Identi cation of the 10- m ammonia ice feature on Jupiter

We present the rst detection of NH3 ice in the thermal infrared in Jupiter’s atmosphere using Cassini CIRS observations in the 10m region obtained on 31 December 2000 and 1 January 2001. We identify a brightness temperature di;erence ≡ TB(1040 cm−1) − TB(1060 cm−1) as an indicator of spectrally identi able NH3 ice, where 1040 cm−1 is an adjacent continuum region and 1060 cm−1 is the NH3 ice feature. Higher values of imply a stronger NH3 ice signature in the spectrum. Using midlatitude zonally averaged CIRS spectra, we demonstrate systematic spatial variations in , 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 (volume equivalent radius = 0:79 m). Particles larger or smaller than 1 m by about a factor of 2 would be unable to duplicate the observed NH3 ice feature at 1060 cm−1: absorption due to larger particles is excessively broadened, and absorption due to smaller particles is hidden by NH3 gas absorption at 1067 cm−1. We also modeled an average spectrum for a second region on Jupiter (14–17◦N and 10–70◦W), nding an upper limit of =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 1m Mie-scattering spheres would also have detectable signatures at 1060 cm−1. We model the 1m NH3 ice cloud with a particle-to-gas scale height ratio Hp=Hg = 1. For both CIRS spectra analyzed, the spectrum at frequencies greater than 1100 cm−1 also requires a second cloud with essentially grey absorption, which we modeled using 10m NH3 ice spheres distributed with Hp=Hg = 1=8 and a cloud base at 790 mbar. ? 2003 Elsevier Ltd. All rights reserved.