An illustration of Callisto, Jupiter's second-largest moon. Credit: Corey Ford/Stocktrek ImagesNASA's Juno spacecraft has spied the final "auroral footprint" of a moon of Jupiter, meaning that it has watched as the moon Callisto interfered with one of Jupiter's planetary auroras. Unlike Earth's moon, Jupiter's four giant moons actually interact with its magnetosphere. When an aurora occurs, Jupiter's atmosphere becomes visible to various instruments—and so do the perturbations caused by the moons. By looking at these perturbations, scientists can discern information about the composition of Jupiter's atmosphere, of the moon's atmosphere, and even of the subsurface structures.