The Moon Io: A Seething
Interior and Active Surface

The adjacent figure (Ref) shows the best Galileo Orbiter image yet of Io's full disk. How did this innermost of the Galilean satellites come to look like a pepperoni pizza? 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.

As we shall see, 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, in an image of stars used to check the positioning of the spacecraft. 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 (Ref) Volcano erupting on Io as observed by Voyager (Ref) 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

Indeed, we now know that Io has the youngest surface in the Solar System. Present estimates are that it is so active geologically that its surface is completely redone on a timescale of about a million years---a twinkling of the eye on geological timescales. This makes Io the most active object geologically in the Solar System. In addition to the intense volcanic activity, Io's surface is being continuously eroded by a bombardment of charged particles trapped in Jupiter's magnetic field.

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. 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 as captured in the adjacent animation (Ref)) of a sequence of Hubble Space Telescope images.

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. Here is a movie simulating the Io Plasma 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.

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