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K. H. Baines, M. L. Delitsky, T. W. Momary, R. W. Carlson, G. S. Orton (Jet Propulsion Laboratory/Caltech)
Analysis of Galileo/NIMS and HST multi-filter maps obtained in 29 distinct colors from UV to IR spanning 0.22 to 5.2 microns indicate unusual spectral properties of cloud particles in the Great Red Spot (GRS). In particular, significant particle absorption exists in two bands, one immediately shortward of 0.40 microns and another near 4.0 microns. Both of these absorptions can be explained by sulfur-bearing particulates, resulting from photolysis of ammonium hydrosulfide (NH4SH) particles which have been vertically transported upwards to the ~330-mbar level within the GRS anti-cyclone. Specifically, photochemical pathways can lead to S and H2S from NH4SH at altitude, which in turn leads to higher-order S molecules and H2S ice particles in the cold Jovian upper troposphere. The H2S ices can explain the 4-micron absorption, while the high-order S particulates may be responsible for the UV-blue (~320 nm) absorption feature. H2S can be further dissociated by solar radiation at < 3170 A (Okabe, Photochemistry of Small Molecules, 1987), yielding additional sulfur atoms which can concatenate and polymerize to form a mixture of chain lengths and ring sizes of sulfur allotropes (S2-S20). The sulfur allotropes range in color from S3 (cherry-red) to S8 (bright yellow) to S12 (pale yellow) which could lend deep hues to the Great Red Spot. Diagnostic absorption bands of these sulfur species (Steudel and Eckert, Topics Curr Chem, 2003) can be used to constrain the composition of the sulfur particulates. Specifically, laboratory spectra reveal absorption immediately shortward of 0.4 microns for S5 and S12, similar to the observed GRS absorption, whereas S3, S4, and S8 do not show such an absorption. Thus, chemical pathways that involve the production of S5 and/or S12 may be important to explain the UV-blue spectral character of particulates within the Great Red Spot.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.