Illustration of aurorae on Jupiter's moon Ganymede (foreground) as it orbits the gas giant (background). Two auroral ovals (blue) can be seen over northern and southern mid-latitudes. As on Earth, Ganymede's aurorae are produced by energetic charged particles causing gases to fluoresce. The aurorae are controlled by Ganymede's magnetic fields, but are also influenced by Jupiter's magnetic field. Measurements of the shift in the aurorae caused by Jupiter have shown that a saline ocean exists under Ganymede's crust.