How high are Jupiter’s clouds? From high-resolution JunoCam images to a multi-wavelength analysis

Last year, we showed that JunoCam images, acquired in the visible, have the resolution necessary to measure the height of clouds from their projected shadows. We focused our analysis on the “Nautilus”, a 3000-km cyclonic vortex seen during Juno’s 14^th periojove. That structure consists mainly of a spiraling counter clockwise white cloud that casts a shadow onto a reddish cloud deck∼20 to 30 km below. Small individual clouds also pop out of the white cloud deck, towering about ~10 to 20 km above it. An analysis of near-simultaneous HST images of the Nautilus confirms that the white region is higher than its surrounding darker, reddish cloud deck. These respective elevations are consistent with the white clouds being made of fresh ammonia ice while most of the reddish clouds underneath are made of ammonium hydrosulfide NH₄SH, as predicted by equilibrium cloud models.

An analysis by F. Biagiotti of a similar region observed by JIRAM during Juno’s 1st perijove identifies the presence of elusive ammonia ice crystals, either pure or mixed with a nitrogen-bearing species similar to Titan’s tholins. In addition, these clouds have altitudes that are consistent with the above interpretation. However, the surrounding material is not much deeper and incompatible with NH₄SH. We discuss a possible solution to the corundum: At least in the gas, the atmosphere's optical thickness is much larger at the wavelengths used for the JIRAM study (2 to 3.2 micron)  than in the visible. The effect of scattering by cloud particles is to be evaluated, but it appears likely that altogether, infrared observations at these wavelengths cannot penetrate as deep as visible ones.

An interpretation of these observations, consistent with spectroscopic observations in the visible, is therefore that, at least in this region close to ~40°N, most of Jupiter's visible cloud deck is made of NH₄SH, that updrafts can locally deliver fresh ammonia ice but that these ammonia ice crystals remain only for a short time either because of downwelling and evaporation or because of coating.