NASA Astrobiology Institute (NAI)

1. Modeling Early Atmospheric Composition and Climate

   Project Investigators: James Kasting

   Other Project Members

   Adam Edson (Doctoral Student)

   Shawn Goldman (Doctoral Student)

   Mike Procopio (Undergraduate Student)

   Jacob Haqq-Misra (Doctoral Student)

   Summary

   We have updated our methane greenhouse model for the early Earth by including the greenhouse effect of ethane and the anti-greenhouse effect of organic haze. We analyzed the mass-independent sulfur isotopic record to find evidence for the existence of such haze during the mid-Archean, between 3.2 and 2.8 Ga. And we worked on the problem of hydrogen escape from the early Earth.

   Astrobiology Roadmap Objectives:

   • Objective 1.1: Models of formation and evolution of habitable planets
   • Objective 4.1: Earth's early biosphere

   Project Progress

   We worked on several aspects of Earth’s early evolution during this time frame. Following earlier work (J. F. Kasting et al., EPSL, 2006), we continued the debate about whether the early Earth was warm (our idea) or hot, i.e., 70°C, as has been claimed by several authors (P. Knauth and D. Lowe, GSA Bull., 2003; F. Robert and M. Chaussidon, Nature, 2006). In a reply to the Robert and Chaussidon article, Graham Shields and I (Shields and Kasting, 2007) argued that the Si isotope trend measured by these authors could be explained without requiring hot early oceans.

   My students and I also published two papers related to the Earth’s early atmosphere. In Haqq-Misra et al. (in press), we redid climate calculations for CH₄-rich atmospheres during the Archean. Our own previous modeling (A. Pavlov et al., JGR, 2000) turns out to have been in error because of a mistake in how the CH₄ absorption coefficients were put into the model. (They were slightly shifted in wavelength from where they should have been.) When we corrected the problem with the absorption coefficients, the amount of greenhouse warming produced by CH₄ diminished considerably. However, we recovered much of that warming by including ethane, C₂H₆, in the climate model. Ethane is produced photochemically from CH₄ in quantities that we can predict using our photochemical model. The story is even more complicated, though, because CH₄ also polymerizes to form hydrocarbon smog, and this smog can cool the surface by creating an anti-greenhouse effect. The bottom line is that a CH₄ greenhouse can still warm the early Earth, but its effectiveness would have been limited because of the formation of organic haze.

   In the second early Earth paper (Domagal-Goldman et al., 2008), we showed that the presence of an organic haze could have explained the glaciation that occurred in the mid-Archean era, around 2.9 Ga. It may also have accounted for the smaller range in Δ³³S values seen in sulfide and sulfate minerals formed near that time. Δ³³S represents the deviation of 33S from the mass-dependent sulfur fractionation line. We know from previous work (J. Farquhar et al., Science, 2000) that such deviations can be caused by UV photolysis of SO₂ in a low-O₂/low-O₃ atmosphere. In our model, the presence of organic haze reduces the rate of SO₂ photolysis and therefore limits the magnitude of the Δ³³S signal.

   In other work with my NAI postdoc, Feng Tian, we developed a 1-D hydrodynamic model of Earth’s upper atmosphere (Tian et al., 2008). We plan to apply this model to study hydrogen escape on the early Earth and on early Venus and Mars, as well. More recently (Tian et al., submitted), we have used this model to study thermal escape of C and O from a hypothetical CO₂-rich atmosphere on early Mars. Because the Sun’s EUV (extreme ultraviolet) luminosity was high during early Solar System history, and because Mars’ gravity is low, these heavier gases may also have been able to escape quite readily. This may be one reason why Mars’ present atmosphere is so thin.

   Mission Involvement

   TPF

   Models of Earth's early atmosphere will provide reference for early-Earth-type planets that may eventually be found around nearby stars.

   Cross-Team Collaborations

   Much of this work is done in collaboration with Vikki Meadows’ NAI group, now headquartered at the University of Washington. Feng Tian did his initial postdoc work under the auspices of Vikki’s group. My ex-Ph.D. student Shawn Goldman is now a postdox with Vikki.

Publications

Catling, D. & Kasting, J.F.  (2007).  Planetary atmospheres and life.  In: W.T. Sullivan & J.A. Baross (Eds.).  Planets and Life: The Emerging Science of Astrobiology (pp. 91-116).  Cambridge: Cambridge Univ. Press.

Domagal-Goldman, S.D., Kasting, J.F., Johnston, D.T. & Farquhar, J.  (2008).  Organic haze, glaciations and multiple sulfur isotopes in the Mid-Archean Era.  Earth and Planetary Science Letters, 269(1-2):29-40  [Online].

Haqq-Misra, J.D., Goldman, S.D., Kasting, P.J. & Kasting, J.F.  (in preparation).  A methane/ethane greenhouse for the early Earth.  Astrobiol..

Kasting, J.F.  ().  Habitable planets around the Sun and other stars.

Kasting, J.F.  (submitted).  The global O2 cycle.  In: K.O. Konhauser, A.H. Knoll & D. Canfield (Eds.).  Fundamentals of Geobiology.  Blackwell.

Lammer, H., Kasting, J.F., Chassefiere, E., Johnson, R.E., Kulikov, Y.N. & Tian, F.  (in press).  Atmospheric evolution of terrestrial planets and satellites.  Space Sci. Rev..

Segura, A., Meadows, V.S., Kasting, J., Cohen, M. & Crisp, D.  (2007).  Abiotic production of O2 and O3 in high-CO2 terrestrial atmospheres.  Astrobiology, 7(3):494-495  [Online].

Selsis, F., Chazelas, B., Borde, P., Ollivier, M., Brachet, F., Decaudin, M., Bouchy, F., Ehrenreich, D., Griessmeier, J.M., Lammer, H., Sotin, C., Grasset, O., Moutou, C., Barge, P., Deleuil, M., Mawet, D., Despois, D., Kasting, J.F. & Leger, A.  (2007).  Could we identify hot ocean-planets with CoRoT, Kepler and Doppler velocimetry?.  Icarus, 191(2):453-468  [Online].

Selsis, F., Kasting, J.F., Paillet, J., Levrard, B. & Delfosse, X.  (2007).  Habitable planets around the star Gl581.  Astron. Astrophys., 476:1373-1387.

Shields, G.A. & Kasting, J.F.  (2007).  Evidence for hot early oceans?.  Nature, 447(7140):E1-E1  [Online].

Tian, F., Kasting, J.F., Liu, H.L. & Roble, R.G.  (2008).  Hydrodynamic planetary thermosphere model: 1. Response of the Earth's thermosphere to extreme solar EUV conditions and the significance of adiabatic cooling.  Journal of Geophysical Research-Planets, 113(E5)  [Online].

Tian, F., Kasting, J.F. & Solomon, S.C.  (submitted).  Fast thermal escape of carbon and oxygen from a dense, CO2-rich early martian atmosphere.  Science.