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Thus, this hotspot that the probe descended into was a relatively clear and dry area. Eight to ten somewhat evenly spaced hotspots are present at this latitude at any given time; located at the boundary between the northern part of the equatorial zone and the southern edge of the northern equatorial belt, between 6° and 8° N. All the hotspots move as a group with a velocity near 103 m per second with respect to System III, at the time of the probe entry. The hotspots are also associated with accompanying equatorial plumes; optically thick cloud features which are believed to be regions of strong convection [123].

The 5-|im hotspots lie in the jet at the boundary between the equatorial zone and the north equatorial belt (NEB), a shear zone believed to be a region of general downwelling air [124]. Not all bluish-gray features are hotspots, but all hot spots are bluish-gray features.

Where the probe entered Jupiter's atmosphere there was no evidence of a deep-water cloud. The favored explanation is that the probe entry hot spot was an extremely dry downdraft [125]. The analysis of Orton et al., of the vertical structure of hotspots is that there is a two-layer cloud structure (as opposed to the normal three-layer one). There is, (1) an upper tropospheric cloud layer above 450 mbar probably consisting of ammonia ice particles less than 1-|im in size with significant opacity in the visible, becoming optically thin in the near-infrared and negligible at mid-infrared wavelengths. (2) A tropospheric cloud below the 1 bar level, probably ammonia hydrosulphide, with small optical depth, < 1.0 at 4.78 |im.

The normal third layer of distinct water clouds is absent. According to Orton et al., comparisons with regions to the north and south of hotspots are consistent with the interpretation that hotspots are regions of reduced cloud opacity, possibly because of a dry downdraft. A thicker cloud of possibly brighter particles in the upper troposphere is required to satisfy the highly reflective cloud features in the visible, as well as particles of size greater than ~3 |im to satisfy the opacity in the mid-infrared. In other words, the high, bright cloud material that must be surrounding the hotspot is absent from within the hotspot itself. Thus, the hotspot is a region of reduced cloud opacity, because of the dry downdraft [126]. Thus, an infrared, or 5 |im, hot spot is actually a hole in the ammonia clouds through which energy at 5 |im can readily escape. It appears dark at visual wavelengths because there is no ammonia to reflect sunlight. We can assume that, had the probe descended into any other region of Jupiter's atmosphere, it would have detected more water, and that the 'typical' atmosphere of Jupiter is, in fact, wetter.

Five micrometer hotspots have a lifecycle. The Galileo probe entered Jupiter's atmosphere on December 7, 1995. Hotspots were being monitored preceding the arrival of the probe, and the expected probe entry site (PES) had been determined long before. During the months preceding the probe's arrival, the morphology of this PES was quite interesting. In September 1995 the PES hotspot apparently had merged with another hotspot and then split again soon after. At the beginning of what Orton et al., referred to as what might be called the beginning of its 'life cycle', between October 3 and 13, 1995, the hotspot reemerged as a small 'wedge shape' evolving into a larger 'comma' shape, with a bright but small round core and a small 'tail'. Then a transient filament-like morphology appeared, after which it evolved into a 'mature phase', when the spot covered an area a few degrees in longitude and had a tail tilted approximately 30°. Subsequently, it reached its most intense state, flattened and expanded enormously. The PES remained in this flat morphology until between November 1995 and July 1996. Following this, it began its life cycle again assuming a small wedge shape, followed by a comma shape. It skipped the mature phase then began again as a small feature in December 1996. By August 1997 it had evolved into a mature phase, and began once again to break up again shortly thereafter [127].

A study spanning more than three years was made of the longitudinal locations, morphology, and evolution of 5-|im hotspots using the Infrared Telescope Facility-National Science Foundation Camera (IRTF-NSFCAM). This three-year period included the date of the Galileo probe entry. According to Orton et al., an analysis of the data shows that within periods of several months to a year, there are eight or nine longitudinal areas with high likelihood of containing a 5-| m hotspot. These areas drift together (approximately at the same rate) with respect to System III at a rate that changes only slowly in time, and they are quasi-evenly spaced, suggesting w __

Ortiz et al., observed that the number of hotspots is actually almost always <D «jg §

higher than 10 or 11. They also observed that there might be different wave modes, y E

or wave numbers, which move at slightly different speeds. This could explain why ^ <D

individual hotspots seem to move a little faster than others. Although their mor- q £ ^

phology is complex, most hotspots seem to show a mature phase in which they are "5 o «*.

larger, with a hot narrow festoon extending south and westward from the eastern U V 0

most edge, tilted about 30°. More observations are needed to provide a complete correlation, if one exists, between the 5-| m hotspots and the bluish-gray festoons we see visually [129].

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