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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.