2011 Annual Science Report
VPL at University of Washington Reporting | SEP 2010 – AUG 2011
Planetary Surface and Interior Models and SuperEarths
We use computer models to simulate the evolution of the interior and the surface of real and hypothetical planets around other stars. Our goal is to work out what sorts of initial characteristics are most likely to contribute to making a planet habitable in the long run. Observations in our own Solar System show us that water and other essential materials are continuously consumed via weathering (and other processes) and must be replenished from the planet’s interior via volcanic activity to maintain a biosphere. The surface models we are developing will be used to predict how gases and other materials will be trapped through weathering over time. Our interior models are designed to predict how much and what sort of materials will come to a planet’s surface through volcanic activity throughout its history.
Weathering Models: We continued to develop and use a reactive transport model to simulate weathering at planetary surfaces. The minerals and solute species in the model were further expanded. Volatile exchange between soil and atmosphere for 8 gases has been tested. Numerical simulation of iron redox kinetics and speciation is quite challenging and additional testing is in progress. We implemented another new implicit method to decrease computational time, which helps speed up calculations by a factor of one million for some mineral sets, but the mineral set for the paleosol currently under consideration has a speed up of only 100. We have extended the one-dimensional model to 2D in order to test the influence of heterogeneity in rocks and ancient soils.
Tidal Heating and Thermal Evolution: We have begun testing of our integrated model to simulate the thermal evolution of planets subjected to tidal heating. The present model calculates non-tidal heat and material fluxes (those arising from mantle convection, conduction, volcanic activity, radioactive decay, etc.) using a simple 1-D numerical model. At the end of each timestep, tidal dissipation is calculated using a 3-D model, tidal dissipation heat fluxes are fed back into the 1-D thermal evolution portion of the code, and the whole suite of models advances to the next timestep. We have added algorithms for varying mantle rigidity in response to extreme tidal heating. These algorithms have greatly enhanced model stability. The present model suite is designed to simulate the evolution bodies as small as Jovian moons and as large as so-called Super-Earths (~10 Earth masses).
Sleep (2011a, 2011b) studied the physics of stagnant-lid convection to aid in finding potentially observable features that would calibrate this process. The increase of viscosity with depth along an adiabat is important. Cool downwelling material tends to pond with deeper more viscous mantle at ~300 km depth. Seismic studies may have detected such downwellings. Oceanic high flow and subsidence data are compatible with weak stagnantlid convection beneath old oceanic lithosphere and hence similar viscosities in the upper asthenosphere beneath oceans and continents. Alternatively an increase of asthenospheric viscosity with depth would allow stagnant-lid convection to supply equilibrium heat flow beneath both old oceans and stable continents.
Earth as a Test Case: Our studies of the gross geochemical signatures related to life continue apace. In particular, we have explored the nature of biological signatures in hard crustal rocks and in the mantle. We have found that life greatly increases sulfur in arc volcanics. Our most recent work has focused on determining what geological environments require life with photosynthesis and thus cannot be pre-biotic candidates.
Sleep et al. (2011) studied changes in ocean and atmospheric chemistry on the early Earth. A geologically brief interval occurred with a massive ~1 bar clement CO2 atmosphere. Oceanic pH was ~6. The pH gradient between the ocean and ~11 at vents from serpentinite provided an attractive energy source for nascent life. Serpentinite occurs in the geological record before 3.8 Ga.
Super-Earths: We examined the creation of magma oceans on the planet CoRoT-7 b (Léger et al., 2011). We showed that a synchronously-rotating, atmosphere-less CoRoT-7 b is close enough to its star that radiation could melt the rocks near the substellar point. We called these planets “Lava Ocean Planets”. We computed the surface temperature as a function of latitude and longitude, and used a linear model to determine magma ocean
depth. We found the shores of the magma ocean extended to 50 degrees from the substellar point and the magma oceans have a depth of 45 km at the substellar point. We did consider the possibility of a habitable terminator, but concluded the water would evaporate very quickly (<1 Earth year).
PROJECT INVESTIGATORS:Edward Bolton
Project InvestigatorNorman Sleep
PROJECT MEMBERS:Rory Barnes
RELATED OBJECTIVES:Objective 1.1
Formation and evolution of habitable planets.
Indirect and direct astronomical observations of extrasolar habitable planets.
Earth's early biosphere.
Co-evolution of microbial communities
Effects of environmental changes on microbial ecosystems