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M.H. Wong, G.L. Bjoraker, M.D. Smith, F.M. Flasar, C.A. Nixon (NASA GSFC)
We present the first detection of NH3 ice in the thermal infrared in Jupiter's atmosphere using Cassini CIRS observations in the 10-\mum region obtained on 31 December 2000 and 1 January 2001.
We quantify the strength of the NH3 ice feature by calculating a brightness temperature difference \alpha defined by subtracting the brightness temperature at 1060 cm-1 from the brightness temperature of an adjacent continuum region at 1040 cm-1. Using midlatitude zonally averaged CIRS spectra, we demonstrate systematic spatial variations in \alpha, with the highest values (implying a stronger NH3 ice signature) at the equator and near 23\circ N.
In one CIRS spectral average (covering 22\circ to 25\circ N and 140\circ to 240\circ W), our models indicate an optical depth of 0.75±.25 for NH3 ice particles modeled as randomly oriented 4:1 prolate spheroids (volume equivalent radius = 0.79 \mum). We choose prolate spheroid particles based on laboratory studies of NH3 ice aerosols, although 1-\mum Mie-scattering spheres would also have detectable signatures at 1060 cm-1. Particles larger or smaller than 1 \mum by a factor of 2 or more would be unable to duplicate the observed NH3 ice feature at 1060 cm-1: absoprtion due to larger particles is excessively broadened, and absorption due to smaller particles is hidden by NH3 gas absorption at 1067 cm-1. In a spectrum of a second region on Jupiter (14\circ to 17\circ N and 10\circ to 70\circ W), we find an upper limit of \tau = 0.2 for the same NH3 ice particle type.
In our models, the 1060 cm-1 feature appears only when NH3 ice is present at P \leq 500 mbar, suggesting that the detected particles are distinct from the deeper higher-opacity grey cloud required to match data from both CIRS and other previous infrared studies.
The author(s) of this abstract have provided an email address for comments about the abstract: mike.wong@gsfc.nasa.gov
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.