The source of 3-μm absorption in Jupiter's clouds: Reanalysis of ISO observations using new NH3 absorption models

A prominent characteristic of jovian near-IR spectra is the widely distributed presence of a strong absorption at wavelengths from about 2.9 μm to 3.1 μm, first noticed in a 3-μm spectrum obtained by the Infrared Space Observatory (ISO) in 1996. While Brooke et al. (Brooke, T.Y., Knacke, R.F., Encrenaz, T., Drossart, P., Crisp, D., Feuchtgruber, H. [1998]. Icarus 136, 1-13) were able to fit the ISO spectrum very well using ammonia ice as the sole source of particulate absorption, Irwin et al. (Irwin, P.G.J., Weir, A.L., Taylor, F.W., Calcutt, S.B., Carlson, R.W. [2001]. Icarus 149, 397-415) noted that their best-fit cloud model implied a strong absorption at 2 μm that was not observed in Galileo NIMS spectra, raising questions about the source of the absorption. Subsequent significant revisions in ammonia gas absorption models (Bowles, N., Calcutt, S., Irwin, P., Temple, J. [2008]. Icarus 196, 612-614) also raised questions about these conclusions because ammonia gas absorption overlaps regions of ammonia ice absorption. Our reanalysis, based on improved ammonia absorption models, finds that the ISO spectrum can be well fit by models that include both NH3 ice and solid NH4SH, with the latter substance providing most of the absorption. The component due to NH3 is very possibly due to NH3 present as a coating on either large (r ∼ 15 μm) NH4SH particles in a deeper layer at ∼550 mb or on small (r ∼ 0.3 μm) photochemical haze particles in a lower pressure layer at ∼370 mb. Neither option creates conflict with the lack of significant NH3 absorption features at thermal wavelengths.