Data from Allen (1976).

Data from Allen (1976).

of hydrogen, helium, and methane at standard temperature and pressure (STP) may be calculated from the semi-empirical formula m — 1 = A(1 + B/X2) (4.39)

and values of the coefficients A, B are also listed in Table 4.3. Since m is close to 1.0 and since from the Clausius-Mossotti relation (e.g., Bleaney and Bleaney, 1976) we expect m2 — 1 to vary linearly with N, we may use m evaluated at STP in Equation (4.37) provided N is also calculated at STP. In practice for the giant planet atmospheres, where composition is dominated by near-solar hydrogen-helium, it is found that Equation (4.37) may be reasonably accurately approximated by (Atreya, 1986)

where the wavelength X is assumed to be in units of angstroms (A) and where the calculated cross-section is in units of m2. The pressure level of unit optical depths at different UV wavelengths for the giant planets is listed in Table 4.4, where it can be seen that longer UV wavelengths penetrate to much deeper levels than shorter wavelengths.

Table 4.4. Pressure level of unit optical depth for Rayleigh scattering in the giant planet atmospheres.


1,000 A

2,000 A

3,000 A


10 mbar

200 mbar

1,000 bar


6 mbar

80 mbar

400 mbar


5 mbar

70 mbar

300 mbar


6 mbar

90 mbar

400 mbar

Photodissociation of important giant planet gases

Now that we have seen how Rayleigh scattering affects the incident fluxes of UV photons we will now discuss the photodissociation of the three most important photo-active gases in the upper tropospheres and stratospheres of the giant planets: namely: ammonia, phosphine, and methane.

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