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 [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).]