2008 Annual Science Report
Pennsylvania State University Reporting | JUL 2007 – JUN 2008
Modeling Early Atmospheric Composition and Climate
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.
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.
PROJECT INVESTIGATORS:James Kasting
PROJECT MEMBERS:Shawn Domagal-Goldman
RELATED OBJECTIVES:Objective 1.1
Models of formation and evolution of habitable planets
Earth's early biosphere