Comparative Climatology of Terrestrial Planets

1. INTRODUCTION Energies of the solar UV photons and energetic particles may be sufficient to break chemical bonds in atmospheric species and form new molecules, atoms, radicals, ions, and free electrons. These products initiate chemical reactions that further complicate the atmospheric composition, which is also significantly affected by dynamics and transport processes. Photochemical products may be tracers of photochemistry and dynamics in the atmosphere and change its thermal balance. Gas exchange between the atmosphere, space, and solid planet also determines the properties of the atmosphere. Studies of the atmospheric chemical composition and its variations are therefore essential for all aspects of atmospheric science, and many instruments on spacecraft missions to the planets are designed for this task. Interpretation of the observations requires photochemical models that could adequately simulate photochemical and transport processes. One-dimensional steady-state global-mean self-consistent models are a basic type of photochemical modeling. These models simulate altitude variations of species in an atmosphere. Vertical transport in the model is described by eddy diffusion coefficient K(z). The solar UV spectrum, absorption cross sections, reaction rate coefficients, eddy and molecular diffusion coefficients K(z) and D(z), and temperature profile T(z) are the input data for the model. The model provides a set of continuity equations for each species in a spherical atmosphere 1 2 2 1 1 1 1 r d r dr P nL K D dn dr n K D H T dT dr H T dT a Φ Φ ( ) = − = − + ( ) −       + + + + α d dr             Here r is radius, Φ and n are the flux and number density of species, P and L are the production and loss of species in chemical reactions, α is the thermal diffusion factor, Ha and H are the mean and species scale heights, and T is the temperature. Substitution of the second equation in the first results in an ordinary second-order nonlinear differential equation. Finite-difference analogs for these equations may be solved using methods described by Allen et al. (1981) and Krasnopolsky and Cruikshank (1999). Boundary conditions for the equations may be densities, fluxes, and velocities of the species. Fluxes and velocities are equal to zero at a chemically passive surface and at the exobase for molecules that do not escape. Requirements for the boundary conditions are discussed in Krasnopolsky (1995). Generally, the number of nonzero conditions should be equal to the number of chemical elements in the system. This type of photochemical modeling is a powerful tool for studying atmospheric chemical composition. For example, in the case of Titan, a model results in vertical profiles of 83 neutral species and 33 ions up to 1600 km using densities of N2 and CH4 near the surface (Krasnopolsky, 2009a, 2012c). Photochemical general circulation models (GCMs; see Forget and Lebonnois, this volume) present significant prog231 Chemistry of the Atmospheres of Mars, Venus, and Titan Vladimir A. Krasnopolsky Catholic University of America Franck Lefèvre Laboratoire Atmosphères et Observations Spatiales (LATMOS), Paris Observations and models for atmospheric chemical compositions of Mars, Venus, and Titan are briefly discussed. While the martian CO2-H2O photochemistry is comparatively simple, Mars’ obliquity, elliptic orbit, and rather thin atmosphere result in strong seasonal and latitudinal variations that are challenging in both observations and modeling. Venus’ atmosphere presents a large range of temperature and pressure conditions. The atmospheric chemistry involves species of seven elements, with sulfur and chlorine chemistries dominating up to 100 km. The atmosphere below 60 km became a subject of chemical kinetic modeling only recently. Photochemical modeling is especially impressive for Titan: Using the N2 and CH4 densities at the surface and temperature and eddy diffusion profiles, it is possible to calculate vertical profiles of numerous neutrals and ions throughout the atmosphere. The Cassini-Huygens observations have resulted in significant progress in understanding the chemistry of Titan’s atmosphere and ionosphere and provide an excellent basis for their modeling. Krasnopolsky V. A. and Lefèvre F. (2013) Chemistry of the atmospheres of Mars, Venus, and Titan. In Comparative Climatology of Terrestrial Planets (S. J. Mackwell et al., eds.), pp. 231–275. Univ. of Arizona, Tucson, DOI: 10.2458/azu_uapress_9780816530595-ch11. 232 Comparative Climatology of Terrestrial Planets ress in the study and reproduction of atmospheric properties under a great variety of conditions. These GCMs are the best models for studying variations of species with local time, latitude, season, and location. Some basic properties of the terrestrial planets and their atmospheres are listed in Table 1. They are discussed in detail in other chapters of this book. Below...