It's a law crying out to be broken: Planets and moons never make their own light, they simply reflect sunlight. Now researchers studying data from the Galileo spacecraft orbiting Jupiter have found a miscreant moon. Galileo observed sulfurous Io as the moon passed into Jupiter's shadow--which gives a better view of the glow of lava belched from its volcanoes. But Galileo found that Io's atmosphere also glows. "The glow is bright enough that someone riding on Galileo could see it with his naked eye," says Paul Geissler, a planetary scientist at the University of Arizona. The greenish glow in the center of Io's disk is thought to be caused by high-speed particles that batter the moon's atmospheric oxygen or sodium gas--the particles are trapped in Jupiter's powerful magnetic field. The bright bluish areas to the left and right are the points closest to and farthest from Jupiter; electricity generated by Jupiter's magnetic field arcs to Io's surface, passes through the moon, and shoots out the other end. The current seems to connect through clouds of sulfur dioxide gas shooting up out of volcanoes at these points.