We have seen already that molecular hydrogen may exist in two distinct ortho and para states. At high temperatures (T greater than roughly 300 K) the equilibrium para-H2 fraction f is 0.25, and this value increases as the temperature decreases, as may be determined from Equation (4.21). The vertical profile of the ortho:para ratio in the atmospheres of the giant planets may be estimated from collision-induced and quadrupole absorptions in the far-IR (the nature of dipole and quadrupole absorption lines are discussed in Chapter 6) and also from visible wavelength hydrogen quadrupole absorption lines. Any departure of the measured para-H2 fraction from the equilibrium value calculated from Equation (4.21) using the local temperature provides information on the vigorousness of vertical convection, with more convectively active atmospheres having an fp closer to the deep value of 0.25 than to the equilibrium value. However, the conversion of ortho-hydrogen to para-hydrogen is catalyzed by the presence of aerosols, as mentioned in Section 4.1.3, and thus knowledge of the aerosol distribution is essential to fully understand the observed fp profile.
The ortho/para fraction in the upper tropospheres of around 300mbar of all the giant planets was estimated as a function of latitude from Voyager IRIS far-IR data by Conrath et a/. (1998), while Fouchet et a/. (2003) used observations of far-IR hydrogen quadrupole lines with the Infrared Space Observatory Short Wavelength Spectrometer (ISO/SWS) to determine disk-averaged values offp in the stratosphere at pressure levels between 1 mbar and 10 mbar. These results and others are discussed in Section 4.4.
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