Evidence of erosion in Io’s southern polar region is visible in this Voyager 1 high-resolution image. The picture has been computer-enhanced to bring out surface detail while suppressing bright markings. A depressed segment of the crust, bounded by faults, is seen near the terminator in the upper right portion of the image. At the lower center are complicated scarps (slopes) and portions of isolated elevated terrain that geologists interpret as “islands” left behind as the scarps eroded. Scientists speculate that sulfur dioxide (as a subsurface liquid) may be a determinant in the creation of these features.
3/4/79 862,000 km (540,000 mi)
Io’s surface, less than ten million years old, is quite young compared to the other Galilean satellites and to other terrestrial bodies, such as Mercury and the Moon. The surface is composed of large amounts of sulfur and sulfur dioxide frost, both of which account for most of the surface color. This picture was taken by Voyager 1. Material deposited by the volcano (see following pictures) can be seen as a white ring near the center of Io.
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.
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.
3/4/79 499,000 km (310,000 mi)