This diagram shows the magnetosphere of Jupiter.
Click on image for full size
Windows to the Universe original artwork

Detecting Planetary Magnetism

A magnetometer is an instrument for measuring magnetic fields. Many spacecraft carry magnetometers to measure the magnetic fields around planets they orbit or fly by. When a spacecraft takes such measurements, what do they tell us about a planet?

The planet might have a global magnetic field surrounding and permeating it. Earth does, which is why compasses work. So do Mercury, Jupiter, Saturn, Uranus, and Neptune. Electrical currents in Earth's core generate its magnetic field. The core consists of iron and nickel, which are good conductors of electricity. Similarly, Mercury has an iron core which produces its field. Areas surrounding the solid cores of Jupiter and Saturn consist of liquid metal hydrogen, which exists only at the intense pressures and temperatures found deep inside massive gas giant planets. Flow of electricity in the liquid metal hydrogen produces the powerful magnetic fields Jupiter and Saturn. The magnetic fields of Uranus and Neptune are generated by current flow in slushy brine deep within, but not at the core of, those frozen gas giants.

The planet might have localized magnetic fields on, through, and around limited areas. Perhaps the planet had a global magnetic field in the past which has now gone extinct as the planet aged and cooled. Remnants of the former field can stay "locked into" rocks formed during those earlier times. Mars and Earth's Moon seem to have such localized magnetism. Large deposits of ores such as iron could influence a magnetometer's readings near the deposits. Such deposits could even be the remains of an iron-rich asteroid that struck the planet, embedding its magnetic field into the planet along with its material!

The spacecraft also might detect magnetism associated with, one way or another, the solar wind or the Sun's magnetic field. The solar wind, the flow of electrically charged particles emitted by the Sun, carries the Sun's magnetic field outward through the Solar System. The spacecraft's magnetometer might directly detect the Sun's magnetic field, even when the spacecraft is near a planet, because the Sun's field is so strong. Charged particles from the solar wind can also become trapped within a planet's magnetosphere, if the planet has one, and the movement of the particles within the magnetosphere can generate magnetic fields detectable by a spacecraft. Also, a planet might emit charged particles from its surface (Mercury does this) or atmosphere (as does Venus). The interaction of the solar wind with such particles is another possible source of magnetic fields that the spacecraft might detect.

Finally, the magnetic field detected by a spacecraft might be a combination of fields produced by several of these effects. For example, the magnetic field near Earth is a result of the combination of Earth's global field, "fossil" magnetism embedded in rocks from eras when Earth's field was different than it is today, charged particles racing around Earth's magnetosphere, and other factors.

Last modified May 30, 2003 by Randy Russell.

You might also be interested in:

Traveling Nitrogen Classroom Activity Kit

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more

The Magnetic Field

The force of magnetism causes material to point along the direction the magnetic force points. This property implies that the force of magnetism has a direction. As shown in the diagram to the left, the...more

Planetary Magnets

The Earth is a good example of a planetary dipole, where the lines of force point in a direction out of the South (magnetic) Pole and into the North (magnetic) Pole. Planets can also show evidence of quadrupoles...more

The Earth's Magnetosphere

The Earth has a magnetic field with north and south poles. The magnetic field of the Earth is enclosed in a region surrounding the Earth called the magnetosphere. As the Earth rotates, its hot core generates...more

An Overview of Jupiter's Magnetosphere

Jupiter's magnetosphere is a unique object in the solar system. It is the biggest object in the entire solar system. Not only is it big enough to contain all of Jupiter's moons, but the sun itself could...more

An Overview of Saturn's Magnetosphere

Saturn's magnetosphere is not as big as Jupiter's, but is very large nonetheless. It extends well beyond the orbits of Saturn's moons. It is probably generated in the same manner as is Jupiter's, which...more

An Overview of Uranus' Magnetosphere

The magnetosphere of Uranus is medium sized, but still much larger than the Earth's. It extends well beyond the orbits of Uranus' moons. It is probably generated in the icy interior layers, rather than...more

Magnetosphere of Mercury

Mercury is the only terrestrial planet other than the Earth that has a significant magnetic field (220 nT). This field, along with the planet's high density and small size relative to the Earth, indicates...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA