[Illustration: One of the most remarkable of all the Voyager discoveries was the arcuate white ridges on Europa. Visible only at very low Sun angle, these curved bright streaks are 5 to 10 kilometers wide and rise at most a few hundred meters above the surface. Their graceful scalloped pattern is unique to this planet and has defied explanation. Also visible in this view, taken by Voyager 2 on July 9 at a range of 225 000 kilometers, are dark bands, more diffuse than the light ridges, typically 20 to 40 kilometers wide and hundreds to thousands of kilometers long. [P-21766]] [Illustration: Shaded relief map of Io as it appeared in early March 1979. [260-634BC]] [Illustration: The great erupting volcanoes on Io produce distinctive surface markings. These are three views of Prometheus (P₃). The bright ring on the surface rims the areas of fallout from the plume, and it is probably an area in which sulfur dioxide frost is being deposited on the surface. [260-451]] [Illustration: Prometheus.] [Illustration: Prometheus.] [Illustration: Prometheus.] [Illustration: These views are of Loki (P₂) as seen by Voyager 1. [260-451]] [Illustration: The asymmetric structure of the plume can be seen.] [Illustration: Loki.] [Illustration: An ultraviolet image has been used to produce a false-color composite; the large ultraviolet halo above the visible-light plume may be due to scattering from sulfur dioxide gas rather than solid particles.] [Illustration: Perhaps the most spectacular of all the Voyager photos of Io is this mosaic obtained by Voyager 1 on March 5 at a range of 400 000 kilometers. A great variety of color and albedo is seen on the surface, now thought to be the result of surface deposits of various forms of sulfur and sulfur dioxide. The two great volcanoes Pele and Loki (upper left) are prominent. [260-464]] In place of impact craters, the surface of Io has a great many volcanic centers, which generally take the form of black spots a few tens of kilometers across. In a few cases, high-resolution pictures show the characteristic shapes associated with volcanic calderas on Earth and Mars, and, if the other volcanic centers are similar, about 5 percent of the entire surface of Io is occupied by calderas. These are extremely black, reflecting less than 5 percent of the sunlight; often they are surrounded by irregular, diffuse halos nearly as black as the central spot. The calderas seem more like the Valles caldera in New Mexico, which is associated with vents that produced large quantities of ash, than with those of Hawaiian-type shield volcanic mountains.