13-5 Jupiter’s magnetic field makes electric currents flow through Io

Most of the material ejected from Io’s volcanoes falls back onto the satellite’s surface. But some material goes on a remarkable journey into space, by virtue of Io’s location deep within Jupiter’s magnetosphere.

The Io Torus

The Jovian magnetosphere contains charged particles, or ions, some of which collide with Io and its volcanic plumes. The impact of these collisions knocks more ions out of the plumes and off the surface, and these additional charged particles become part of Jupiter’s magnetosphere. Indeed, material from Io is the main source of material in Jupiter’s magnetosphere as a whole. Some of the ejected material forms the Io torus, a huge doughnut-shaped ring of ionized gas that circles Jupiter at the distance of Io’s orbit. A gas that is hot enough for electrons to dissociate from atoms forms a gas of charged particles called a plasma. The plasma’s glow can be detected from Earth (Figure 13-9a).

Figure 13-9: RIVUXG
The Io Torus (a) An Earth-based telescope recorded this false-color infrared image of Io’s plasma torus of charged particles. Because of Jupiter’s glare, only the outer edge of the torus could be photographed; an artist added the visible-light picture of Jupiter and the yellow line indicating the rest of the torus. (b) A tube of electric current carries a charged-particle plasma between Jupiter and Io, forming an immense electric circuit.

(a: Courtesy of J. Trauger; b: Alfred T. Kamajian and Torrence V. Johnson, “The Galileo Mission to Jupiter and Its Moons,” Scientific American, February 2000, p. 44)

Jupiter’s magnetic field has other remarkable effects on Io. As Jupiter rotates, its magnetic field rotates with it. This field sweeps over Io at high speed and generates a voltage of 400,000 volts across the satellite, causing 5 million amperes of electric current to flow through Io.

In fact, a current flows not only through Io but also through the sea of charged particles in Jupiter’s magnetosphere and through Jupiter’s atmosphere. This forms a gigantic, oval-shaped electric circuit that connects Io and Jupiter in the same way that wires connect the battery and the lightbulb in a flashlight (Figure 13-9b). Jupiter’s aurora, which we discussed in Section 12-7, is particularly strong at the locations where this current strikes the upper atmosphere of Jupiter.

This important phenomenon occurs on all the Galilean moons, leading to profound insights and discoveries, so let us summarize what we have learned for further use in the chapter. As Jupiter’s magnetic field sweeps over an electrically conductive medium (in this case, a plasma concentrated around Io), there is an induced flow of electricity in the conductive medium called an induced electric current. In addition to inducing the flow of electricity, all electricity produces its own magnetic field, which, in this scenario, is called an induced magnetic field.

Probing Io’s Interior

The electric current within and near Io also creates a weak magnetic field, which was first detected by Voyager 1. But what about an intrinsic magnetic field generated by Io’s interior? Galileo data determined that Io does not generate a magnetic field. This means that Io does not contain a convecting iron core.

If Io has undergone enough tidal heating to melt its interior, chemical differentiation must have taken place and the satellite should have a dense core. (We described chemical differentiation in Box 7-1 and Section 8-5.) To test for a dense core, scientists measured how Io’s gravity deflected the trajectory of Galileo as the spacecraft flew past. From these measurements, they could determine not only Io’s mass but also the satellite’s oblateness (how much it deviates from a spherical shape because of its rotation). The amount of oblateness indicates the size of the core; the greater the fraction of the satellite’s mass contained in its core, the less oblate the satellite will be. The Galileo observations suggest that Io has a dense core composed of iron and iron sulfide (FeS), with a radius of about 900 km (about half the satellite’s overall radius). Surrounding the core is a mantle of partially molten rock, on top of which is Io’s visible crust.