Io's crescent
Io's crescent New Horizons captured this photo of Io with its Long Range Reconnaissance Imager (LORRI) as it sped away from the Jupiter system on March 3, 2007 at 06:11 UTC. NASA / JHUAPL / SwRI

The plume of the giant volcano Tvashtar is brightly backlit by the Sun.  On the sunlit limb, there is a bright flash off of the smooth surface of the flat lava flows of Loki Patera; these flows look quite dark when seen with the Sun overhead, but in this lighting geometry the sunlight glances off the smooth surface as off a mirror, producing a flash of light.