While imaging Io's night side under illumination from Jupiter-shine on Dec. 30, 2023, the Stellar Reference Unit (SRU) on NASA's Juno spacecraft observed an unprecedented glow from active lava at the base of a mountain on Io (red arrow, right panel). The thermal emission signature was located at the base of the western flank of South Zal Mons. Sunlit imagery of the region by captured by NASA's Galileo mission circa 1999 (at left) shows a vertical mountain fracture running from the top of the mountain to the location of the SRU-observed "glow" (red arrow, left panel). One hypothesis is that an extension of the mountain fracture created a fissure vent that allows lava to escape to the surface at this location.

More information about Juno is at https://www.nasa.gov/juno and https://missionjuno.swri.edu.