Jupiter’s 3.3-micron CH4 polar brightening: Retrieval of methane effective temperature in the jovian auroral regions using Juno/JIRAM data

Despite the multiple evidence of the diffuse presence of methane in Jupiter’s auroral regions, the mechanisms leading to the CH4 brightening observed both from ground- and space-based platforms are not yet fully understood. During the first NASA/Juno’s orbit, the on-board imager/spectrometer JIRAM (Jovian Infrared Auroral Mapper) detected the 3.3-µm methane emission on both Jupiter’s poles. The signal was found to be mostly confined within the main auroral ovals, although the lack of spectral coverage over 80°S prevented a deep investigation of the southern methane spot. The CH4 polar emissions at 3.3 µm are likely originated by non-thermal excitation mechanisms occurring above the 1 µbar level, such as auroral particle precipitation and/or Joule heating. However, aurorally driven upwelling of methane inside the main oval might also explain the enhanced concentrations of CH4 observed at the jovian poles. To address this controversy, we derive the effective temperature of methane in Jupiter’s auroral regions, which is a key information to understand the origin of the detected fluorescence. The goal is achieved by exploring three Juno’s orbits and focusing on the spectra with the highest methane emissions and the smallest contribution from other auroral features due to H3+. JIRAM measurements from the first perijove are used to investigate the northern methane brightening, while observations from perijoves 7 and 8 are examined for its southern counterpart. The analysis reveals similar temperatures in the north- and south-polar spots, mainly ranging between 400 K and 670 K.