Historical Photo of the Month - September 2007

Io Plumes
Photograph Number 260-736

One of the most significant discoveries of the Voyager mission was the detection of volcanic activity on Jupiter's moon, Io. The illustration above, and a similar image were used in the Smith and Johnson articles cited below. They showed what was believed to make up the interior of the moon, including sulphur, sulphur dioxide, and silicates. Similar illustrations were also used at press conferences and at public talks about the discoveries.

In March 1979, Voyager I took many photographs of Jupiter and its moons. While looking at one of the photos, navigation team member Linda Morabito noticed a faint crescent on the edge of Io and reported it to the imaging team. It turned out to be the plume of a volcanic eruption. A total of nine active volcanos were eventually found on Io.

The gravitational forces of Jupiter, Europa, and Ganymede combine to flex and heat the interior of Io. Io's atmosphere has such a low density and the moon has such a low gravity, that erupted material encounters little resistance, sometimes spraying up hundreds of kilometers before coming back down to the surface in a roughly circular pattern. Many of these volcanoes may be more like Old Faithful geyser on Earth, while others have lava flows like terrestrial volcanoes.

To see photos from the Voyager mission, see the online exhibit Historical Images of Voyager's Grand Tour. For more information about Voyager or about the history of JPL, contact the JPL Archives for assistance.

Archival and library sources:
- Photo albums and indexes.
- Smith, B. A., et al, Nature, 280, 30 August 1979.
- Johnson, T.V. and L. A. Soderblom, Scientific American, 249 (6), 1983.
- Dethloff, H. C. and Schorn, R. A., Voyager's Grand Tour, Smithsonian, 2003.
- Evans, Ben, NASA's Voyager Missions, Springer, 2003.