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Jupiter I

Nothing endures but change.
- Heraclitus


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Io [EYE-oh or EE-oh] can be classified as one of the most unusual moons in our solar system. Active volcanism on Io was the greatest unexpected discovery at Jupiter. It was the first time active volcanoes had been seen on another body in the solar system. The Voyagers observed the eruption of nine volcanoes on Io altogether. There is also evidence that other eruptions occurred between Voyager encounters. Plumes from the volcanoes extend to more than 300 kilometers (190 miles) above the surface, with material being ejected at speeds up to a kilometer (.6 miles) per second.

Io's volcanoes are apparently due to heating of the satellite by tidal pumping. Io is perturbed in its orbit by Europa and Ganymede, two other large satellites nearby, then pulled back again into its regular orbit by Jupiter. This tug-of-war results in tidal bulging as great as 100 meters (330 feet) on Io's surface.

The temperature on Io's surface is about -143° C (-230° F); however, a large hot spot associated with a volcanic feature measured about 17° C (60° F). Scientists believe the hot spot may be a lava lake, although the temperature indicates the surface is not molten. This feature is reminiscent of lava lakes on Earth.

Io is composed primarily of rocky material with very little iron. Io is located within an intense radiation belt of electrons and ions trapped in Jupiter's magnetic field. As the magnetosphere rotates with Jupiter, it sweeps past Io and strips away about 1,000 kilograms (1 ton) of material per second. The material forms a torus, a doughnut shaped cloud of ions that glow in the ultraviolet. The torus's heavy ions migrate outward, and their pressure inflates the Jovian magnetosphere to more than twice its expected size. Some of the more energetic sulphur and oxygen ions fall along the magnetic field into the planet's atmosphere, resulting in auroras.

Io acts as an electrical generator as it moves through Jupiter's magnetic field, developing 400,000 volts across its diameter and generating an electric current of 3 million amperes that flows along the magnetic field to the planet's ionosphere.

Io Statistics
 Discovered bySimon Marius & Galileo Galilei 
 Date of discovery1610 
 Mass (kg)8.94e+22 
 Mass (Earth = 1)1.4960e-02 
 Equatorial radius (km)1,815 
 Equatorial radius (Earth = 1)2.8457e-01 
 Mean density (gm/cm^3)3.55 
 Mean distance from Jupiter (km)421,600 
 Rotational period (days)1.769138 
 Orbital period (days)1.769138 
 Mean orbital velocity (km/sec)17.34 
 Orbital eccentricity0.004 
 Orbital inclination (degrees)0.040 
 Escape velocity (km/sec)2.56 
 Visual geometric albedo0.61 
 Mean surface temperature-143°C 
 Magnitude (Vo)5.02 

Animations of Io

Views of Io

See also: Galileo Images of Io.

Color Image of Io
This color image of Io was created by combining the color channels of the low resolution USGS Voyager controlled color mosaic with Tayfun Oner's high resolution Galileo mosaic. The brown, orange areas are probably covered by sulphur or a mixture containing sulphur. The light areas may be sulphur dioxide snow and the pock-marks are mostly volcanic calderas up to 200 kilometers (124 miles) across. Mountainous regions exist near both poles, with some features rising 8 kilometers (5 miles) or more above their surroundings. (Courtesy A.Tayfun Oner)

Interior of Io The Interior of Io
This picture shows a cutaway view of the possible internal structure of Io. The images were created from a mosaic of images obtained by NASA's Galileo spacecraft. Io is the most volcanically active body in the solarsystem. The interior characteristics of the moon are inferred from gravity field and magnetic field measurements by the Galileo spacecraft. Io's radius is 1821 km, similar to the 1738 km radius of our Moon; Io has a metallic (iron, nickel) core (shown in gray) drawn to the correct relative size. The core is surrounded by a rock shell (shown in brown). Io's rock or silicate shell extends to the surface. (© Copyright Calvin J. Hamilton)

Io Feature Map
This is the highest resolution color global mosaic of Io. It was created by combining the color channels of the low resolution USGS controlled color mosaic with the high resolution B&W controlled USGS mosaic. It was then projected to an ortographic projection centered at 0 degrees latitude and 315 degrees longitude. (Courtesy A.Tayfun Oner)

Io Shown in Lambertian Equal Area Projection
Voyager 1 computer color mosaics, shown in approximately natural color and in Lambertian equal-area projections, show the Eastern (left) and Western (right) hemispheres of Io. Almost all the features visible here have volcanic origins, including several calderas and eruption plumes that were active at the time of the Voyager 1 encounter.

Global Mercator Mosaic
Io's volcanic plains are shown in this Voyager 1 image mosaic which covers the area roughly from latitude 60 degrees N. to latitude 60 degrees S. and longitude 100-345. North is up. Numerous volcanic calderas, lava flows, and volcanic eruption plumes are visible here. The composition of Io's volcanic plains and lava flows has not been determined, but they could consist dominantly of sulfur with surface frosts of sulfur dioxide or of silicates (such as basalt) encrusted with sulfur and sulfur dioxide condensates. The bright whitish patches probably consist of freshly deposited sulfur dioxide frost.

Hubble Resolves Volcanoes on Io
This image of Jupiter and its satellite Io were taken on March 5, 1994. Io is too small for ground-based telescopes to see the surface details. Several of the volcanoes periodically are active because Io is heated by tides raised by Jupiter's powerful gravity. The volcano Pele appears as a dark spot surrounded by an irregular orange oval in the lower part of the image. The orange material has been ejected from the volcano and spread over a huge area. Though the volcano was first discovered by Voyager, the distinctive orange color of the volcanic deposits is a new discovery in these Hubble Space Telescope images. (Voyager missed it because its cameras were not sensitive to the near-infrared wavelengths where the color is apparent). (Credit: John Spencer, Lowell Observatory; NASA)

Hubble Discovers Bright New Spot on Io
This pair of Hubble images of Jupiter's volcanic moon Io shows the surprising emergence of a 320-kilometer (200-mile) diameter, large yellowish-white feature near the center of the moon's disk (photo on the right). Scientists suggest the spot may be a new class of transient feature on the moon. For comparison, the photo on the left was taken in March 1994 before the spot emerged, and shows that Io's surface had undergone only subtle changes since it was last seen by the Voyager 2 probe in 1979. The new spot in the July 1995 Hubble image replaces a smaller whitish spot seen at about the same location in the March 1994 image. "The new spot surrounds the volcano Ra Patera, which was photographed by Voyager, and is probably composed of material, probably frozen gas, ejected from Ra Patera by a large volcanic explosion or fresh lava flows," according to John Spencer of Lowell Observatory in Flagstaff, Arizona. (Credit: J. Spencer, Lowell Observatory/NASA)

Volcanic Plumes
Voyager 2 took this picture of Io on the evening of July 9, 1979, from a range of 1.2 million kilometers (745,700 miles). On the limb of Io are two blue volcanic eruption plumes about 100 kilometers (62 miles) high. These two plumes were first seen by Voyager 1 in March 1979 and are designated Plume 5 (upper) and Plume 6 (lower). They apparently had been erupting for a period of at least four months and probably longer. A total of six plumes were seen by Voyager 2, all of which were first seen by Voyager 1. Plume 1, the largest volcano viewed by Voyager 1, was no longer erupting when Voyager 2 arrived. Plume 4 was not viewed on the edge of Io's disc by Voyager 2 and, therefore, it is not known whether it was still erupting. (Courtesy NASA/JPL)

P3 Prometheus
Voyager 1 took this picture of the P3 Prometheus volcano on March 4, 1979. The volcanic eruption can be seen on the limb of Io. (Credit: Calvin J. Hamilton)

Loki Patera
This is a close up view of Io's northern hemisphere. The central feature has been named Loki Patera. The large dark area might be a lake of liquid sulphur with a raft of solid sulphur inside. (Credit: Calvin J. Hamilton)

Venting Gasses
This photograph of Io shows what appears to be a volcanic caldera that is venting gasses (the bright blue patch at left center). In the photo is a network of volcanic calderas with dark floors linked by bright red materials. The northernmost caldera has a bright blue patch on the floor. Scientists believe the bright blue patch may be clouds of gas issuing from volcanic vents. The gas clouds may condense to form extremely fine particles that appear blue. Since Voyager 1's infrared spectrometer has discovered sulfur dioxide on Io, it is possible that sulfur dioxide is the main component of the clouds. Sulfur dioxide clouds would rapidly freeze and snow back to the surface. It is also possible that dark areas in the floors of the calderas are pools of molten sulfur, a very dark form of sulfur. The image was taken March 5, 1979, as Voyager 1 approached Io, and was taken from 129,600 kilometers (80,500 miles).

Haemus Mons
Haemus Mons is a mountain located near the south pole and in this image near the terminator of Io. Its base measures about 200 by 100 kilometers (124 by 62 miles). Several mountain peaks are found on Io some measuring as high as 10 kilometers (6.2 miles) tall. (Credit: Calvin J. Hamilton)

Volcano Pele
The heart shaped feature in the center of this image was caused by volcanic ejecta thrown out of the eruptive Pele. (Credit: Calvin J. Hamilton)

Ra Patera
Ra Patera is a large shield volcano with multi colored flows. This image shows at least a dozen dark flows originating from the central dark vent. Some of these flows are 300 kilometers (186 miles) in length. (Credit: Calvin J. Hamilton)


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