O O M

uuo and is higher in the east than in the west, yielding an overall tilt as predicted. Other studies have previously implied that the GRS is tilted. Indeed, some have described the GRS as a tilted pancake! NW and SE regions of the GRS contained cloud sheets at about the same pressure, but differed substantially in haze structure. The tropospheric hazes for both bright and dark locations in the GRS were more-blue absorbing than those seen in the belt regions. The core locations also required redder stratospheric hazes, when modeled, than seen anywhere else on the planet, consistent with the possibility of a second differing coloring agent in the GRS [113].

It is common to see darker, bluish-gray features on the southern edge of the NEB at visible wavelengths (Fig. 4.5). For years we have referred to them as projections or the bases of festoons. It has been observed that the larger, dark patches are very warm at wavelengths of 5 |im, indicating that these are actually areas of thin clouds, allowing the warmth beneath to be detected in the infrared. These bluish-gray NEBs features are often referred to as 5-|im hotspots [114].

These dark bluish-gray features are regions of suspected strong downwelling, which clear out clouds and hazes allowing 5-|im radiation to escape from below [115] (Fig. 4.6). We see down into this hole, thus the darker appearance. David M. Harland states that an infrared hotspot is actually a hole in the ammonia clouds through which energy at a wavelength of 5 | m can readily escape. It appears dark at visual wavelengths because there is no ammonia to reflect sunlight [116]. Thus, it would appear that these dark areas, rather than being a feature, would instead be the absence of features, or clearings in the clouds. A hotspot appears bright in the

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