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Project Progress

Weathering: Our weathering models were made more efficient and flexible this year. The mineral set included in the reactive-transport codes expanded significantly. Gaseous diffusion and aqueous phase flow and reaction codes are being linked to create a state-of-the-art model for vadose-zone weathering. Our weathering model will soon compute O₂ and CO₂ fluxes through the surface. These fluxes will be integrated into the broader VPL model this year.

Our black-shale-weathering model examines weathering-related oxygen fluxes. When the boundary layer is resolved, most ancient organic matter and pyrite is oxidized before reaching the surface, except under very rapid erosion rates (Bolton et al., in press). Oxidation at depth implies a weaker feedback between P_O² and oxygen consumption by organic matter oxidation at the surface than predicted using box models. Thus, processes within the weathering boundary layer can profoundly affect long-term atmospheric evolution. Explicit modeling of these processes is necessary.

Internal Processes and Habitability: S, C, H, and O all are redistributed by life and by internal planetary processes. Our generic terrestrial planet thermal evolution code provides inputs that allow us to explore the critical links between these processes and planetary habitability.

We are working to better understand Earth’s biologically mediated elemental cycles, and how these cycles may differ for lifeless planets. For example, the rapid turnover of sulfur in the Earth’s oceans today is an indirect result of photosynthesis, which provides both the oxidant to convert sulfur dioxide to sulfate and the energy that produces raw materials used by sulfate-reducing bacteria to sequester sulfide in marine sediments, to be subducted and then re-emitted via arc volcanoes. We postulate that volcanic sulfur fluxes on non-photosynthesizing planets (e.g. Hadean Earth) will be much lower.

Intraplate stress maintains the uppermost stable continual crust near frictional failure. Small earthquakes open permeable cracks and allow water to circulate. These quakes also tap sealed-off regions where hydrogen has built up. The release of hydrogen-rich water and its mixing with CO₂-rich water may produce abiotic methane or subsurface microbial plumes.

On early Earth, hydrogen from hot vents at ridge axes, hydrothermal circulation in serpentine, and arc volcanoes were comparable sources of usable energy. On planets with reducing atmospheres photolysis of methane provides a comparable surface niche based on CO and soot. We are able to explore how these energy sources will vary in strength depending on the initial composition and thermal evolution of a planet.

Atmospheric Escape: We modeled atmospheric escape from terrestrial planets using a multi-dimensional hydrodynamic escape model. This year we applied this model to a range of planetary applications including hydrogen escape for early Earth and Venus. These results were given at several conferences and submitted for publication.