Juno Microwave Radiometer Measurements of the Depths of Spatial and Temporal Variability in Jupiter

Juno’s Microwave Radiometer (MWR) has provided an unprecedented opportunity to explore the dynamical properties and composition of Jupiter’s deep atmosphere. Its visible atmosphere is arguably the most heterogeneous and time-variable in the solar system. Since Juno’s arrival on 2016 August 27, the MWR has observed microwave emission at wavelengths between 1.3 and 50 cm, sensing from 0.7 bar to over 100 bars of pressure, at over 75 close approaches to the atmosphere. A concerted effort has collected MWR-supporting contextual information from other Juno instruments, as well as ground- and space-based observations, sensing the upper atmosphere at complementary wavelengths.

     We report here observations of long-term variability in Jupiter over 2016-2025, limited to data with spatial resolutions no worse than 2° in latitude, which have been subject to careful corrections to the calibration drift for all MWR’s channels with an improved relative calibration uncertainty of 0.5% or better over the zonal mean for the entire mission to date. This has allowed us to evaluate long-term variability with confidence that observed variability is not an artifact of receiver drift. The North Equatorial Belt, (NEB: 12°N-16°N) shows the greatest variability with a standard deviation of 2% of the time-averaged mean at all levels sensed by the MWR except for the 50-cm channel that senses variability in temperature and ammonia and water composition at pressures in excess of 100 bars of pressure. Among the strongest variability associated with discrete features in the atmosphere is a major upwelling and subsequent clearing of cloud cover in the North Temperate Belt (NTB: 20°N-26°N) in August-September of 2020.  In general, the microwave brightness temperature variability often but not always correlates with visible or near- to mid-infrared variability. In some regions, such as the Equatorial Zone (EZ: 3°S-6°N), substantial variability is detected not only in regions above the level of the water-condensate cloud (~10 bars) but also at great depth (>100 bars). Because the radiances emitted by Jupiter in the 5-µm spectral region are largely modulated by cloud cover in the 0.7- to 5-bar region, we use such observations as a reference to the variability of cloud-top weather. These observations were made by Juno’s JIRAM instrument and ground-based observations.  In general, 5-µm brightness temperatures are anticorrelated with MWR brightness temperatures, explained by an upward motion in a stably stratified atmosphere decreasing NH₃ vapor as a function of altitude. The NEB/NTrZ and NTB disturbances are most likely to be caused by baroclinic instabilities that grow fast, return to their initial unperturbed state more slowly and require a lateral density contrast. They do not affect the atmosphere significantly below the 9-bar H₂O condensation level. In contrast, the EZ disturbances extend deeper in the atmosphere and are likely to be caused by convective instability, which is more symmetric between its growth and decaying phases and requires a vertical entropy contrast. Current work includes convolving the MWR fields of view over maps of 5-µm radiances to assess whether the measured microwave variability is associated with spatial rather than temporal variability.