Neptune is the most remote and most difficult to observe of the giant planets (Figure
4.22). The mean composition profiles of tropospheric and stratospheric gases in Neptune's atmosphere are shown in Figure 4.23. Just like Uranus, Neptune appears to have a higher abundance of methane in its atmosphere than Jupiter and Saturn, and a much greater D/H ratio. Voyager radio occultation profiles and ground-based observations of hydrogen quadrupole lines indicate a deep methane v.m.r. of 2.2% (Baines and Hammel, 1994), indicating a C/H enrichment of ~50x the solar value. Since interior models of Neptune suggest that it contains a greater proportion of heavy elements than Uranus, it is likely that Neptune's methane mixing ratio is indeed greater than that of Uranus. Again the hydrogen-helium atmosphere observed is merely a thin shell, accounting for roughly 15% of the radius and 6% of the mass.
As for Uranus, ECCM calculations predict that clouds such as water and NH4SH condense at very deep levels (as can be seen in Figure 4.24) with methane condensing near 2 bar. The deep abundances are here assumed to be: O/H = 100 x the
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