Jupiter's ammonia clouds—localized or ubiquitous?

Abstract

From an analysis of the Galileo Near Infrared Imaging Spectrometer (NIMS) data, Baines et al. (Icarus 159 (2002) 74) have reported that spectrally identifiable ammonia clouds (SIACs) cover less than 1% of Jupiter. Localized ammonia clouds have been identified also in the Cassini Composite Infrared Spectrometer (CIRS) observations (Planet. Space Sci. 52 (2004a) 385). Yet, groundbased, satellite and spacecraft observations show that clouds exist everywhere on Jupiter. Thermochemical models also predict that Jupiter must be covered with clouds, with the top layer made up of ammonia ice. For a solar composition atmosphere, models predict the base of the ammonia clouds to be at 720 mb, at 1000 mb if N/H were 4 Â solar, and at 0.5 bar for depleted ammonia of 10 À2 Â solar (Planet. Space Sci. 47 (1999) 1243). Thus, the above NIMS and CIRS findings are seemingly at odds with other observations and cloud physics models. We suggest that the clouds of ammonia ice are ubiquitous on Jupiter, but that spectral identification of all but the freshest of the ammonia clouds and high altitude ammonia haze is inhibited by a combination of (i) dusting, starting with hydrocarbon haze particles falling from Jupiter's stratosphere and combining with an even much larger source-the hydrazine haze; (ii) cloud properties, including ammonia aerosol particle size effects. In this paper, we investigate the role of photochemical haze and find that a substantial amount of haze material can deposit on the upper cloud layer of Jupiter, possibly enough to mask its spectral signature. The stratospheric haze particles result from condensation of polycyclic aromatic hydrocarbons (PAHs), whereas hydrazine ice is formed from ammonia photochemistry. We anticipate similar conditions to prevail on Saturn. r

Flux and polarization signals of spatially inhomogeneous gaseous exoplanets

“…As put forward by e.g. Atreya et al (2005), this apparent contradiction could be explained if the ammonia ice particles are coated by in particlar hydrocarbon haze particles settling from the stratosphere. Thus, only the highest and freshest ammonia ice clouds would show identifiable spectral features.…”

Jupiter’s interior and deep atmosphere: The initial pole-to-pole passes with the Juno spacecraft

“…Combined with a model of the atmosphere, JAMRT computes radiances from profiles of temperature, ammonia and water from the cloud tops down to pressures of 1000 bars. The model uses experimental data on the opacity of ammonia and water in hydrogen-helium mixtures at temperatures up to 600 K and pressures up to 100 bars (18,(41)(42)(43). The uncertainties of the opacity models have been determined to be about 5% to 7% from low pressures up to 100 bars.…”

JIRAM, the Jovian Infrared Auroral Mapper

“…Sounding in the near-infrared between 2.0 and 5.0 µm by an orbiting spacecraft is necessary to determine the deep morphology of water clouds that characterize storms and, hence, moist convective processes (Atreya et al 2005). Three-dimensional mapping of water variations and cloud morphology will quantify the nature of water moist convection.…”

“…Ammonia clouds have been observed only in very limited number and areas (Baines et al 2002) and, following Atreya et al ( , 2005, these clouds would be ubiquitous on Jupiter, their observation being difficult as their original signature is masked by the effect of the stratospheric hydrocarbon aerosols, that sediment and mix with clouds. Again, as noted above, Fry (2010a, 2010b) have both reported the spectroscopic signature of ammonia hydrosulfide ice in Jupiter.…”