Exploring the Depth of Planetary-Scale Changes in Jupiter from Juno Microwave Radiometer Observations

Abstract

The Juno Microwave Radiometer (MWR) has extended our knowledge of the structure and composition of the atmosphere down to several hundred bars, revealing meridional variability to great depth (e.g. Li et al. 2017 Geophys. Res. Lett. 44, 5317; Fletcher et al. 2021. J. Geophys. Res. 126, E06858). The MWR has revealed that some cyclonic and anticyclonic vortices may have roots at depths of tens of bars of pressure (Bolton, et al. 2021. Science 374, 968.), but 5-µm hot spots and associated plumes appear to be restricted to shallow depths above the water cloud (Fletcher et al. 2020, J. Geophys. Res. 125, e06399). We report ongoing work on evolution of the microwave brightness of Jupiter’s axisymmetric bands over 2016-2022. We have examined the regions where changes have taken place at visible wavelengths, as documented by images from professional and amateur observers, to judge their depth. Preliminary results show that microwave brightness variability from channels sensitive to depths corresponding to 9-50 bars of atmospheric pressure are generally much lower than those at pressures of 0.7-3 bars. One exception to this is in the northern component of Jupiter’s Equatorial Zone (2°N-6°N), whose measured variability at depth does not correspond to any visible or infrared feature in the upper atmosphere, although it might be considered a precursor to the short-lived 2018-2019 Equatorial Zone disturbance. At the lower pressures, a decrease in the antenna temperature in the northern component of the North Equatorial Belt (12°N-15°N) is coincident with its visible brightening and drop of 5.1-µm radiance, both implying increased cloud and NH3 opacity in 2021. Even though the visibly dark North Equatorial Belt expanded northward into latitudes more typically associated with visibly bright regions that are cold at 5.1 µm (16°N-19°N), known as the North Tropical Zone (Fletcher, et al. 2017. Geophys. Res. Lett. 44, 7140), we do not detect any corresponding change of the MWR antenna temperature.  Although there are substantial changes in the visible and 5.1-µm appearance of the northern component of the North Temperate Belt (24°N-26°N) as well as in the MWR antenna temperatures, the two do not appear to be correlated with one another. An important part of our next steps in this research will be to examine which of the MWR variabilities in the zonal-mean microwave brightness are the result of zonally discrete features in the atmosphere, particularly the North Equatorial Belt (6°N-15°N).