Crescent Io seen in approximately "natural" color by Voyager 2 in July 1979 from a distance of 1.1 million kilometers.


The innermost of Jupiter's four planet-sized moons, Io is also one of the most volcanically active bodies in the solar system. Voyager observed eight active volcanic plumes in March and July 1979 and some of these have been monitored since using infrared telescopes on Earth. Lava flows and irregularly shaped mountains are also observed, but no craters (consistent with ongoing volcanic activity, which would erase impact craters not long after they form). The composition of these lavas and plumes remains uncertain. Are they silicate lavas similar to those seen on Earth (for example Hawai'i and Mt. Etna)? Or are they composed of liquid sulfur (which might account for Io's yellowish color) or a combination? Io is the same size and density as Earth's Moon. Why, then, is Io so volcanically active? The answer is that tides on Io raised by both Europa and massive Jupiter are so great that they continually heat and partially melt the interior of Io.

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Io & Jupiter

Stereo image of Io floating above the cloud decks of Jupiter, showing the true difference in size between the planet and its moon. These images were taken three minutes apart, and the stereo effect results from the orbital motion of Io about Jupiter.

Io in "Natural" Color

Io appears in shades of pastel yellow and white to the naked eye. This view, using orange, green, and blue filter images, shows approximately how Io would appear to passing humans. Features as small as 7 kilometers are visible. (Gaps are due to missing data.)

Io in "Enhanced" Color

This view (of the same area as the image above) takes advantage of Voyager's greater sensitivity in the blue and violet wavelengths to produce an "enhanced-color" view. Color differences on Io are more pronounced at these wavelengths, and using them allows geologists to better discriminate individual features.

Volcanos and Mountains

This color mosaic extends from near the equator (top of image) to the pole (bottom). A variety of lava flows and mountains can be seen. Haemus Montes is the rectangular-shaped mountain at lower left and Euboea Montes is the elliptical-shaped mountain at center. Creidne Patera, a large dark volcanic complex at image center, is a site of effusive volcanism similar to volcanic calderas on Earth (such as Hawai'i). Just above Creidne is Maasaw Patera, a series of radiating brownish lava flows emanating from a volcanic caldera (or collapse pit). For scale, this image is approximately 1900 kilometers from top to bottom, or slightly larger than the state of Texas.

The Io Movie

This first-cut movie is based on stereo-derived topography of Io (generated at LPI) combined with the above color mosaic to provide this 3-D rendering of the surface. An improved version is in production.

Haemus Montes

Haemus Montes, located near Io's south pole, rise approximately 10 kilometers above the surrounding plains (Mount Everest rises almost 9 kilometers above sea level). The origin of these and other similar mountains, which cover about 2% of Io's surface, is uncertain. They are possible evidence of tectonic deformation predating most of the volcanic material that surrounds them.

Haemus Montes in 3-D

Red-green stereo glasses can be used with this image to see Haemus Montes in 3-D.

More Volcanos and Mountains

Enhanced color mosaic of a portion of Io from approximately 0° to 60° latitude and from -60° to 10° longitude. A persistent thermal hotspot named "Kanehekili" is located just north of the rectangular bright orange patch in the upper left quadrant of the scene. Voyager also imaged an active volcanic plume in the vicinity of the large Y-shaped black patch (a lava flow?) at bottom left.


Loki & Loki Patera

Loki is one of the most persistent volcanic areas on Io and has been monitored from Earth almost continuously since it was discovered by Voyager 1 in March 1979. The dark, horseshoe-shaped feature is thought to be a lava lake, partially filled with dark, possibly basaltic, lava. An active volcanic plume, arcing over 150 kilometers into space, can be seen near image center (the blue-gray wispy features extending from the left end of the long, dark, gash-like feature). This is one of eight active plumes observed by Voyager in 1979.


Prometheus, another plume observed by Voyager in 1979, is seen in this triptych of three views taken over a 6-hour span. The Promethus plume extends upward 70 kilometers into space before falling on the surface 125 kilometers from the central vent. The plume somewhat resembles an umbrella, with several prominent dark radial spokes comprised of optically thick concentrations of plume material. The plume may be composed of sulfur dioxide explosively heated within the crust of Io. The first image is a black-and-white image, the second shows the area in approximately natural color, the third view shows the area in enhanced color (emphasizing blues and violets where color difference between geologic units on Io is more pronounced). Prometheus can also be viewed in 3-D. Focus on two adjacent images, and allow your eyes to cross until the two images merge. (Red-green stereo glasses do not work on this image.)

Prometheus in 3-D

Red-green stereo glasses can be used on this image to see the Prometheus plume in 3-D.


Vent source of the Pele plume, observed by Voyager in March 1979. This was the largest plume observed by Voyager, extending ~500 kilometers in all directions from the source (the plume itself extends beyond the limits of this image). The eruption source is the small dark spot at the eastern (right) end of the linear fissure near the center of the frame.

Ra Patera

Ra Patera is the largest shield volcano on Io. Shield volcanos, like Hawai'i, are large volcanic edifices composed of numerous overlapping lava flows. They are called shields because they form broad convex domes with low slopes. Dark linear flows radiate from the center of Ra. Some have speculated that these flows are composed of sulfur, although many do not agree with this interpretation. Ra Patera was not active when Voyager passed in 1979, but Hubble Space Telescope observations in 1995 and 1996 indicate that a major change has taken place at this location, quite likely a major volcanic eruption. Galileo should obtain new images of this area that will hopefully reveal the origin of these changes.

All images by Paul M. Schenk, Lunar and Planetary Institute, Houston, TX.

©Lunar and Planetary Institute, 1997

Supplemental Reading Materials

Beatty J. K., O'Leary B., and Chaikin A., eds. (1990) The New Solar System. Sky Publishing Corporation, Cambridge, Massachusetts; Cambridge University Press, New York. 326 pp.

Moons and Rings (1991) Voyage Through the Universe series. Time-Life Books, Alexandria, Virginia. 144 pp.

Rothery D. (1992) Satellites of the Outer Planets. Clarendon Press, Oxford. 208 pp.

McEwen A. et al. (1989) Dynamic geophysics of Io. In Time-Variable Phenomenon in the Jovian System, pp. 11-46, NASA Special Publication 494.

Morrison D., ed. (1982) Satellites of Jupiter. University of Arizona Press, Tucson. 972 pp.

Nash D. et al. (1986) Io. In Satellites (J. Burns and M. Matthews, eds.), p. 629, University of Arizona Press, Tucson.

Spencer J. and Schneider N. (1996) Io on the eve of the Galileo mission. Annual Reviews of Earth and Planetary Science, 24, 125.

Schenk P. et al. (1997) Geology and topography of Ra Patera, Io, in the Voyager era: Prelude to eruption. Geophys. Res. Lett., 24, 2467-2470.