The most amazing thing about the light ridges is their form. Instead of being straight, they form scallops or cusps, with smooth curves that repeat regularly on a scale of 100 to a few hundred kilometers. In some of the low-Sun-angle pictures, the surface of Europa seems to be covered with a beautiful network of these regular curving lines. The impression is so bizarre that one tends not to believe the reality of what is seen. Nothing remotely like it has ever been seen on any other planet. At present the geology of Europa remains beyond our understanding. Presumably there is a thick ice crust, perhaps floating on a liquid water ocean. Presumably there is sufficient heat coming from the interior to have produced cracking or motion in the ice crust, and the light and dark streaks preserve a pattern in some way related to this internal activity. However, the actual mechanisms for producing the observed features so lightly traced on this smooth white world remain for scientists to decipher. [Illustration: Europa looks like a cracked egg in this computer mosaic of the best Voyager 2 images. In this presentation, the variation of surface brightness due to the angle of the Sun has been removed by computer processing, so that surface features can be seen equally well at all places. The many broad dark streaks show up well, but this presentation does not bring out the much fainter and more enigmatic light streaks. These pictures were taken from a distance of about 250 000 kilometers and show features as small as 5 kilometers across. [260-686]] Io The most spectacular of the Galilean satellites is Io. Even in low-resolution images, its brilliant colors of red, orange, yellow, and white set it apart from any other planet. The dramatic scale of its volcanic activity confirms that Io is in a class by itself as the most geologically active planetary body in the solar system. The diameter of Io is 3640 kilometers, and its density is 3.5 grams per cubic centimeter. Both values are nearly identical to those of our Moon. Were it not for Io’s proximity to Jupiter, it would probably be a dead, rocky world much like Earth’s satellite. Careful examination of all the Voyager images of Io, some of which have resolutions as good as 1 kilometer, has failed to reveal a single impact crater. Yet the flux of crater-producing impacts at Io must be even greater than for the other Galilean satellites, because of the focusing effect of Jupiter’s gravity. The absence of craters alone would indicate that Io has an extremely young and dynamic planetary surface, even without the observation of active volcanoes. Calculations indicate that craters on Io must be filled in or otherwise obliterated at a rate corresponding to the deposition of at least 100 meters per million years, and quite probably a factor of ten greater, or 1 meter every thousand years.