NASA's Juno aircraft are in Jupiter's study mission, but this is not the only heavenly object in the range of its instruments. Jupiter has many months, and Juno can peek into those as he spins around the gas giant. In his latest orbit Jupiter, Juno saw something interesting on Moon Io: volcanic activity.
In 2011, Juno launched a mission to study Jupiter's atmosphere and internal structure, which began after the 2016 gas giant. Juno is currently on a 53-day orbit of Jupiter. NASA originally intended to significantly shorten that orbit, but aircraft engine problems have made it too risky. One outcome of a longer orbit is that NASA gets more opportunities to observe other elements of the Jovian system such as Io.
Scientists have known that Io has been geologically active since 1979 when Voyager 1 passed on its way out of the Solar System. I is somewhat larger than the Earth's moon, but its internal heating triggers the tidal forces of Jupiter's massive gravity. Over the years it has become clear how much Io is active – with more than 400 volcanoes, Io is considered to be geologically the most effective object in the solar system. So it is not unthinkable that Juno would spy on the eruption if he wanted to.
During his 16th orbit Jupiter (halfway through the mission), the scientists turned Juno Instruments on Io just when he began to pass through the shadow of Jupiter. Four Juno's cameras saw the flash over the terminator on the dark side of Io (see above), indicating a volcanic eruption.
Juno first noticed an eruption at 12:00 UTC on December 21. The ground is already in the shadows, but a light spot indicates that the volcanic cloud is high enough to reflect the sun's light. The top image comes from JunoCam in a visible range, reconstructed from red, blue and green pictures. Below you can see the eruption through the Jivian Infrared Auroral Map Maker (JIRAM). The bright point of the eruption points to extremely high temperatures. Though it was not intended for JIRAM, the team was pleasantly surprised at how well it worked to observe the buds on Io.
NASA expects to complete Juno's primary science mission in July 2021 when the probe will complete its Jupiter map. Juno has already helped scientists understand Jupiter's atmosphere and magnetic field. The second half of the mission could illuminate the zonal winds and the inner structure of the planet.