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L. A. Sromovsky, P. M. Fry (University of Wisconsin - Madison)
Three uranian cloud layers were identified by West et al. (Uranus, Univ. Arizona Press, 1991): an optically thin stratospheric haze, an optically thicker methane haze (0.4 < \tau < 1) primarily in the 1.2-1.3 bar region, and a cloud of unknown composition near 3 bars. Using improved methane band models of Irwin et al. (BAAS, this issue) we were able to test this paradigm using near-IR spectra covering 1.1-1.8 \mum, a range well suited for distinguishing the main cloud levels. We assumed a 2-cloud model in which the lower cloud is opaque and the upper cloud consists of broken opaque elements. The pressure and fractional coverage of the upper cloud and the pressure and albedo of the lower cloud were adjusted to fit the 1975 geometric albedo spectrum of Fink and Larsen (ApJ \bf 233, 1021-40, 1979), with the following results (first two rows):
| Fit Range (\mum) | Upper P (bars) | Upper Fraction(%) | Lower P (bars) | Lower Albedo (%) | \chi2 |
| 1.175-1.34 | 2.2±0.15 | 2.8±0.4 | 6.6+1.2-0.7 | 6.5±0.8 | 206 |
| 1.450-1.70 | 1.8±0.10 | 2.2±0.3 | 5.2+0.8-0.4 | 3.3±0.3 | 223 |
| 1.175-1.34 | 1.25 (fixed) | 0.15±0.08 | 3.1 (fixed) | 6.6±0.8 | 296 |
| 1.450-1.70 | 1.25 (fixed) | 0.66±0.05 | 3.1 (fixed) | 4.4±0.1 | 281 |
Fixing clouds at paradigm pressures of 1.25 bars and 3.1 bars yields a significant reduction in fit quality and a very small upper cloud contribution (last two rows). The paradigm-violating best-fit results are consistent with an analysis of seven-band Keck AO imaging observations (Sromovsky and Fry, in preparation), which concludes that the 1.2-bar cloud is at best a minor contributor to Uranus' reflectivity and that latitudinal variations in brightness are mainly controlled by deeper clouds. How prior results can be explained in the context of these new results remains to be determined.
This research was supported by a grant from NASA's Planetary Astronomy Program.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.