Conversion of para and ortho hydrogen in the Jovian planets

A mechanism is proposed which partially equilibrates the para and ortho rotational levels of molecular hydrogen in the atmospheres of Jupiter, Saturn, and Uranus. Catalytic reactions between the free-radical surface sites of aerosol particles and hydrogen molecules yield significant equilibration near 1 bar pressure, if the efficiency of conversion per collision is between 10^?8 and 10^?10 and the effective eddy mixing coefficient is 10⁴ cm²/sec. At lower pressures the ortho-para ratio retains the value at the top of the cloud layer, except for a very small effect from conversion in the thermosphere. The influence of conversion on the specific heat and adiabatic lapse rate is also investigated. The effect is found to be generally small, though it can rise to 10% inside the aerosol layer.