The temperature/pressure profiles observed and inferred in the upper parts of the outer planet atmospheres in the equatorial regions were shown previously in Figure 4.1. The stratospheric temperatures calculated from Equation (4.13) are 104.6 K, 79.9 K, 49.7 K, and 49.9 K, respectively, for Jupiter, Saturn, Uranus, and Neptune, and it can be seen that the observed tropopause temperatures are indeed close to these values. Below the radiative-convective boundary, the temperature profiles are observed to follow an adiabatic lapse rate, and in this figure the SALRs have been assumed. For Jupiter and Saturn it is not possible to determine the hydrogen ortho:para ratio from the measured lapse rate since the heat capacities (and hence lapse rates) of both equilibrium and frozen (3:1) hydrogen are indistinguishable from the measured lapse rate at the observable tropospheric temperatures of these planets (Figure 4.2). However, Uranus and Neptune are much colder and so there is a big difference between the two calculated lapse rates. The atmospheric circulation of Uranus is apparently very sluggish and, as we shall see in Section 4.4.3, the ortho:para ratio is found to be roughly in equilibrium. However, the observed lapse rate is more consistent with the frozen 3:1 ortho:para ratio. It has been suggested that the two observations may be compatible if in general the ortho:para ratio is equal to the equilibrium value at a particular level, but vertical displacements are small and rapid enough such that negligible ortho:para redistribution occurs during the motion of individual parcels. Thus
f =_even_ (42n yeqm £(2/ + 1) exp[—/(/ + 1)0*/T]+3^(2/ + 1) exp[-/(/ + 1)®*/T]' '' ;
This "intermediate" hydrogen (de Pater and Massie, 1985) has a very similar heat capacity to "frozen" 3:1 ortho:para ratio hydrogen and may thus explain why the observed lapse rate on Uranus is more consistent with "frozen" ortho-para hydrogen than with equilibrium ortho-para hydrogen.
All the planets have clearly defined upper tropospheres, which start at approximately 500 mbar to 600 mbar for Jupiter, 400 mbar to 500 mbar for Saturn, and 1 bar to 2 bar for both Uranus and Neptune. The tropopauses occur at 100 mbar, 60 mbar, 100 mbar, and 50 mbar, respectively. The stratospheric temperatures in Saturn's atmosphere are generally lower than those found in Jupiter's, which might be expected from Saturn's increased distance from the Sun. However, the stratospheric temperatures of Uranus and Neptune are noticeably and puzzlingly different. Although the temperature profiles of these planets is similar in the lower troposphere, the stratospheric temperatures in the 10 mbar to 0.01 mbar pressure region is of the order of 40 K warmer in Neptune's atmosphere than in Uranus. The difference may be due to the abundance of stratospheric hazes (see Section 4.4.4), which perhaps absorb more sunlight in Neptune's atmosphere than in Uranus'. Some absorption of solar radiation by stratospheric aerosols in Uranus' atmosphere does occur, however, leading to the small temperature maxima observed by Voyager 2 radio occultation in Uranus' atmosphere near 10 mbar. Neptune's stratosphere may also be warmer than Uranus' due to the higher abundance of stratospheric methane in Neptune's atmosphere, which absorbs sunlight in the visible and near-IR.
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