NASA Astrobiology Institute (NAI)

In this project, scientists from different disciplines model the conditions likely to have been found on the Early Earth, prior to 2.3 billion years ago. Specific areas of research include understanding the gases, many biologically produced, and mechanisms that controlled early Earth’s surface temperature, the nature of hazes that shielded the planetary surface from UV and may be responsible for signatures in sulfur isotopes that were left in the rock record, the chemical nature of the Earth’s environment during and after a planet-wide glaciation (a “Snowball event”), the evolution of planetary atmospheres over time due to loss of atmosphere to space, and the use of iron isotopes as a tracer of the oxidative state of the Earth’s ocean over time.

We published a series of papers on the early Earth, primarily focusing on the climatic and redox evolution of Earth from the Archean through the early Proterozoic.

The Early Earth’s Climate

Climatic studies included a follow-up on earlier work (J. F. Kasting et al., EPSL, 2006), where we continued the debate about whether the early Earth was warm (our idea) or hot, i.e., 70°C (P. Knauth and D. Lowe, GSA Bull., 2003; F. Robert and M. Chaussidon, Nature, 2006). In a reply to Robert and Chaussidon (2006), we argued (Shields and Kasting, 2007) that the Si isotope trend measured by these authors could be explained without requiring hot early oceans. In a separate study (Haqq-Misra, et al), we ran climate calculations for methane (CH4) rich atmospheres. In this work, we corrected a mistake in previous work (A. Pavlov et al., JGR, 2000) in how the CH₄ absorption coefficients were put into the model. After this correction, these models predict that a CH₄/C₂H₆ greenhouse can still warm the early Earth, but its effectiveness would have been limited because of the formation of organic haze.

The photochemical effects of organic haze were examined by another paper (Domagal-Goldman et al., 2008), in which we showed that its presence could have explained both a glaciation and the smaller range in Δ³³S values seen in the mid-Archean. In our model, the presence of organic haze shields SO₂ from UV photolysis and limits the magnitude of the Δ³³S signal, while creating an anti-greenhouse effect that cools the surface. A separate study (Catling, Claire, and Zahnle) also examined feedbacks between the sulfur cycle and climate by modeling how a biological feedback between oxygen production and oceanic sulfate would throttle methane production, a positive feedback on the rise of atmospheric oxygen.

Chemical Evolution of the Early Earth

We also examined other links between climate and Archean surface chemistry during “Snowball Earth” type events. Specifically, we ran models to show how a weak hydrological cycle coupled with photochemical reactions involving water vapor could have given rise to the sustained production and sequestration of hydrogen peroxide. This compound would have been buried in the snow/ice and would have accumulated over the lifetime of the Snowball and would have been released upon melting. This could explain global oxidation events in the aftermath of the Snowball seen in the rock record. Additionally, low levels of peroxides generated during Archean and earliest Proterozoic non-Snowball glacial intervals could have driven the evolution of oxygen-mediating enzymes and thereby paved the way for the eventual appearance of oxygenic photosynthesis. Other work relating to the chemical composition of the Archean atmosphere involved the development of a 1-D hydrodynamic model of Earth’s upper atmosphere (Tian et al., 2008). 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 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, perhaps contributing to the thinness of Mars’ present atmosphere.
Fe Isotope Fractionation and Evolution of the Earth’s Oceans

We also used molecular modeling techniques to examine the theoretical basis for using Fe isotopes as a tracer of the evolution of the oxidation state of Earth’s oceans. Two papers on this have recently been accepted for publication (Domagal-Goldman and Kubicki, accepted; Domagal-Goldman et al., accepted) that predict equilibrium Fe isotope fractionations caused by oxidation and by organic complexation. Our results agree with the sentiment that the main control on Fe isotope fractionations is redox reactions, and therefore support with the use of Fe isotopes as a tracer of oceanic redox state.