Figure 36.23 Summary of Galileo probe results on Jupiter's clouds. Two experiments on the probe detected clouds using different techniques. One experiment, based on oscillation of sky brightness, detected a cloud layer near the 0.6-bar pressure level. Based on its location, this cloud layer is believed to be composed of ammonia ice particle. The other experiment, based on observing scattered laser light from cloud particles, found one well-defined, tenuous cloud structure with a base at the 1.6-bar pressure level, as well as faint traces of cloud particles at a wide range of altitudes. No other cloud layers were found--in particular, the one cloud layer detected by the first experiment was not seen. The simplest explanation for this apparent contradiction between the experiments is that the upper cloud layer near 0.6 bar is patchy and the probe went through a clear area. The profound difference between the entry site's cloud structure and the cloud structure expected for "average" Jupiter is illustrated at right. The "expected" picture is based on theoretical modeling and interpretation of Earth-based and spacecraft-based observations of large regions of Jupiter. Layers of ammonia, ammonium hydrosulfide, and water clouds were predicted. However, no thick, dense clouds were found, and no water cloud was detected. The entry of the Galileo probe into an infrared "hot spot" may account for most, if not all, of the deviations from expectations, because "hot spots" are believed to represent regions of reduced cloud cover and are not common on Jupiter. (NASA/Ames)

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