The Evolution of an Upper-Atmospheric Haze During a Coloration Episode in Jupiter’s Equatorial Zone from Near-Infrared Observations, 2017-2021

From 2018 to 2019 Jupiter’s Equatorial Zone (EZ) appeared to begin a quasi-periodic disturbance, an event characterized by the appearance of an increasing number of regions of minimal cloudiness that are revealed by uncharacteristically bright 5-μm emission (Antunano et al. 2018, Geophys. Res. Lett. 45, 10987). At visible wavelengths, these deep-cloud disturbances are often associated with a transformation in the upper cloud levels from a bright, white color to a redder ochre shade. Although a few bright 5-μm regions appeared in the EZ in early 2019, it regained its almost uniformly dark appearance at this wavelength within a few months. However, even this weak 5-μm event was accompanied by the characteristic and dramatic transformation of color from a visibly white to a strongly ochre shade, particularly in the central part of the EZ. Although some subtle lightening of the EZ disturbed color was noted in mid 2020, its stronger reddish color returned in later in that year. During this time, our images of the region at near-infrared wavelengths using the IRTF SpeX and Gemini North NIRI instruments noted a remarkable change in the central part of the EZ, with a prominent brightening at wavelengths with strong gaseous CH₄ and H₂ absorption, which continue to this writing. While this was taking place, the EZ remained dark at 5 μm. This near-infrared brightening is consistent with an increase of particle density in the upper troposphere and lower stratosphere of the EZ. The formation of a high-altitude haze that is contemporaneous with visible color changes could be interpreted as the result of lofting of such particles at sufficient altitudes and for sufficient amounts of time to allow chemical changes in the haze particles to take place as a result of increased exposure to solar ultraviolet irradiation. The connection between this thickening of high-altitude haze and the weak cloud-clearing event at depth, signified by 5-μm brightening, remains unclear. We will discuss constraints on this chemistry from an understanding of the latitude distribution of these particles and color changes in the EZ from imaging by the Hubble Space Telescope and Juno.