The source of widespread 3-$\mu$m absorption in Jupiter's clouds: Constraints from 2000 Cassini VIMS observations

The Cassini flyby of Jupiter in 2000 provided spatially resolved spectra of Jupiter's atmosphere using the Visual and Infrared Mapping Spectrometer (VIMS). These spectra contain a strong absorption at wavelengths from about 2.9 $\mu$m to 3.1 $\mu$m, previously noticed in a 3-$\mu$m spectrum obtained by the Infrared Space Observatory (ISO) in 1996. While Brooke et al. (1998, Icarus 136, 1-13) were able to fit the ISO spectrum very well using ammonia ice as the sole source of particulate absorption, Sromovsky and Fry (2010, Icarus 210, 211-229), using significantly revised NH$_3$ gas absorption models, showed that ammonium hydrosulfide (NH$_4$SH) provided a better fit to the ISO spectrum than NH$_3$ , but that the best fit was obtained when both NH$_3$ and NH$_4$SH were present. Although the large FOV of the ISO instrument precluded identification of the spatial distribution of these two components, the VIMS spectra at low and intermediate phase angles show that 3-$\mu$m absorption is present in zones and belts, in every region investigated, and both low- and high-opacity samples are best fit with a combination of NH$_4$SH and NH$_3$ particles at all locations. The best fits are obtained with a layer of small ammonia-coated particles ($r\sim0.3$ $\mu$m) overlying but often close to an optically thicker but still modest layer of much larger NH$_4$SH particles ($r\sim 10$ $\mu$m), with a deeper optically thicker layer, which might also be composed of NH$_4$SH. Although these fits put NH$_3$ ice at pressures less than 500 mb, this is not inconsistent with the lack of prominent NH$_3$ features in Jupiter's longwave spectrum because the reflectivity of the core particles strongly suppresses the NH$_3$ absorption features, at both near-IR and thermal wavelengths.