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Project Progress

Weathering Models
We continued to develop and use a reactive transport model to simulate weathering at planetary surfaces. We expanded the mineral set. We added atmosphere/soil water exchange and surface flux computation for the volatiles CH₄, H₂, H₂S, SO₂, NH₃, and N₂. We solved difficulties with the pH dependence of Al speciation. We implemented iron redox kinetics and speciation. We formulated an implicit method to decrease computational time. Coding is in progress, and testing is planned for the immediate future.

Tidal Heating
We seek to model how tidal heating may affect volcanism and on terrestrial planets in orbit about M stars. All planets in eccentric orbits experience tides. If a planet is close enough to its star tidal energy may be dissipated through deformation, heating, and even melting of a portion of the planet’s interior. Many terrestrial planets within the nominal habitable zone about M stars will experience such tidal forces and resultant volcanism. Results to date indicate that tidal heating may affect habitability in two important ways: 1) Massive tidal heating may lead to rapid resurfacing (as on Io) and sterilization; 2) Lower levels of tidal heating may initiate plate tectonics on a planet with little radiogenic heating and maintain habitability for billions of years. We therefore propose that habitable planets must have less tidal heating than Io, but more than the minimum needed for plate tectonics (Barnes et al., 2009; Jackson et al., 2008)

To further pursue the role of tidal heating in determining habitability we are combining and modifying two extant models recently provided by non-VPL colleagues: 1) a 1-D planetary thermal evolution model (without tidal heating) and, 2) a tidal dissipation (heating) model.

Earth As a Test Case
We examined the linkages between global geochemical cycles, biological evolution (e.g. metabolisms), and the environment on early Earth. Model results indicate that volcanic activity and serpentinization were reliable sources of energy (via H2) for early pre-photosynthetic organisms: such processes may provide support for life on extrasolar planets. Results also suggest biological enhancement of weathering led to the development of sandstones, shales, carbonates, and (indirectly) granites (Sleep and Bird, 2008). This process needs to be incorporated into weathering models of extrasolar planets.

Our studies of Earth’s mantle carbon cycle during and after the Earth-Moon impact provide strong support for the notion that the Earth’s mantle sequesters a biological record (Sleep 2009). This observation will lead to consideration of how biology may affect the interiors of other planets.

Work has started on predicting the chemistry of mafic rocks exposed to 350°C rain after an ocean boiling impact and of rocks exposed to ~200°C rain during an epoch of the 100 bar CO₂. Candidate rocks exist along Hudson Bay in Canada. Analog rocks exist within fresh water hydrothermal systems in Iceland.