Photodissociation of molecules by solar UV radiation is an extremely important driver on the composition profiles measured in the upper parts of the giant planet atmospheres. It is beyond the scope of this book to provide a detailed exposition on the finer points of this topic and the reader is referred to books such as Atreya (1986) for a more complete discussion. Here we shall limit ourselves to the main photochemical reactions governing the upper atmospheric composition profiles of important giant planet gases. However, before we can discuss photodissociation, we must briefly introduce Rayleigh scattering, and discuss how it affects the flux of UV photons reaching the upper atmospheres of the giant planets.
Photolysis of different molecules requires UV photons of different frequencies. For example, photolysis of methane requires photons with wavelengths less than 160 nm, whereas photolysis of ammonia, phosphine, and hydrazine requires photons in the wavelength range 160 nm to 230 nm. Solar photons with wavelengths less than 160 nm are dominated by Lyman-a emission at 121.6 nm. The penetration of UV photons into planetary atmospheres is strongly regulated by the Rayleigh scattering of air molecules, which is strongly wavelength-dependent. In general, the Rayleigh scattering cross-sectional area per dipole (Goody and Yung, 1989) is given by a« = 3 (^J M2 (4.34)
where a is the polarizibility, which relates the electric dipole induced on a molecule or atom to the local electric field strength by p = aElocal; and A is the wavelength. It can immediately be seen that shorter wavelength photons are much more efficiently scattered than longer wavelength photons and indeed this is why the Earth's sky appears blue from the ground. For atoms and molecules where the polarizibility is independent of the molecule's orientation with respect to the incident electric field, the polarizibility is related to the refractive index m via the equation
where N is the number of molecules per unit volume. Hence, substituting into Equation (4.34) we find
where the last approximation assumes m is close to unity, which for a gas it is. For all but spherical top molecules (Chapter 6), the polarizibility is not actually independent of the molecule's orientation with respect to the incident electric field and thus Equation (4.36) must be modified to
where /anisotropic is a parameter describing the non-isotropy of the atom or molecule (Goody and Yung, 1989). Assuming that a number of atoms/molecules are randomly orientated with respect to the incident electric field, the correction factor /anisotropic may be shown to be equal to
/anisotropic 36 TAJ (4.38)
where A is known as the depolarization factor, which may be measured in the laboratory and is listed in Table 4.3 for relevant Jovian gases. The refractive indices
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