Identification of the 10-μm ammonia ice feature on Jupiter

Details Identification of the 10-μm ammonia ice feature on Jupiter Identification of the 10-μm ammonia ice feature on Jupiter

Apr 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004p%26ss...52..385w&link_type=abstract

Planetary and Space Science, Volume 52, Issue 5-6, p. 385-395.

Physics

23

Atmospheres, Cassini, Ices, Infared Spectroscopy

Scientific paper

We present the first detection of NH3 ice in the thermal infrared in Jupiter's atmosphere using Cassini CIRS observations in the 10-μm region obtained on 31 December 2000 and 1 January 2001.
We identify a brightness temperature difference α≡TB(1040cm-1)-TB(1060cm-1) as an indicator of spectrally identifiable NH3 ice, where 1040cm-1 is an adjacent continuum region and 1060cm-1 is the NH3 ice feature. Higher values of α imply a stronger NH3 ice signature in the spectrum. Using midlatitude zonally averaged CIRS spectra, we demonstrate systematic spatial variations in α, with the highest values at the equator and near 23°N.
In one CIRS spectral average (covering 22-25°N and 140-240°W), our radiative transfer models are consistent with an optical depth of 0.75+/-0.25 for NH3 ice particles modeled as randomly oriented 4:1 prolate spheroids (volumeequivalentradius=0.79μm). Particles larger or smaller than 1μm by about a factor of 2 would be unable to duplicate the observed NH3 ice feature at 1060cm-1: absorption due to larger particles is excessively broadened, and absorption due to smaller particles is hidden by NH3 gas absorption at 1067cm-1. We also modeled an average spectrum for a second region on Jupiter (14-17°N and 10-70°W), finding an upper limit of τ=0.2 for the same NH3 ice particle type. The choice of prolate spheroid particles is based on laboratory studies of NH3 ice aerosols, although 1-μm Mie-scattering spheres would also have detectable signatures at 1060cm-1. We model the 1-μmNH3 ice cloud with a particle-to-gas scale height ratio Hp/Hg=1. For both CIRS spectra analyzed, the spectrum at frequencies greater than 1100cm-1 also requires a second cloud with essentially grey absorption, which we modeled using 10-μmNH3 ice spheres distributed with Hp/Hg=1/8 and a cloud base at 790mbar.

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