The giant planet Jupiter has four large moons, discovered by Galileo and visible through good binoculars. It has in addition many smaller ones, as well as a narrow ring like Saturn's, observed by the spacecraft Pioneer 11.

    Of the large moons--comparable to our own moon or bigger--the outer three are icy spheres, but the innermost one, Io, is heated by tides, and as a result has volcanoes and an ionosphere which is a fair conductor of electricity. Jupiter itself like Earth is a magnet, but one that is 20,000 times stronger; as a result it has a large magnetosphere and a very intense radiation belt.

    A dynamo is created in a magnetic field by an electric circuit, part of which is moving relative to the rest (additional conditions must also be met). The circuit may consist entirely of fluids (as in sunspots), but solid conductors can also be involved.

    The conditions for a dynamo are fulfilled in the case of Io and and Jupiter. Both are conductors, and they move quite differently--Io orbits, Jupiter rotates. Furthermore, the plasma between them conducts electricity very well along its magnetic field lines, which act as if they were wires connecting Io and the planet (drawing). One expects a continuous current to flow in this circuit, feeding on Io's orbital energy.

    The drawing is not to scale. Actually Io is much smaller. Watchful readers may notice that the north-south magnetic polarity of Jupiter is the reverse of what it is for Earth. They might also note that if the drawing views Jupiter from the Sun's side, Io actually orbits in a direction opposite to that of the arrow. However, the plasma which fills space around it rotates with Jupiter and moves much faster, overtaking Io. Relative to the plasma, therefore, Io moves backwards.

    The path of the space probe Voyager 1 was designed to check out this dynamo, by flying close to where its currents were expected to flow. It did so on March 5, 1979, and its magnetometer very clearly detected the signature of a current of about a million amperes. Previous to that it was noted that unlike any other moon of Jupiter, Io had a strong influence on radio emissions from Jupiter's magnetosphere, which depended on its position: it could be that the moon's unique electric currents were involved in this.

Postscripts

See here for a picture of Io "in its true colors" and links to sites about it.