1955

Jupiter’s Magnetic Field

Just as in a standard wire-wound electric motor, rotating electrically conductive (metallic) interiors and/or cores of planets and satellites are prime locations for the generation of magnetic fields. The magnetic fields of Earth and Mercury are thought to be generated by electrical currents within their spinning, partially molten, iron-rich cores. High electrical conductivity in a metallic core deep within Ganymede may explain that moon’s magnetic field as well.

Our solar system’s gas- and ice-giant planets have been found to have even stronger magnetic fields. The first such discovery was in 1955, when radio astronomers from the Carnegie Institution in Washington, D.C., noticed strong radio frequency emissions coming from Jupiter. Starting with Karl Jansky’s pioneering discovery of the strong radio source at the center of the Milky Way galaxy in 1931, radio astronomers had been scanning the skies looking for other natural sources of extraterrestrial radio waves. The Crab Nebula had been identified as another strong radio source. In fact, Jupiter’s radio emission, interpreted to come from a strong magnetic field, was accidentally discovered in radio telescope observations intended to study the Crab Nebula.

Significantly more details about Jupiter’s field came from flying spacecraft through it, first with the Pioneers in the 1970s, then the Voyagers in the 1980s, and then the Galileo orbiter and Cassini Jupiter flyby in the 1990s and early 2000s. Sensitive magnetometers on these missions were able to discover that Jupiter’s magnetic field is about 10 times stronger than Earth’s, and generates about 100 terawatts of power, or more than a million times Earth’s magnetic field radio power. The field is generated by electrical currents flowing through metallic hydrogen in Jupiter’s outer core. It interacts with the solar wind (as does Earth’s field), but also interacts with the moons—especially Io, where volcanic eruptions inject sulfur dioxide into a doughnut-shaped “plasma torus” that encircles Io and is ionized by Jupiter’s magnetic field.

And it is enormous: the volume of space encompassed by Jupiter’s magnetic field—its magnetosphere—is the largest continuous structure in the solar system (not counting the Sun’s own magnetic envelope). If we could see Jupiter’s magnetosphere with our eyes, it would be five times larger than the full Moon!

A schematic cartoon illustration of Jupiter’s magnetic field. Strong magnetic fields generated in Jupiter’s deep interior extend outward and interact with the satellites and rings. The field is connected to Io and its plasma torus, for example, via a stream of high-energy particles called a flux tube.