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K. H. Baines, R. W. Carlson, L. W. Kamp (JPL/CalTech)
We report the first spectral detection of ammonia ice in the clouds of Jupiter shortward of 2.5 micron wavelength. The spectral signature of ammonia is thus far uniquely observed within a single, high-altitude cloud feature, imbedded within the turbulent wake region to the northwest of the Great Red Spot (GRS), as recorded in Galileo/NIMS long-map spectral observations on May 3, 1999 during Galileo's 20th orbit. Band depths of about 30% and 50% respectively, are observed for the 1.5 and 1.9 micron ammonia bands. An additional absorption is also observed near 2.7 micron, a spectral region where ammonia ice has been previously observed in regionally-averaged spectra of jovian clouds (Brooke et al., 1998, Icarus 136, 1-13). When spectra of this turbulent wake region feature are ratioed to spectra of nearby GRS clouds located at similar high altitudes, spectral features of homogeneously-distributed gas species such as hydrogen and methane null out, as expected, while the ammonia features remain. We identify these ammonia features as ice features rather than ammonia gas absorptions due to the high altitude of these clouds, some ten kilometers above the ammonia condensation level. A working hypothesis is that both anomolous ammonia and water clouds are freshly-forming clouds of large ice particles, lofted relatively-rapidly above their respective condensation levels by vertical advection and perhaps convection induced by wave motions in the turbulent wake region. The persistence of the 2.73-micron-dark ammonia cloud seen over the Galileo mission indicates that vertical motions are relatively fixed relative to the GRS, and suggests a quasi-stationary wave in the three-dimensional flow around the GRS. The SSI water cloud downstream of the NIMS ammonia cloud could then represent the crest of another wave in the now ammonia-dry downstream flow, enabling the deep water cloud to be observed through the clear, ammonia-dry atmosphere above. In turn, the power liberated by the condensation of water may help to maintain the local circulation, including the Great Red Spot itself. In this case, the GRS may be a self-sustained system which is largely maintained by the wake-induced condensation of uplifted water vapor generated by turbulent interaction of the GRS itself with the surrounding zonal flow.