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Nothing endures but change.
- Heraclitus
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.
| Animations of Io |
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| Views of Io |
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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) 
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)
Copyright © 1997-2001 by Calvin J. Hamilton. All rights reserved. Privacy Statement.
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