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2013 Annual Science Report

VPL at University of Washington Reporting  |  SEP 2012 – AUG 2013

Climates and Evolution of Extrasolar Terrestrial Planets

Project Summary

Planetary climate results from the interplay of a large number of different physical processes, including radiative heating and cooling, advection and dynamics, latent heating and cloud effects, atmosphere-interior interactions, and the presence of life. Atmospheres and climate then evolve through time due to interplay between these processes and longer-term effects, such as atmospheric escape, orbital evolution, and other dynamical interactions. Since planetary climate determines surface habitability, we can better understand how planets maintain habitability over long time periods by studying and modeling the large network of interactions that determine the atmospheric state of a planet and how it changes through time.

4 Institutions
3 Teams
5 Publications
0 Field Sites
Field Sites

Project Progress

This year we made significant progress on the development and use of a hierarchy of planetary climate models, which can be used as tools to understand the factors affecting terrestrial planetary climate, and how surface habitability may evolve over time. Team members Robinson and Catling derived an analytic radiative-convective model for planetary atmospheres (Robinson and Catling 2012), which is an intuitive tool for gaining insight into fundamental processes in planetary climate. This model was recently used to explain why Earth, Jupiter, Saturn, Titan, Uranus, and Neptune all share a common tropopause temperature minimum in their atmosphere at 0.1 bar pressure (Robinson and Catling 2013). The explanation lies in the physics of infrared radiative transport, and should apply to countless worlds outside the Solar System. Furthermore, the assumption of a 0.1 bar tropopause can be used to help constrain surface pressure or surface temperature on an exoplanet, the combination of which determine habitability.

Shields, Meadows, Bitz, Pierrehumbert and collaborators used a hierarchy of climate models to explore the effect of the interaction between the parent star’s radiation and the planet’s wavelength dependent reflectivity (from surface ice and snow, and atmospheric absorption) on planetary climate. Their results indicate that planets orbiting cooler, redder (M-dwarf) stars are less sensitive to decreases in stellar insolation (as shown in Figure 2 below), and episodes of low-latitude glaciation may be less likely to occur on M-dwarf planets in the habitable zone than on planets orbiting stars with high visible and near-UV output. This is due to absorption of near-infrared radiation by lower-albedo surface ice and atmospheric absorption by CO2 and water-vapor. However, at the outer edge of the habitable zone, high levels of CO2 mask the ice-albedo effect, leaving the traditional limit of the outer edge of the HZ unaffected by the spectral dependence of ice and snow albedo (Shields et al., 2013). Ongoing simulations also indicate that the amount of increased stellar flux required to melt a planet out of a snowball state is highly sensitive to host star SED. We find that a distant frozen M-dwarf planet orbiting beyond the outer edge of its star’s habitable zone without a continuously active carbon cycle is likely to melt more easily out of global ice cover as its host star ages and its luminosity increases (Shields et al., in prep).

In ongoing development in this task, Robinson, Crisp, and Meadows have continued to develop a new, accurate and versatile 1-D model of terrestrial planetary climate, which is based on the VPL’s line-by-line radiative transfer model, SMART (developed by D. Crisp, see Meadows and Crisp 1996). By computing how radiative fluxes evolve due to changes in the atmospheric state, this new model can both accurately and quickly determine atmospheric thermal equilibrium states (Robinson and Crisp, in prep.). The model is currently being validated against the challenging problem of Venus’ climate. As part of this effort, Gao et al. (in press) implemented and validated the latest version of the Community Aerosol and Radiation Model for Atmospheres (CARMA), and used this model to successfully simulate the clouds of Venus. This sophisticated cloud microphysical model will now be used to simulate clouds in the VPL 1-D Climate Model. Team member Brown contributed significantly to the release of HITRAN 2012 gas absorption database (Rothman et al. 2013) which is used as input to all our radiative and climate modeling efforts. Pierrehumbert and colleagues started work on modifying the Princeton FMS dynamical core to become a generalized 3-D climate model for exoplanet studies.

Effect of Ice-Albedo Feedback on Ice Coverage for Planets Orbiting Different Stellar Types.
Mean ice line latitude 
as a function of percent of the modern solar 
constant (stellar flux) for planets orbiting stars of different spectral types at equivalent flux distances (Shields et al., 2013)

Pressure of the tropopause temperature minimum over a large range of atmospheric conditions.  The location of the tropopause depends on two key parameters---the strength at which sunlight is attenuated in a planet's stratosphere (shown as contours labeled k_strato), and the ratio of the energy flux budgets of the upper atmosphere to the lower atmosphere (shown as the horizontal axis).  Both the upper and lower atmosphere absorb solar energy, and the lower atmosphere may have an internal energy source.  Values for solar system worlds are indicated.  Note that the tropopause pressure is near 0.1 bar over a range of parameter space that exceeds what is seen in the solar system.
Comparison between the forthcoming VPL climate model (solid), the Venus International Reference Atmosphere (VIRA, dashed), and data from the Venus Express mission (dotted). Differences tend to be within 5-10%, and the model is systematically colder than the data. Improvements in techniques and model input data will further reduce this discrepancy, and also indicate key factors and processes that influence Venus' climate.