When the twin probes Voyager 1 and Voyager 2 passed by Io in 1979, their more advanced imaging system allowed for far more detailed images.
Voyager 1 flew past Io on 5 March 1979 from a distance of 20,600 km (12,800 mi).
The images returned during the approach revealed a strange, multi-colored landscape devoid of impact craters.
The highest-resolution images showed a relatively young surface punctuated by oddly shaped pits, mountains taller than Mount Everest, and features resembling volcanic lava flows.

Shortly after the encounter, Voyager navigation engineer Linda A. Morabito noticed a plume emanating from the surface in one of the images.
Analysis of other Voyager 1 images showed nine such plumes scattered across the surface, proving that Io was volcanically active.
This conclusion was predicted in a paper published shortly before the Voyager 1 encounter by Stan Peale, Patrick Cassen, and R. T. Reynolds.
The authors calculated that Io's interior must experience significant tidal heating caused by its orbital resonance with Europa and Ganymede (see the "Tidal heating" section for a more detailed explanation of the process).
Data from this flyby showed that the surface of Io is dominated by sulfur and sulfur dioxide frosts.
These compounds also dominate its thin atmosphere and the torus of plasma centered on Io's orbit (also discovered by Voyager).

Voyager 2 passed Io on 9 July 1979 at a distance of 1,130,000 km (700,000 mi).
Though it did not approach nearly as close as Voyager 1, comparisons between images taken by the two spacecraft showed several surface changes that had occurred in the four months between the encounters.
In addition, observations of Io as a crescent as Voyager 2 departed the Jovian system revealed that seven of the nine plumes observed in March were still active in July 1979, with only the volcano Pele shutting down between flybys.
When the twin probes Voyager 1 and Voyager 2 passed by Io in 1979, their more advanced imaging system allowed for far more detailed images. Voyager 1 flew past Io on 5 March 1979 from a distance of 20,600 km (12,800 mi). The images returned during the approach revealed a strange, multi-colored landscape devoid of impact craters. The highest-resolution images showed a relatively young surface punctuated by oddly shaped pits, mountains taller than Mount Everest, and features resembling volcanic lava flows.
Shortly after the encounter, Voyager navigation engineer Linda A. Morabito noticed a plume emanating from the surface in one of the images. Analysis of other Voyager 1 images showed nine such plumes scattered across the surface, proving that Io was volcanically active. This conclusion was predicted in a paper published shortly before the Voyager 1 encounter by Stan Peale, Patrick Cassen, and R. T. Reynolds. The authors calculated that Io's interior must experience significant tidal heating caused by its orbital resonance with Europa and Ganymede (see the "Tidal heating" section for a more detailed explanation of the process). Data from this flyby showed that the surface of Io is dominated by sulfur and sulfur dioxide frosts. These compounds also dominate its thin atmosphere and the torus of plasma centered on Io's orbit (also discovered by Voyager).
Voyager 2 passed Io on 9 July 1979 at a distance of 1,130,000 km (700,000 mi). Though it did not approach nearly as close as Voyager 1, comparisons between images taken by the two spacecraft showed several surface changes that had occurred in the four months between the encounters. In addition, observations of Io as a crescent as Voyager 2 departed the Jovian system revealed that seven of the nine plumes observed in March were still active in July 1979, with only the volcano Pele shutting down between flybys.
Io (moon)