZ conservatively when designing our space telescopes, so that we don’t underestimate the problem Credit: Sonny Harman

19 Updated habitable zone (Kopparapu et al., 2013, 2014)
Optimistic HZ Once we’ve got these telescopes built and launched, we can afford to be more optimistic, so that we don’t overlook interesting planets Credit: Sonny Harman

20 Updated habitable zone (Kopparapu et al., 2013, 2014)
The latest version of this figure includes the Trappist-1 planets, along with Proxima Centauri b Credit: Sonny Harman

21 Part 3—Climate limit cycling

22 A (relatively) new paper by Kristen Menou shows that planets near the outer edge of the habitable zone should not have stable, warm climates, despite the influence of the carbonate-silicate cycle See also Kadoya and Tajika (ApJ, 2014), along with earlier papers by Tajika, referenced therein Menou used a parameterized version of an energy-balance climate model (EBM) 

23 Energy Balance Models (EBMS)
EBMs are sometimes referred to as 1.5-D climate models They treat latitudinal heat transport like diffusion The diffusion coefficient is adjusted to get the right equator-to-pole T gradient The surface heat capacity is adjusted to get the right seasonal cycle Ice albedo feedback is included in such models Caldeira & Kasting, Nature (1992) Stable solutions Unstable solutions

24 Menou’s limit cycling model
One needs to simultaneously solve for surface temperature, Tsurf, as a function of pCO2 and for pCO2 as a function of Tsurf The radiation balance is done using a fit to Darren Williams’ 1997 EBM The EBM parameterization itself was created by fitting results from our own 1-D radiative-convective climate model

25 Menou’s model (cont.) The CO2 model balances removal by weathering, W, with production from volcanism, V The weathering rate parameterization is from Berner and Kothavala (2001). It applies to an abiotic Earth

26 When Menou put all of this together, he found that stable steady states are not achieved under some circumstances, e.g., low stellar heating or low rates of volcanism Instead, the planet’s climate is predicted to undergo limit cycles of global glaciation followed by deglaciation 

27 Limit cycles on poorly lit planets
An Earth-like planet at 1 AU from its parent star has a stable, warm climate state. Snowball climate states exist, but they go away because of volcanic CO2 buildup An Earth-like planet at 1.6 AU has no stable states but, rather, cycles between warm and cold (Snowball) climate states Present Earth Unstable Snowball Earth Limit cycles IR cooling Solar heating Different weathering rates K. Menou, EPSL (2015)

28 Our new limit cycle figure
We have tried to illustrate this behavior in a different way Stable climate states are achieved when the surface temperature curves (in blue for different solar luminosities) intersect the weathering rate curve above the freezing point of water Limit cycles are predicted when the intersection occurs below the freezing point Present Mars Haqq-Misra et al., ApJ (2016)

29 Example of a limit cycle
Tmax pCO2 Haqq-Misra et al., ApJ (2016) This shows limit cycling behavior in our model for a planet orbiting a G star with 70% solar luminosity and 1/10th the present volcanic outgassing rate Tmax represents the maximum surface temperature, which occurs in the tropics in this model

30 Limit cycling near the HZ outer edge
Figure credit: Sonny Harman Limit cycling should be most pronounced for small planets (low V/V) orbiting in the outer HZs of F and G stars (because ice albedo feedback is strongest when the star’s output peaks in the visible) This phenomenon may have implications for the existence of complex (animal) life, including intelligent life Haqq-Misra et al., ApJ (2016)

31 Part 4—Application to Mars

32 Did climate limit cycling occur on Mars?
Mars is a small planet near the outer edge of the HZ of a G star (the Sun)  it could have experienced climate limit cycling early in its history Actually, early Mars is slightly beyond the conventional maximum greenhouse limit for the outer edge of the HZ  Warrego Vallis

33 Maximum greenhouse limit
Mars S/S0 = 0.75 at 3.8. b.y. ago, when most of the valleys formed The minimum solar flux at which you can warm the mean surface temperature of Mars to >OoC is about 82% of the present solar flux at Mars’ orbit, or ~36% of the present solar flux at Earth R. Ramirez et al., Nature Geosci. (2014)

34 As shown by Wordsworth and Pierrehumbert (2013), H2 can be an
Science, Jan. 4, 2013 As shown by Wordsworth and Pierrehumbert (2013), H2 can be an excellent greenhouse gas when present in sufficient concentrations Collisions excite the pure rotational spectrum of H2

35 Warming early Mars with H2 and CO2 A combination of 1.3-4 bar
of CO2 and 5-20% H2 by volume is enough to keep early Mars warm Indeed, one might be able to get by with even less… Ramirez et al., Nature Geosciences (2014)

36 (2016) A new paper by Robin Wordsworth and colleagues shows that collision-induced absorption (CIA) by H2 is much more effective when the broadening gas is CO2, rather than N2 --The stronger electric quadrupole moment of CO2 (a linear molecule) is to blame Collision-induced absorption is also important for CH4-CO2

37 Collision-induced absorption of H2 and CH4 induced by CO2 vs. N2
Wordsworth et al., GRL (2016) CIA coefficients for H2 are 5-6 times higher for CO2 than for N2

38 Wordsworth et al. propose a fairly complicated model in which CH4 produced early in the planet’s history by outgassing or serpen-tinization was stored in clathrates and then released episodically during periods of high obliquity But the same type of episodic behavior could be caused by limit cycling  Wordsworth et al., 2017, Fig. 3

39 Limit cycling on early Mars
Limit cycling occurs on early Mars for some choices of volcanic outgassing rates and weathering rate parameters This requires a fair amount of H2 and CO2 in our model These calculations need to be redone using the new CO2-H2 CIA coefficients from Wordsworth et al. (2017) Batalha et al., EPSL (2016)

40 Limit cycling on early Mars
Batalha et al., EPSL (2016) The limit cycle duration depends on multiple parameters. For reasonable choices, one can get warm periods lasting millions to tens of millions of years

41 Conclusions Detectable life requires, at a minimum, a planet with a solid (or liquid) surface, sufficient availability of carbon, and surface liquid water Habitable zones should be defined conservatively if they are being used to generate design parameters for future space-based telescopes Climate limit cycling is predicted to occur on planets with low volcanic outgassing rates located near the outer edge of the HZs around F and G stars Climate limit cycling is also predicted to have occurred during some time periods early in Mars’ history

42 Backup slides

43 pCO2 dependence of the weathering rate
The key parameter here, is , the power law dependence of the weathering rate on pCO2 W  pCO2  = 0.5 if weathering is proportional to dissolved [H+]   0 today because soil pCO2 is thought to be independent of atmospheric pCO2 The terrestrial biological pump: Vascular plants enhance soil pCO2 through root respiration and release of humic acids

44 Spectral distribution of stellar radiation
When we do this calculation for planets around other stars, we need to account for the spectral distribution of the star’s radiation Hotter, blue stars emit more radiation in the visible and UV Cooler, red dwarf stars emit more of their radiation at longer, near-infrared wavelengths Segura et al., Astrobiology (2005)

45 Ice albedo vs. wavelength
This makes a difference because the albedo of snow and ice is much lower in the near-infrared than in the visible Ice albedo feedback is therefore much stronger on planets orbiting F and G stars Consequently, their planets are more susceptible to limit cycling  Warren et al., JGR 107, 3167 (2002)

46 Collision-induced absorption of H2 and CH4 induced by CO2 vs. N2
For a 1.5-bar back-ground atmosphere, replacing N2 by CO2 lowers the required concentration of H2 from 5% to about 2.5% This lowers the required amount of volcanic outgassing Slowing H escape below the diffusion limit might also help Wordsworth et al., GRL (2016)

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