NASA Astrobiology Institute (NAI)

This project develops models of planetary atmospheres and surface temperature to allow us to model extra solar terrestrial planetary environments and to understand what they would look like to distant observers.

VPL employs a spectrum-resolving surface/atmosphere radiative transfer model, called the Spectral Mapping Atmospheric Radiative Transfer (SMART) model. SMART has been used to simulate the reflected stellar and emitted thermal spectra of a wide range of realistic terrestrial planetary environments (c.f. Meadows and Crisp, 1996; 2004; Crisp, 1997; Segura et al. 2003; 2005; Tinetti et al. 2005; 2006; Kiang et al. 2007). This model would be ideal for generating the radiative heating and cooling rates used by the VPL climate model, but it was too computationally slow for this application. Over the past year, we implemented a new approach that improves the speed of SMART, while preserving its accuracy and range of validity. This approach employs spectrally dependent “radiance Jacobians,” similar to the “weighting functions” used in remote sensing algorithms. Jacobians specify the rate of change of the radiance at any wavelength with changes in the temperature or optical properties of the surface or any layer of the atmosphere. In a time-marching climate model, the radiative fluxes and heating rates for the initial time step are based on a time-consuming, SMART calculation. For subsequent steps, wavelength-dependent fluxes are approximated by a first order Taylor series expansion, using the Jacobians to adjust values for changes in the atmospheric and surface thermal structure and optical properties. The adjusted fluxes are then integrated to yield heating and cooling rates at a tiny fraction of the cost of a full SMART calculation. This approach was validated using the 1-D VPL Climate model. We found that if the thermal structure or optical properties change dramatically as the environment relaxes to equilibrium, a full SMART calculation must be repeated from time to time to preserve the accuracy of the heating rates. However, this approach still dramatically improves the computational speed.

In addition to upgrading SMART for use in climate applications, this year SMART was upgraded to model non-Local Thermodynamic Equilibrium (non-LTE) effects and upper atmospheric airglow phenomena (Martin-Torres et al., 2008). This upgrade has been validated on observations of the planet Venus, and additional observations of Venus and Mars airglow have been acquired from the Kitt Peak Observatory (PI: Tom Slanger) and are being analyzed with the new model. Work is also currently underway to add speed dependence to the existing line-mixing modules in SMART. This will give SMART additional versatility in modeling a range of extrasolar planet environments and atmospheric condition.