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Project Progress

Results from Virtual Planetary Laboratory (VPL) Task 1 indicate that both the habitability and the detectability of biosignatures in the disk averaged spectrum of a terrestrial planet depend crucially on the planet’s surface temperature and the vertical distribution of temperatures in its atmosphere. Hence, as the first step in our efforts to simulate a broad range of plausible extrasolar terrestrial planetary environments, we have implemented a simple, but versatile one-dimensional climate model to provide a globally-averaged description of their vertical temperature distributions. Given assumptions about a planet’s stellar type, orbital properties (mean distance, eccentricity, obliquity), and the composition and optical properties of its surface and atmosphere, the climate model yields a globally-averaged description of the vertical temperature distribution that is in thermal equilibrium with these imposed conditions. This model includes all of the major physical processes that contribute to the deposition and vertical transport of heat in the planet’s atmosphere, surface, and near sub-surface. These processes include stellar radiative heating, thermal radiative cooling, vertical convective and conductive heat transport within the sub-surface and atmosphere, and latent heat transport associated with the evaporation, condensation, and transport of volatile species.

The radiative heating and cooling rates are simulated using the same spectrum-resolving atmospheric radiative transfer model (SMART), used to generate the high-resolution spectra in VPL Task 1. Vertical convective heat transport in the atmosphere is simulated using a formulation adapted from a 1-dimensional boundary layer model (Savijarvi, 1991). This model incorporates classical mixing length theory, Ekman pumping, and Richardson-number-dependent eddy mixing to provide an explicit physical link between the surface-atmospheric thermal structure and the vertical transport of heat and volatile species. Vertical diffusive heat transport within the near-surface layer is simulated by solving the one-dimensional heat diffusion equation using a time-dependent 4th-order Runge-Kutta scheme (Sertorio and Tinetti, 2002). Cloud and aerosol particle nucleation, condensation, evaporation, transport, and the associated latent heat transport are modeled using a formulation originally developed for simulations of the Martian climate (A. Inada personal communication, 2002). These processes are incorporated into the time-dependent thermodynamic energy equation of the system, which is solved numerically as an initial value problem using time stepping.

References Cited:
Savijarvi, H. 1991, “A Model Study of the PBL Structure on Mars and Earth, Contrib. Atmos. Phys., 64, 103-112.

L. Sertorio and G. Tinetti, Constraints in the Coupling Star-Life, Il Nuovo Cimento, 25 C, N. 4, 457-499 July-August 2002