Global variation of the para hydrogen fraction in Jupiter's atmosphere and implications for dynamics on the outer planets

Abstract

Infrared spectra obtained by the Voyager spacecraft indicate that the para hydrogen fraction near the 300-mbar pressure level on Jupiter is not in thermodynamic equilibrium. Analysis of the global mapping data sequences from Voyagers 1 and 2 shows that the para fraction is smallest at equatorial latitudes, and approaches equilibrium at high latitudes. The sampled atmospheric level is near 125°K and the equatorial para fraction would represent thermal equilibrium at about 160°K. There are small-scale variations superposed on the global pattern, and these do not correlate with albedo, flow velocity, or 5-μm brightness. Lack of correlation of cloud indicators with the para fraction suggests that catalysis of ortho-para conversion does not occur on aerosol surfaces, at least near the 300 mbar level. The fact that dynamics alters the para fraction from equilibrium while not affecting temperatures to a large degree suggests that the para hydrogen equilibration rate is slower than radiative thermal adjustment. A survey of the mechanisms for equilibration suggests that H 2H 2 paramagnetic interaction is dominant. The slow equilibration rate has dynamical implications for all the outer planets. A mixing length model is used to demonstrate that within the convective lower tropospheres of the giant planets there is very slow overturning. The mean structures are close to equilibrium para fraction, the thermal structures are equilibrium adiabats, and they are statically stable to high frequency dynamical perturbations. The para hydrogen conversion greatly increases the efficiency of convection. Within Jupiter's stably stratified upper troposphere, where the infrared spectra originate, the global variation of the para fraction appears most likely to be produced by upwelling at equatorial latitudes in response to solar heating. If this is true, there is compensating downward motion in polar regions.