The ISO mission discovered the presence of gaseous H20 in the stratospheres of all four giant planets (Feuchtgruber et al., 1997), and the abundances are listed in Tables 4.6-4.9. Since the tropopauses of these planets are very cold, there is effectively a "cold trap'', which in the absence of any other sources should keep the stratospheric water abundance very low, as discussed in Section 4.3.3. The fact that significant levels of water, and in addition oxygenated species such as C02, exist at these levels suggests that there is an external source of oxygen. The source of this stratospheric H20 and C02 was initially thought to be due to the arrival of interplanetary icy micrometeoroids, and a flux of approximately 106molcm~2 s_1 was modeled to be required for all four giant planets. Species such as CO and C02 may be contributed directly from these micrometeoroids or through subsequent reactions between water vapor and stratospheric hydrocarbons. Note that the presence of C0 does not on its own suggest external sources since rapid convection from the interior may inject this disequilibrium species into the stratosphere. The presence of stratospheric C02, however, does imply the presence of stratospheric water vapor.
Although an approximately similar flux rate of interplanetary dust (IPD) was required for all four giant planets, Feuchtgruber et al. (1997) noted that Jupiter and Saturn actually required a flux approximately 10 times greater than Uranus and
Neptune. It was initially suggested that this might be due to variations in IPD density as a function of distance from the Sun. However, it is now thought that the differences are due to additional sources of water. For Saturn, the extra source would appear to be the erosion of ring material, which becomes ionized and then spirals into the atmosphere along connecting magnetic field lines causing increased abundances of water at midlatitudes (Prange et al., 2006). For Jupiter, the extra source of water, and CO2, would appear to have been the collision of Comet Shoemaker-Levy 9 (SL-9) with Jupiter in 1994, which injected large abundances of oxygen-rich molecules into the stratosphere (Lellouch et al., 2002). While the latitude dependence of Jovian stratospheric water remains unclear, the abundance of stratospheric CO2 ten years after the impact of SL9 was observed (Kunde et al., 2004) to decrease by a factor of 7 from southern midlatitudes, where SL9 struck, to northern midlatitudes (Section 5.4.2). Further exploration of the latitudinal variation of such stratospheric gases will require more observations, either from ground-based microwave observatories, or more likely from future projects such as SOFIA and Herschel, which are described in Chapter 8.
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