New evidence for chemical depletion of ammonia in the 1 to 2 bar region of Jupiter's atmosphere
Abstract:It has long been known that the vertical profile of ammonia within Jupiter's cloud layers is not well-described by a simple equilibrium profile, with saturated vapor above the cloud base and the well-mixed deep abundance below the cloud base. An additional depletion of ammonia by a factor of 4-10 is required by global microwave spectra at p < 6 bar [e.g., 1]. Dynamical effects, ranging from cloud layer circulation between belts and zones  to molecular differentiation following convective activity  might be sufficient to explain the global microwave data.
However, in situ cloud density measurements by the Galileo Probe  suggest a large gap in our understanding of cloud chemistry in Jupiter, especially when combined with other tracers such as volatile mixing ratios  and static stability . Using the “fresh clouds” method of modeling cloud density , and assuming that cloud-forming advection was weak at all levels in the probe site, we find that NH4SH formation cannot explain cloud densities between 1 and 1.4 bar in situ. The composition of additional chemical species, or adsorption of ammonia on other ices, are candidate processes that strongly require further laboratory study of the H2O-NH3-H2S volatile system at temperatures of 150 to 300 K . Spectral features near 3 microns suggest widespread NH4SH in the visible cloud decks of Jupiter , but additional species may also contribute to absorption at these wavelengths. Infrared spectroscopy at high angular resolution in the future---performed by Juno, JWST, or 30-m class ground-based telescopes---may be able to observe ammonia depletion mechanisms in action.
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[This material is supported by the NASA Juno Project through a SWRI subcontract (SKA), and by NASA Grant No. NNX11AM55G issued through the Outer Planets Research Program (MHW).]