DPS 35th Meeting, 1-6 September 2003
Session 48. Outer Planets/Gas Giants II
Oral, Chairs: L. A. Young and H. B. Hammel, Saturday, September 6, 2003, 1:30-3:00pm, DeAnza I-II

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[48.06] Identification of Jupiter's 10-\mum Ammonia Ice Feature in CIRS Data

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
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