Exploring Jupiter’s upper atmosphere using Juno/JIRAM data: retrieval of H3+ and CH4 from polar to equatorial latitudes

As the largest and oldest planet in the Solar System, Jupiter has long captured the interest of scientists. Its vast size and primordial nature suggest it holds crucial insights into the formation and evolution of our planetary system, making it a focal point of scientific research and exploration. In response to this interest, NASA designed the Juno mission to uncover the mysteries of this massive gas giant and, by extension, deepen our understanding of the Solar System as a whole. Launched in 2011, Juno entered Jupiter’s orbit in 2016 and, thanks to mission extensions, continues to gather invaluable data on the planet’s origin, internal structure, atmosphere, and magnetosphere. The findings from Juno not only advance our knowledge of Jupiter, but also provide essential information applicable to the study of exoplanetary systems. One of the principal goals of the mission is to investigate Jupiter’s puzzling aurorae, which are atmospheric emissions resulting from the interplay between charged particles, the planet’s magnetic field, and its atmosphere. The particles responsible for Jupiter’s aurorae originate from various regions of the magnetosphere and are accelerated by multiple mechanisms, including interactions with the Galileans moons, magnetosphere-ionosphere coupling, and solar wind-driven processes. As these particles interact with the gases in the planetary ionosphere, they generate emissions observable across a wide range of wavelengths, from X rays to radio. The aurorae reflect the properties of the atmospheric layers in which they occur as well as the magnetospheric conditions, making them valuable probes for studying Jupiter’s magnetosphere. To support this investigation, the Juno spacecraft is equipped with several instruments, including the Jovian InfraRed Auroral Mapper (JIRAM), an imager-spectrometer designed to explore the Jovian aurorae and the planet’s atmospheric structure, dynamics, and composition at infrared wavelengths. JIRAM observations in the L-band (3.3–3.6 𝜇m) have provided unprecedentedly detailed images of the auroral structures, revealing finer details of the emission patterns. The spectrometer, operating in the 2–5 𝜇m range, has enabled estimates of the temperature and abundance of molecules responsible for the detected signals. In the infrared, auroral emissions are primarily due to H+ 3 , which appears exceptionally bright in the 3–4 𝜇m range due to the near-total absorption by CH4 of incoming solar radiation from the lower layers of Jupiter’s atmosphere. In the same spectral interval, the spectrometer also detects the 3.3-𝜇m emission band of methane, allowing for a more in-depth analysis of this molecule, which significantly influences the chemistry, dynamics, and composition of the Jovian atmosphere at the 𝜇-bar pressure level. This thesis investigates the extensive, largely unexplored volume of JIRAM spectra within the 3–4 𝜇m range, employing a Bayesian inversion approach to analyze H+ 3 and CH4 emissions across Jupiter’s upper atmosphere, from the poles to the equator. L-band images are also utilized to provide morphological context for the spectra and facilitate data selection for analysis. To enhance the quality of the spectral data, this work introduces filtering tools specifically designed to identify measurements with the highest signal-to-noise ratio for the H + 3 and CH4 emission lines. The investigation includes spectra acquired in both nadir and limb viewing geometries, with distinct objectives tailored to each observation type. For nadir observations, this study aims to accurately assess the column density and temperature of the target gases. To this end, a primary goal of this project was to refine the retrieval code originally developed for JIRAM nadir spectra, addressing its limitations. This is explored in the first part of this thesis, which details modifications made to the code to improve the fitting of the spectral continuum and identifies potential challenges in modeling the emission bands around the 3.3 𝜇m that require further investigation. The refined code is then applied to three case studies. The first study examines spectra collected along the main auroral ovals across multiple orbits, yielding insights into the properties of H+ 3 associated with the main emissions and revealing temporal variations in the auroral signal. Next, spectra from the footprints of the Galilean moons Io, Europa, and Ganymede are analyzed, marking the first investigation of these auroral structures using infrared spectroscopy. This analysis uncovers new details about the properties of moon-induced aurorae, contributing to a deeper understanding of the mechanisms driving their formation. Finally, the thesis explores polar emissions, analyzing bright spots within the auroral ovals linked to intense methane emissions at 3.3 𝜇m. The results reveal that methane in these regions exists at temperatures significantly higher than those typically recorded for stratospheric CH4. This suggests that these features may arise from non-thermal processes at the 𝜇-bar pressure level, such as particle precipitation or Joule heating, or from upwelling of CH4 due to enhanced vertical mixing. For the limb spectra, the inversion process provides the vertical distribution of volume mixing ratios and temperatures of the gases of interest. JIRAM limb measurements have revealed two distinct emission layers for H+ 3 and CH4, enabling separate analyses of these molecules. Initially, this thesis presents the results of the inversion of JIRAM H+ 3 limb spectra, providing vertical profiles of the ion’s temperature and volume mixing ratios at equatorial latitudes. Subsequently, it details efforts to model the CH4 signal under Non-LTE (Non Local Thermodynamic Equilibrium) conditions using the GRANADA (Generic Radiative transfer AnD non-LTE population Algorithm) and KOPRA (Karlsruhe Optimized and Precise Radiative Transfer Algorithm) codes, along with advancements in the configuration of the retrieval code RCP (Retrieval Control Program). The ultimate goal is to achieve accurate mapping of methane distribution in Jupiter’s atmosphere around the 𝜇-bar pressure level. Collectively, these investigations deepen our understanding of the upper layers of Jupiter’s atmosphere, offering a comprehensive overview of the H+ 3 and CH4 properties on a global scale. Furthermore, they highlight the importance of ongoing exploration and research in this field, as such efforts will further expand the existing knowledge and help clarify the processes that shape Jupiter’s atmosphere and its magnetospheric environment.