Modelling the full 2-5 µm Juno JIRAM spectral range with NEMESIS: Zonal Profiles of Jupiter’s Aerosols, Condensables, and Disequilibrium Species

Since 2016 the Juno spacecraft has been in orbit around Jupiter, gathering unprecedented data from its highly inclined 53-day orbit. The Jupiter Infrared Auroral Mapper (JIRAM) is an imager and spectrograph with spectral coverage between 2 and 5 µm. This region is dominated by reflected sunlight by aerosols and hazes, with distinct absorptions by ammonia, phosphine, germane and other minor species in Jupiter's troposphere, as well as ionospheric H₃⁺ at high altitude. Here, we outline the process undertaken to model the full spectral coverage of JIRAM with NEMESIS, our radiative transfer and retrieval code (Irwin et al., 2008). This includes altering the NH₃ aerosol and haze properties, updating the molecular line-list, and testing the sensitivity to the abundance of the molecular species that are within the 2-5 µm range offered by JIRAM. 

This study builds on previous models for JIRAM spectra in thermal emission (Grassi et al., 2020) and reflected sunlight (Grassi et al., 2021), by attempting to fit the entire 2-5 µm range simultaneously with a single consistent aerosol model.  The model includes two aerosol layers, a NH₄SH type layer at 1.3 bars, and a NH₃ type layer at 0.7 bars, as well as a tholin type haze layer that extends from the troposphere to the stratosphere. We demonstrate that JIRAM observations of both reflected sunlight and thermal emission cannot be reproduced simultaneously using standard refractive indices available in the literature.  We build a simple model of the refractive indices for the three aerosol layers, adapting the technique of Sromovsky et al. (2010), and demonstrating the improvement in the fits at each step.  As a proof of concept we present the analysis of meridionally averaged zonal profiles, investigating how aerosols, ammonia, and phosphine vary with latitude during the early perijoves of the mission.