Evidence of pure ammonia clouds in Jupiter's Northern Temperate domain from Juno/JIRAM infrared spectral data

Document Type

Article

Publication Date

4-1-2025

Abstract

In this work, we analyse data from the Jovian Infrared Auroral Mapper (JIRAM) imaging spectrometer on board the NASA mission Juno, to investigate the presence of spectrally identifiable ammonia clouds (SIACs). Focusing on the data from the first perijove passage, we found that a white vortex structure near 40 N provides the best candidate. Implementing atmospheric retrieval thanks to the planetary spectrum generator, we fit the JIRAM spectra (in the 2.5-3.1 m range) inside and outside the vortex, varying the gaseous ammonia profiles, and the clouds and hazes properties and composition. We found that outside the vortex, the best fit is achieved using main clouds composed of tholins (approximation of an unknown contaminant material). Inside the vortex the best fit is achieved when main cloud decks are composed of pure ammonia ice, or ammonia-coated tholins. We therefore claim the detection of 38 SIACs, all detected over the white vortex structure. With respect to the external regions, the retrieved parameters distributions inside the vortex show: (i) higher altitude hazes and clouds, (ii) smaller haze's effective radii, and (iii) higher gaseous ammonia relative humidity values. Both the detection of pure ammonia ice clouds and the retrieved physical parameters are consistent with the vortex being the result of a moist convection storm that uplifted fresh ammonia from the deep troposphere which in turn either condensed or became a possible source of coating material for existing cloud particles. This work confirms the trend established by space and ground-based observations, for which ammonia clouds on Jupiter are rare and connected to strong convective episodes.

Publication Title

Monthly Notices of the Royal Astronomical Society

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