Jovian Satellite and Ring Observations from the Juno Stellar Reference Unit, plus Plans for the Dark Side Perijoves

Juno’s low-light sensitive Stellar Reference Unit (SRU) navigation camera has embarked on Juno’s Extended Mission as a full-fledged member of the science payload. SRU images from Juno’s Prime Mission led to multiple discoveries within the Jovian system, inspiring new objectives that the orbit’s evolution will soon put within reach. SRU images of Jupiter’s dark side revealed the presence of “shallow lightning” flashes with origins in high altitude ammonia-water clouds above the 2 bar level. These observations fortify theories in which ammonia-water hailstones (“mushballs”) transport ammonia into Jupiter’s deep atmosphere, accounting for the ammonia variability observed at depth by Juno’s Microwave Radiometer (MWR). Images of Jupiter’s faint dust ring have been acquired from unique vantage points while inside the ring looking out, and while observing the shadow line cast by Jupiter across the dust. In June 2021, the SRU imaged Ganymede’s dark side at >79 degrees illumination angle and <920 m/pixel resolution while the surface was illuminated only by Jupiter-shine. This novel use of a low-light star tracker resolved multiple craters, surface features (such as grooved terrain), and an unusual elongated ejecta deposit in Xibalba Sulcus. These features are not resolved in the Voyager and Galileo imagery used for the USGS global geologic map of Ganymede. Our discussion will focus on proposed updates to Ganymede’s geologic record and SRU ring science. A preview of upcoming SRU observations to investigate the influence of Jupiter’s shallow thunderstorms on Jupiter’s deep atmospheric dynamics (during the dark side perijoves starting in 2023) will also be included.