Thermal Structure and Para Hydrogen Fraction on the Outer Planets fromVoyagerIRIS Measurements

Voyagerinfrared spectra from Jupiter, Saturn, Uranus, and Neptune are used to infer latitude-height cross sections of temperature and para hydrogen fraction. A new inversion algorithm is developed that simultaneously retrieves both quantities. It uses all portions of the spectra containing information about temperature and para fraction and not influenced by other properties, such as cloud opacity. The sensitivity of spectra to temperature and para hydrogen values at different heights is calculated and presented. Retrievals based on artificial data are carried out. The sensitivity studies and the artificial retrievals are used to determine the information content of the spectra. Temperature and para hydrogen retrievals are presented for a layer about a scale height deep, centered near the 300-mb level, but varying in position from planet to planet. Temperature cross sections show influence of seasonal solar forcing on Saturn and Uranus. On all the planets except Uranus there are also thermal anomalies correlated with zonal flows in the lower tropospheres. Para hydrogen cross sections show complicated patterns. On Uranus there is north–south hemispheric asymmetry. On Neptune there is correlation with latitudinal gradients of the zonal flow. Fractional para hydrogen anomalies are larger than those of temperature, and the ratio suggests a para hydrogen relaxation time of about a century near the 200-mb pressure level.