Identification of the 10-μm ammonia ice feature on Jupiter

摘要

We present the first detection of NH_3 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 ≡ T_B (1040 cm~(-1)) -T_B(1060 cm~(-1)) as an indicator of spectrally identifiable NH_3 ice, where 1040 cm~(-1) is an adjacent continuum region and 1060 cm~(-1) is the NH_3 ice feature. Higher values of α imply a stronger NH_3 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 NH_3 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 NH_3 ice feature at 1060 cm~(-1): absorption due to larger particles is excessively broadened, and absorption due to smaller particles is hidden by NH_3 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 NH_3 ice particle type. The choice of prolate spheroid particles is based on laboratory studies of NH_3 ice aerosols, although 1-m Mie-scattering spheres would also have detectable signatures at 1060 cm~(-1). We model the 1-μm NH_3 ice cloud with a particle-to-gas scale height ratio H_p/H_g = 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 NH_3 ice spheres distributed with H_p/H_g = 1/8 and a cloud base at 790 mbar.