New evidence for chemical depletion of ammonia in the 1 to 2 bar region of Jupiter's atmosphere

Michael H Wong1,2, Sushil K Atreya3, Paul N Romani4, Imke De Pater5, William R Kuhn1 and Konstantinos S Kalogerakis6, (1)University of Michigan Ann Arbor, Ann Arbor, MI, United States, (2)University of California, Berkeley, United States, (3)University of Michigan Ann Arbor, Climate and Space Sciences and Engineering, Ann Arbor, United States, (4)NASA - GSFC, Greenbelt, MD, United States, (5)University of California Berkeley, Departments of Astronomy and of Earth and Planetary Science, Berkeley, United States, (6)SRI International, Center for Geospace Studies, Menlo Park, United States
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 [2] to molecular differentiation following convective activity [3] might be sufficient to explain the global microwave data.

However, in situ cloud density measurements by the Galileo Probe [4] suggest a large gap in our understanding of cloud chemistry in Jupiter, especially when combined with other tracers such as volatile mixing ratios [5] and static stability [6]. Using the “fresh clouds” method of modeling cloud density [7], 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 [1]. Spectral features near 3 microns suggest widespread NH4SH in the visible cloud decks of Jupiter [8], 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.

References:
[1] de Pater et al. (2001), Icarus 149, 66-78.
[2] Showman and de Pater (2005), Icarus 174, 192-204.
[3] Sugiyama et al. (2011), GRL 38, L13201.
[4] Ragent et al. (1998), JGR 103, 22891-22909.
[5] Wong et al. (2004), Icarus 171, 153-170.
[6] Magalhães, Seiff, and Young (2002), Icarus 158, 410-433.
[7] Wong et al. (2014), Icarus, submitted.
[8] Sromovsky et al. (2010), Icarus 210, 211-229 and 230-257.

[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).]

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