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0 20 JO 60 SO 100 120 140 Tempered ure (K)

O 50 100 150 200 250 300 J50 Temperature (K)

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0 20 40 60 SO 100 120 MO Tempered ure (K)

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Figure 4.13. Mean observed/modeled cloud profiles of the giant planets.

The only in situ measurements that have been made of Jupiter's cloud structure with the Galileo entry probe are unfortunately ambiguous since the probe sampled an unrepresentative 5 ^m hotspot, which has abnormally low cloud cover and a low abundance of volatiles (we will return to the phenomenon of 5 ^m hotspots in Chapter 5). The cloud structure measured by the Galileo probe nephelometer experiment was found to be very tenuous with a main cloud layer based at 1.4 bar, a thinner cloud below at 1.6 bar and the suggestion of a cloud base at 0.4 bar (Figure 4.9, see p. 107). The three clouds have tentatively been identified as ammonium hydrosulfide, water ice, and ammonia ice, respectively, but they are at higher altitudes than expected from ECCMs. As we shall see in Chapter 5 these hotspots have been modeled either as regions of extremely rapid downdraft or more recently, and probably more plausibly, as part of an atmospheric wave system, where the air column is vertically stretched and compressed with the hotspots occurring at the stretching phase. Both explanations are consistent with low volatile abundances and low cloud cover.

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