Evidence of pure ammonia ice clouds from Juno/JIRAM infrared spectral data

Ammonia is historically thought to be the main source of condensable species for Jupiter's main cloud layer (0.5-1 bar level). However, measurements from Galileo first [1] and Juno later [2] showed that the spectral features connected to ammonia clouds are rare (less than 2 % of the entire planetary disk) and not ubiquitous. Using infrared spectra collected by the JIRAM instrument on board the NASA Juno mission we investigated the possible presence of SIACs (spectrally identifiable ammonia clouds) in PJ1 data.

As a preliminary step, we used two spectral indicators sensible to the absorption of ammonia ice particles in the 2.97-3.01 micron range and ran a PCA+GMM (Principal Component Analysis + Gaussian Mixture Models) clustering analysis. The two indicators showed higher values in a high-latitude region in which cross-referenced JunoCam images highlight the presence of a Nautilus-shaped cloud, already noticed in PJ14 by previous work [3]. The PCA-GMM analysis identified the spectra in this region as belonging to a specific cluster, different from the surroundings. Performing optimal estimation atmospheric retrievals using the powerful NASA PSG (Planetary Spectrum generator) suite as the forward model, we tried to model all the spectra of this region (considering only the 2.5-3.1 micron range). We first used a toy model with a variable ammonia profile and parametrized pure reflecting hazes (complex refractive index 1.4+0i) and tholin clouds. It is important to stress that Titan’s like tholins must not be intended as a realistic candidate for Jupiter’s aerosol clouds but as an approximation of the real amorphous unknown material that exhibits an evident N-H-bond-like absorption. We found that the described toy model fits well the majority of the spectra outside the Nautilus, whereas the spectra near and inside the Nautilus require more complex assumptions on cloud compositions and so have been re-modeled.

As a result, we noticed that a total of 20 spectra are best fitted by a pure ammonia ice cloud model and so have been identified as SIACs. The SIACs are located at the center of the Nautilus-shaped cloud and in correspondence with the nearby swirls. In most cases, the SIACs are surrounded by spectra best fitted by a cloud deck composed of tholin particles coated with ammonia ice. Our results in correspondence with the Nautilus suggest: (I) higher altitude hazes and clouds, (II) higher values of ammonia relative humidity that also reach super-saturation conditions, and (III) smaller effective radii for the haze particles. Such results are compatible with the presence of pure ammonia ice clouds, formed at these latitudes as a consequence of an uplifting event from the lower troposphere that brought a large fraction of fresh ammonia up to reach super-saturation conditions, triggering condensation and/or coating of mixed particles.

[1] Baines K. H. et al. (2002) Icarus, 159, 1, 74-94. [2] Grassi D. et al. (2021) MNRAS, 503, 4, 4892-4907. [3] Guillot T. et al. (2023) EGU23, the 25th EGU General Assembly, EGU-17178.