Cloud structure and atmospheric composition of Jupiter retrieved from Galileo near-infrared mapping spectrometer real-time spectra

The first four complete spectra recorded by the near infrared mapping spectrometer (NIMS) instrument on the Galileo spacecraft in 1996 have been analyzed. These spectra remain the only ones which have been obtained at maximum resolution over the entire NIMS wavelength range of 0.7–5.2 μm. The spectra cover the edge of a “warm” spot at location 5°N, 85°W. We have analyzed the spectra first with reflecting layer models and then with full multiple scattering models using the method of correlated‐k. We find that there is strong evidence for three different cloud layers composed of a haze consistent with 0.5‐μm radius tholins at 0.2 bar, a cloud of 0.75‐μm NH3 particles at about 0.7 bar, and a two‐component NH4SH cloud at about 1.4 bars with both 50.0‐ and 0.45‐μm particles, the former being responsible for the main 5‐μm cloud opacity. The NH3 relative humidity above the cloud tops is found to decrease slightly as the 5‐μm brightness increases, with a mean value of approximately 14%. We also find that the mean volume mixing ratio of ammonia above the middle (NH4SH) cloud deck is (1.7±0.1) × 10−4 and shows a similar, though less discernible decrease with increasing 5‐μm brightness. The deep volume mixing ratios of deuterated methane and phosphine are found to be constant and we estimate their mean values to be (4.9±0.2) × 10−7 and (7.7±0.2) × 10−7, respectively. The fractional scale height of phosphine above the 1 bar level is found to be 27.1±1.4% and shows a slight decrease with increasing 5‐μm brightness. The relative humidity of water vapor is found to be approximately 7%, but while this and all the previous observations are consistent with the assumption that “hot spots” are regions of downwelling, desiccated air, we find that the water vapor relative humidity increases as the 5‐μm brightness increases.