[Illustration: The first active volcanic eruptions other than on Earth were discovered on Io. These volcanoes are extremely explosive with ejection velocities of more than one kilometer per second (2200 miles per hour), which is more violent than Etna, Vesuvius, or Krakatoa on Earth. Both pictures below were taken by Voyager 1.] [Illustration: 3/4/79 450,000 km (280,000 mi) In this picture, the plume visible on the right edge extends more than 100 kilometers (60 miles) above the surface.] [Illustration: 3/4/79 499,000 km (310,000 mi) The same volcano is shown in this picture, photographed one hour and 52 minutes earlier.] [Illustration: 3/4/79 490,000 km (304,000 mi) Special color reconstruction by means of ultraviolet, blue, green, and orange filters allowed scientists to study the amount of gas and dust and the size of the dust particles that erupted from the volcano on Io shown in this Voyager 1 image. The region that is brighter in the ultraviolet (blue area) is about 210 kilometers (130 miles) high, over twice the height of the denser, bright yellow core. The vent area is visible on page 18 as a dark ring in the upper left region of Io.] [Illustration: 7/10/79 1.2 million km (750,000 mi) Of the eight active volcanoes discovered on Io by Voyager 1, six of the seven volcanoes sighted by Voyager 2 were still active. The giant volcano observed by Voyager 1 over the “hoofprint” region (see page 18) had become inactive. Scientists, therefore, believe that the satellite is undergoing continuous volcanic activity, making Io’s surface the most active in the solar system. This Voyager 2 photograph, which shows three active volcanoes, was one of the last of an extensive sequence of “volcano watch” pictures planned as a result of Voyager 1’s volcano discovery. The black dots are calibration points on the camera.] [Illustration: 7/8/79 1.2 million km (750,000 mi) Europa, approximately the same size and density as our Moon, is the brightest Galilean satellite. The surface displays a complex array of streaks, indicating that the crust has been fractured. In contrast to its icy neighbors Ganymede and Callisto, Europa has very few impact craters. The relative absence of features and low topography indicate that the crust is young and probably warm a few kilometers below the surface. The warmth is probably due to a combination of radioactive and tidal heating. The tidal heating within Europa is estimated to be ten percent that of the stronger tidal heating effect within Io. The regions that appear blue in this Voyager 2 image are actually white.] [Illustration: 7/9/79 240,000 km (150,000 mi)