Orbit: 422,000 km from Jupiter
Diameter: 3630 km
Mass: 8.93e22 kg
Discovered by: Galileo and Marius in 1610.
Io ( "EYE oh" ) is the fifth of Jupiter's known satellites and the third largest,Io is slightly larger than Earth's Moon.
How did this innermost of the Galilean satellites come to look like a pepperoni pizza(above)? The answer is that Io is the most active object geologically in the Solar System.
It is covered with volcanic activity that spews various sulfur compounds onto the surface and accounts for the color and pocked nature of the surface.
This volcanic activity does not originate in plate tectonic activity the way that much volcanic activity originates on the Earth.
For Io, we shall see that instead it is tidal forces associated with the 500 pound gorilla next door (Jupiter) that are the culprit.
The Volcanoes of Io
In 1979 an engineer responsible for navigation of the Voyager I spacecraft noticed a strange mushroom-like object on the limb of Io. After ruling out trivial explanations, it was realized that the camera had captured a volcano erupting on the surface of Io, the first live volcano found in the Solar System beyond the Earth. Subsequent spacecraft (Voyager II and Galileo) have documented extensive vulcanism on the surface of Io, as illustrated in the following three images.
Volcano on Io's limb discovered by Voyager
Volcano erupting on Io as observed by Voyager
Galileo image of volcano erupting on Io.
The rightmost image above is of the large shield volcano Ra Patera, as recently photographed by the Galileo spacecraft. Some volcanoes seen by Voyager are no longer erupting, but there are new volcanoes that have begun erupting in the 17 years between Voyager and Galileo.
The image adjacent left shows the region near the volcano Pele, as photographed by Galileo in 1996. The large red ring around Pele is thought to indicate recent volcanic activity. The red color indicates the presence of Sulfur, although how the Sulfur was produced is not precisely known.
The Surface and Interior of Io
Io's surface is radically different from any other body in the solar system. It came as a very big surprise to the Voyager scientists on the first encounter.
They had expected to see impact craters like those on the other terrestrial bodies and from their number per unit area to estimate the age of Io's surface. But there are very few, if any, impact craters on Io (left). Therefore, the surface is very young.
Instead of craters, Voyager 1 found hundreds of volcanic calderas. Some of the volcanoes are active! Striking photos of actual eruptions with plumes 300 km high were sent back by both Voyagers (right) and by Galileo (bottom left image on this page) This may have been the most important single discovery of the Voyager missions; it was the first real proof that the interiors of other "terrestrial" bodies are actually hot and active.
The material erupting from Io's vents appears to be some form of sulfur or sulfur dioxide. The volcanic eruptions change rapidly. In just four months between the arrivals of Voyager 1 and Voyager 2 some of them stopped and others started up.
The deposits surrounding the vents also changed visibly.
The source of this geological activity is the tidal effect of Jupiter on Io as it orbits the giant planet. These tidal forces are so large that Io's surface is pulled upwards and downwards by hundreds of meters in each rotation. Just like bending a paper clip heats it, these tidal distortions keep much of the interior of Io in a molten state (perhaps mixed with iron sulfide)with a radius of at least 900 km.
Thus, Io is a thin crust (primarily of sulfur) sitting on a molten interior. It is this molten interior breaking through to the surface that produces the volcanoes of Io, and the spectacular surface appearance.
In contrast to most of the moons in the outer solar system, Io and Europa may be somewhat similar in bulk composition to the terrestrial planets.
Recent images taken with NASA's Infrared Telescope Facility on Mauna Kea, Hawaii show a new and very large eruption (right).
A large new feature near Ra Patera has also been seen by HST. Images from Galileo also show many changes from the time of Voyager's encounter. These observations confirm that Io's surface is very active indeed.
Some of the hottest spots on Io may reach temperatures as high as 2000 K though the average is much lower, about 130 K. These hot spots are the principal mechanism by which Io loses its heat.
The Io Torus and the Io Flux Tube
As we have already seen Io influences and is strongly influenced by Jupiter's magnetic field. The moon orbits within the field, and material ejected from its surface by volcanoes (and by the charged particle bombardment from the material already trapped in Jupiter's magnetic field) is a primary source of the charged particles in the Jovian magnetic field.
These particles (primarily sulfur and oxygen) have a high concentration in a doughnut shaped region surrounding Io's orbit called the Io Torus.
As Io moves around its orbit in the strong magnetic field of Jupiter and through this plasma torus, a huge electrical current is set up between Io and Jupiter in a cylinder of highly concentrated magnetic flux called the Io Flux Tube. The Flux Tube has a power output of about 2 trillion watts, comparable to the amount of all manmade power produced on Earth. It is responsible for bursts of radio frequency radiation long detected on Earth.
Io has a thin atmosphere composed of sulfur dioxide and perhaps some other gases.
Unlike the other Galilean satellites, Io has little or no water. This is probably because Jupiter was hot enough early in the evolution of the solar system to drive off the volatile elements in the vicinity of Io but not so hot to do so farther out.