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Chemistry of the Atmospheres of Mars, Venus, and Titan

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Chemistry of the Atmospheres of Mars, Venus, and Titan 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. XX–YY. Univ. of Arizona, Tucson, DOI: XXXXXXXXXXXXXXXXXXXXXX. Chemistry of the Atmospheres of Mars, Venus, and Titan Vladimir A. Krasnopolsky Catholic University of America Franck Lefèvre Service d’Aeronomie 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. Photo- chemical 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. 1. INTRODUCTION 2 1 dr()Φ =−PnL Energies of the solar UV photons and energetic particles r2 dr may be sufficient to break chemical bonds in atmospheric dn 11dT 11+α dT species and form new molecules, atoms, radicals, ions, and Φ =−()KD+ − nK ++ + D dr HTa dr HTddr free electrons. These products initiate chemical reactions that further complicate the atmospheric composition, which is also significantly affected by dynamics and transport Here r is radius, Φ and n are the flux and number density processes. Photochemical products may be tracers of pho- of species, P and L are the production and loss of species tochemistry and dynamics in the atmosphere and change in chemical reactions, α is the thermal diffusion factor, Ha its thermal balance. Gas exchange between the atmosphere, and H are the mean and species scale heights, and T is the space, and solid planet also determines the properties of temperature. Substitution of the second equation in the first the atmosphere. results in an ordinary second-order nonlinear differential Studies of the atmospheric chemical composition and equation. Finite-difference analogs for these equations may its variations are therefore essential for all aspects of at- be solved using methods described by Allen et al. (1981) mospheric science, and many instruments on spacecraft and Krasnopolsky and Cruikshank (1999). missions to the planets are designed for this task. Interpre- Boundary conditions for the equations may be densities, tation of the observations requires photochemical models fluxes, and velocities of the species. Fluxes and velocities that could adequately simulate photochemical and transport are equal to zero at a chemically passive surface and at the processes. exobase for molecules that do not escape. Requirements One-dimensional steady-state global-mean self-consis- for the boundary conditions are discussed in Krasnopolsky tent models are a basic type of photochemical modeling. (1995). Generally, the number of nonzero conditions should These models simulate altitude variations of species in an be equal to the number of chemical elements in the system. atmosphere. Vertical transport in the model is described by This type of photochemical modeling is a powerful tool for eddy diffusion coefficient K(z). The solar UV spectrum, studying atmospheric chemical composition. For example, absorption cross sections, reaction rate coefficients, eddy in the case of Titan, a model results in vertical profiles of 83 and molecular diffusion coefficients K(z) and D(z), and neutral species and 33 ions up to 1600 km using densities of temperature profile T(z) are the input data for the model. N2 and CH4 near the surface (Krasnopolsky, 2009a, 2012c). The model provides a set of continuity equations for each Photochemical general circulation models (GCMs; see species in a spherical atmosphere Forget and Lebonnois, this volume) present significant prog- 1 2 Comparative Climatology of Terrestrial Planets ress in the study and reproduction of atmospheric properties for the UV ozone detection (1 μm-atm = 10–4 cm × 2.7 × under a great variety of conditions. These GCMs are the 1019 cm–3 = 2.7 × 1015 cm–2). The observations at low and best models for studying variations of species with local middle latitudes beyond the polar caps and hoods were time, latitude, season, and location. below this limit. Some basic properties of the terrestrial planets and their The upper atmosphere was studied by the UV spectrom- atmospheres are listed in Table 1. They are discussed in eters at Mariner 6, 7, and 9, as well as mass spectrometers detail in other chapters of this book. and retarding potential analyzers on the Viking landers Below we will briefly discuss observational data on and (Barth et al., 1972; Nier and McElroy, 1977; Hanson et al., photochemical modeling of the atmospheric chemical com- 1977). Mass spectrometers on the Viking landers and more positions on Mars, Venus, and Titan. Earth’s photochemistry recently on the Mars Science Laboratory (MSL) measured is strongly affected by life and human activity (Yung and the compositions of the lower atmosphere, especially N2 and DeMore, 1999) and is beyond the scope of this chapter. noble gases Ar, Ne, Kr, and Xe. Helium was later observed Previous reviews of the photochemistry of Mars, Venus, using the Extreme Ultraviolet Explorer (Krasnopolsky and and Titan may be found in Levine (1985), Krasnopolsky Gladstone, 2005). The first detailed study of the H2O global (1986), and Yung and DeMore (1999). distribution was made by the Viking orbiters (Jakosky and Farmer, 1982). 1 2. MARS Noxon et al. (1976) detected the O2( Δg) dayglow at 1.27 μm and properly explained its excitation by photoly- 2.1. Chemical Composition of Mars’ Atmosphere and sis of ozone and quenching by CO2. Traub et al. (1979) Its Variations repeated their observations at three latitude bands. Using groundbased IR heterodyne spectroscopy at 9.6 μm, Espe- Mars’ obliquity of 24° and the elliptic orbit with peri- nak et al. (1991) detected ozone at low latitudes with the helion at 1.381 AU (LS = 251°) and aphelion at 1.666 AU column abundance of ~1 μm-atm at LS = 208°. These ob- (LS = 71°) induce a great variety of conditions in the servations have been continued (Fast et al., 2006, 2009). To comparatively thin atmosphere of Mars. A summary of improve the ozone detection at large airmass, Clancy et al. observational data on the chemical composition of the (1999) observed the UV spectra of Mars near the limb using martian atmosphere is given in Table 2. Groundbased de- the Hubble Space Telescope (HST). Some data from Fast tections of the parent species CO2 (Kuiper, 1949) and H2O et al. (2006) and Clancy et al. (1999) are shown in Fig. 2. (Spinrad et al., 1963) and the main photochemical products Clancy and Nair (1996) predicted significant seasonal CO (Kaplan et al., 1969) and O2 (Barker, 1972; Carleton variations of ozone above 20 km at the low and middle and Traub, 1972) were a basis for the first photochemical latitudes. Krasnopolsky (1997) argued that the O2 dayglow models. Observations of O3 by the Mariner 9 ultraviolet at 1.27 μm is the best tracer for that ozone, because the (UV) spectrometer (Barth et al., 1973) revealed strong dayglow is quenched by CO2 below ~20 km, and started variations in Mars’ photochemistry (Fig. 1). The ozone groundbased mapping observations of this dayglow (Figs. 3, UV absorption band at 255 nm has a halfwidth of 40 nm, 4). Interpretation of the groundbased observations of ozone the solar light varies within the band by a factor of ~15, and the O2 dayglow at 1.27 μm does not require any as- the surface reflection and dust properties may vary as well, sumptions regarding reflectivities of the surface rocks and and the Mariner team adopted an upper limit of 3 μm-atm atmospheric dust. TABLE 1. Basic properties of Venus, Earth, Mars, and Titan. Property Venus Earth Mars Titan Mean distance from Sun (AU) 0.72 1.0 1.52 9.54 Radius (km) 6052 6378 3393 2575 Mass (Earth = 1) 0.815 1 0.1075 0.0225 Length of year 224.7 d 365.26 d 1.881 yr 29.42 yr Gravity (cm s–2) 887 978 369 135 Mean atmospheric pressure (bar) 92 1.0 0.0061 1.5 Mean temperature near surface (K) 737 288 210 94 Mean exobase altitude (km) 200 400 200 1350 Mean exospheric temperature (K) 260 1000 230 160 Escape velocity (km s–1) 10.3 11.2 5.0 2.64 Main gas CO2 (0.96) N2 (0.78) CO2 N(0.96) 2 (0.95) Second gas N2 (0.034) O2 (0.21) N2 (0.019) CH4 (0.05) –4 Third gas SO2 (1.3 × 10 ) Ar (0.0093) Ar (0.019) H2 (0.001) Atmospheric composition is given near the surface; mixing ratios are in parentheses. Krasnopolsky and Lefèvre: Chemistry of Atmospheres 3 Hydrogen (H2) was predicted by photochemistry as a Peroxide H2O2 was detected using groundbased mi- long-living product on Mars, and it had been detected using crowave and infrared instruments, and a summary of the the Far Ultraviolet Spectroscopic Explorer (FUSE) (Kras- current data is in Fig. 6. Along with ozone and the O2 nopolsky and Feldman, 2001). Continuous monitoring of dayglow, H2O2 is a sensitive tracer of Mars’ photochemistry, temperature profiles, 2H O, water ice, and dust atmospheric especially odd hydrogen chemistry.
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