NASA Astrobiology Institute (NAI)

In this task we are developing and using models of a terrestrial planet's surface and interior to understand the evolution of planetary environments. These models allow us to understand how interactions between the planetary surface and interior, and life, affect a planet's atmosphere. New models are also exploring the possible habitability of "super-Earths", rocky planets that have been found around other stars that can be up to 10 times more massive than our own Earth.

Weathering Models

This task involves modeling the weathering of rocks, an important component of elemental cycling on habitable planets. Exchange of elements and gases between the solid planet, oceans, and the atmosphere control the atmospheric composition over time. Although much of the exchange between the solid and the fluid portions of the planet may occur via volcanism, long-term exchange due to weathering of rocks and soils is also important. Through detailed modeling of weathering of various rock types we sought to elucidate the exchange of gases with the atmosphere and the exchange of elements between the solid surface and the hydrologic system of terrestrial planets.

Our work here focused on model development and reactive transport modeling in Earth’s crust and weathering at planetary surfaces. We expanded the mineral set included in reactive-transport codes that include mineral reactions, aqueous phase speciation, and surface erosion (Bolton et al., 2007) Codes for diffusion of gaseous species are being linked to aqueous phase flow and reaction codes to create a state-of-the-art model for weathering in the vadose zone. The main focus has been on a one-dimensional weathering model with infiltrating flow through a developing “soil”. A natural drainage rate condition has been coded and tested. Some difficulty with Al speciation has required extra attention due to the extreme pH dependence of which species dominates in solution. Additional options to deal with this problem are in progress.

Our black shale weathering model, which re-examines oxygen fluxes and the boundary condition relevant for long-term atmospheric oxygen evolution, was published in the prior year. When the boundary layer is resolved, most of the ancient organic matter and pyrite would be oxidized before reaching the surface, except for very rapid erosion rates, unlike the predictions of previous simple box models. This would imply less oxygen concentration feedback than previous workers found.

We have assembled mineralogic compositions for typical igneous rock types in preparation for modeling weathering of abiotic terrestrial planets, with special focus on fluxes of volatile gases that could, over time, change planetary atmospheric compositions. We are exploring the implications of hypothesized high-temperature Archean conditions on granite and basalt weathering, as compared to present-day condition (Rye et al., 2007). Both O₂ and CO₂ exchange at the surface has been implemented, so that fluxes to the surface can be computed by our weathering model and will be useful components of the broader VPL.

Plate Tectonics and Super-Earth models

In this task we wrote a review of plate tectonics through the lifetimepl of a terrestrial planet (Sleep 2007) and developed theories that link early geological events and processes with biological evolution, including the possibility that early microbial life may have aided weathering, increasing access to ferrous iron (Sleep and Bird, 2008). We also quantitatively explored the hypothesis that episodic faulting on early cratons releases batches of H2-rich or CH4-rich water into rock environments, putting the reactants well above the level that abiotic processes can consume them. This then provides nutrients for microbial populations. Consequently strain and faulting are attractive mechanisms for maintaining habitability in the upper 5km or so of the cratonic crust (Sleep and Zoback, 2007).

In addition to this exploration of habitability on the very early Earth, we explored the effect of tectonics on habitability of silicate super-Earths, concluding that ***VIGOROUS tectonics is likely to be a characteristic of terrestrial planets lager than the Earth, but our current understanding of the Earth poorly constrains the mode of tectonics***. We also explored other effects on planetary habitability that scale with planetary mass including asteroidal impact and atmospheric escape (Sleep, 2008). We postulate that a super-Earth with 1 bar of H2 is conceivable and that life on it would need to operate in a highly reduced atmosphere.