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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(1040 cm−1)−TB(1060 cm−1) as an indicator of spectrally identifiable NH3 ice, where 1040 cm−1 is an adjacent continuum region and 1060 cm−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 (volume equivalent radius=0.79 μm). Particles larger or smaller than 1 μm 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), 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 1060 cm−1. We model the 1-μm 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 10-μm NH3 ice spheres distributed with Hp/Hg=1/8 and a cloud base at 790 mbar. [Copyright &y& Elsevier]