DPS 2001 meeting, November 2001
Session 22. Outer Planet Atmospheres II: Chemistry and Thermal Structure
Oral, Chairs: K. Rages, J. Moses, Wednesday, November 28, 2001, 3:00-4:30pm, Regency E

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[22.06] The Hydrogen Ortho-to-Para Ratio in the Stratosphere of the Giant Planets

T. Fouchet (AOPP, Univ. Oxford), E. Lellouch (DESPA, Obs. Paris-Meudon), H. Feuchtgruber (MPIE, Garching)

The Voyager/IRIS observations have demonstrated that the H2 ortho-to-para ratio in the upper tropospheres of the giant planets departs from thermodynamic equilibrium (Conrath et al.\ 1998). This disequilibrium sheds light on the structure and dynamics of these planets. In this presentation we will demonstrate that disequilibrium also occurs in their lower stratospheres.

Between 1995 and 1997, the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO) observed the S(0) and S(1) H2-quadrupolar lines (respectively 354 and 587~cm-1) in the atmospheres of the four giant planets. These lines probe the temperature profiles in the stratospheres between 10 and 1~mbar. In addition, ISO-SWS observed the CH4 emission in the \nu4 band at 1307~cm-1 both on Jupiter and Saturn, which provides additional contraints on the temperature at the same pressure levels. We will show that each planet dataset cannot be reconciled with a single stratospheric thermal profile: the best fits respectively underestimate and overestimate the observed fluxes in the S(0) and S(1) line. This discrepancy can only be explained by a departure of the ortho-to-para ratio from thermodynamic equilibrium in the lower stratospheres. The best fits are obtained with para fraction higher than the equilibrium fraction, which corresponds to a thermodynamic equilibrium value at lower temperatures than that of the pressure levels probed.

Assuming a time constant for relaxation of the ortho-to-para ratio towards thermodynamic equilibrium, we will show that these observations can provide constraints on the eddy mixing coefficient in the lower stratospheres (p~~mbar) of the giant planets. Unfortunately, the uncertainties on the relaxation time remain quite large.

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