Temperature planetary atmospheres

In this chapter, we describe the current status of theoretical kinetics for chemical reactions at low temperature, i.e., from 1 to 200K. The desire to understand the chemistry of interstellar space and of low temperature planetary atmospheres provides the general motivation for studying chemical kinetics at such temperatures. For example, the chemistry of Titan s atmosphere is currently a topic of considerable interest. This motivation led to the development of novel experimental techniques, such as the CRESU (cinetique de reaction en ecoulement supersonique uniforme) method, which allows for the measurement of rate coefficients at temperatures as low as lOK (see Chapter 2 by Canosa et ai). Such measurements provide important tests for theory and have sparked a renewed interest in theoretical analyses for this temperature range. ... 

The CH3 + H and CH3 - - CH3 reactions are two prototypical radical-radical reactions. The latter reaction is of some interest to the modeling of low temperature planetary atmospheres. For the CH3 - - H reaction, the long-range interactions are much weaker and the reduced mass is smaller. Both of these factors suggest that short-range interactions should be more important for the H atom case. However, for the CH3 - - CH3 case, there are three more modes that transform from free rotations to bending motions... 

Through spectroscopic observations and sometimes tenuous deductions there has accumulated a significant picture of the makeup of the planetary atmospheres. Doubt pervades much of this picture, yet it represents our starting place in knowledge as we venture outside our own atmosphere for the first time. Table 25-V summarizes a part of this information—the maximum surface temperatures and the chemical compositions. Naturally, these compositions are incomplete ... 

The temperature profile of a planetary atmosphere depends both on the composition and some simple thermodynamics. The temperature decreases with altitude at a rate called the lapse rate. As a parcel of air rises, the pressure falls as we have seen, which means that the volume will increase as a result of an adiabatic expansion. The change in enthalpy H coupled with the definition of the specific heat capacity... 

At the lowest temperatures, the three-body moments are relatively strong, Table 6.4. A density of only 10 amagat will modify the observed moments by roughly 10%. The strong temperature dependence of the three-body moments at low temperature may be quite important for some applications, for example for the spectroscopic modeling of planetary atmospheres. It seems to be related to the formation of dimers and, consequently, to monomer-dimer interactions which are three-body processes by our definition. Of course, at 45 K, quantum corrections are substantial and the numbers quoted must be considered rough estimates. Nevertheless, the general trend of the temperature dependence seems clear. 

The reported halflife of ethenol (vinyl alcohol) in the gas phase at room temperature is ca. 30 min [221], far shorter than our calculated 1028-1029 s. However, the 30 min halflife is very likely that for a protonation/deprotonation isomerization catalyzed by the walls of the vessel, rather than for the concerted hydrogen migration (Fig. 5.30) considered here. Indeed, the related ethynol has been detected in planetary atmospheres and interstellar space [222], showing that that molecule, in isolation, is long-lived. Even under the more confined conditions of the lab, ethenol can be studied in the gas phase [221, 223] and in solution [224]. All three methods predict very long halflives for the uncatalyzed reaction. 

Ip and Fernandes [ 101] calculated that 6x lO to 6x lO s g of cometry material could liave been delivered to Earth at the time of the formation of the great Oort Cloud of comets. This amount is equivalent to 4-40 times the present mass of the oceans, assuming about 50% of the cometaiy mass is ice. Owen and Bar-Nun [102] examined tlie ability of amorphous ice formed at temperatures below lOOK to trap ambient gases. By comparison of the compositions of gases trapped by ice with tlie compositions of the interstellar medium, comets, and planetary atmospheres, Owen and Bar-Nun [102] concluded that icy comets delivered a considerable fraction of the volatiles to the imier planets. Owen [83] emphasized that Uie potential supply of cometaiy materials is more than adequate. 

As already discussed in Section 14.1, the CN-I-C2H2 reaction is alleged to be the most important step in the formation of cyanopolyynes in different environments, such as planetary atmospheres and the interstellar medium. Its potential importance has been confirmed by kinetic studies which have found this reaction to be very fast in a large range of temperatures [2 5]. The possible reaction channels include ... 

The inputs to the chemical equilibrium calculations are the temperature, pressure, bulk elemental composition, and thermodynamic data for all compounds included in the calculations. The temperatures and pressures used in the calculations depend on the system being studied, e.g., a protoplanetary accretion disk, the photospheric region of a cool star, the ejecta from a nova or supernova, a planetary atmosphere, and so on. The bulk elemental composition is the set of elemental abundances that are appropriate for the system... 

An algebraic expression for the atmospheric temperature profile may be found by solving a simplified differential equation of infrared radiative transfer. Some of the assumptions may seem extreme but are appropriate for a conceptual model. Nevertheless, the end result is instructive and even somewhat realistic for a cloud-free planetary atmosphere. 

The presence of a separate section on the reactions between radicals and unsaturated molecules reflects two facts (i) the reactions are important, or potentially important, in a number of environments, combustion systems, planetary atmospheres and interstellar clouds, and they have been widely studied and (ii) they do not fall comfortably into the categories of unim-olecular or bimolecular reactions. The reactions generally proceed via the initial formation of a chemically bound radical adduct, which may be collisionally stabilised or breakdown to products. The competition between different reaction channels can depend on the nature of the radical, the size and nature of the unsaturated molecule, the total pressure and the temperature. 

Smith GP. (2003) Rate theory of methyl recombination at the low temperatures and pressures of planetary atmospheres. Chem. Phys. Lett. 376 381-388. 

Reactions between neutrals include atom/radical + radical and atom/radical + molecule reactions. As discussed above, the intermolecular forces are shorter range than is the case with ion-molecule reactions, so that it is necessary to consider chemical interactions explicitly when modelling a reaction. After a section on experimental methods, the ideas behind transition state (TS) theory and its variational modification are discussed, together with theories of reactions where the TS switches, as the temperature increases, from A-B distances mainly controlled by the potential arising from electrostatic interaction to shorter distances where chemical forces are important. While the pressure in the ISM is too low for pressure dependent reactions, this topic is important in the conditions used to measure rate coefficients and in the chemistry of planetary atmospheres, including those of the exoplanets (see Chap. 5). This topic is discussed in Sect. 3.4.4, which also introduces the ideas that lie behind master equation models, which are widely used for such reactions. These models can also be used for reactions in which the adduct AB from an A + B reaction dissociates into several products, and these ideas are discussed in Sect. 3.4.5. Section 3.4 concludes with discussion of two examples of neutral + neutral reactions. 

As a next step, a higher resolution spectrum can be used for interesting planetary targets to identify the compounds of the planetary atmosphere, constrain the temperature in the IR and radius of the observed exoplanet. In that context, we can test if we have an abiotic explanation of all compounds seen in the atmosphere of such a planet. If we do not, we can begin to consider the exciting biotic hypothesis. 

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