Planetary Magnetic Fields

Type of physical science: Astronomy; Astrophysics

Field of study: Planetary systems

Magnetic fields are ultimately produced by moving electric charges such as electrons. Since planets have magnetic fields, astronomers must explain how those planetary magnetic fields are produced, which involves the description of processes taking place deep inside a planet.

Overview

With the development of spacecraft, such as the Mariners, Vikings, Pioneers, and Voyagers, that are capable of traveling to all the planets but Pluto, the strength and orientation of the magnetic fields possessed by those planets have become known. Scientists have determined three basic categories: planets such as Venus, with low-level fields; planets such as Earth, with intermediate values; and high-strength fields for Jupiter and Saturn. The orientation of each planetary field is also unique.

When an electrically charged particle moves, a magnetic field is created that forms concentric circles around the path of the charge. An electrical current can be passed through a wire wound around a cylinder, thereby concentrating the magnetic field within the cylinder. The field spreads out at one end of the cylinder, arcs around the cylinder, and enters the other end. A dipolar magnetic field is produced, which surrounds the coil of wire. The field is called dipolar because it has two poles (the north and the south poles). The north end will attract the south end of another magnet such as a compass needle, but it will repel the north pole of another magnet.

The north end of a compass is attracted to Earth's magnetic pole that is in the Northern Hemisphere which is the south pole of Earth's magnet. It is located in the islands north of Canada. In other words, the field is tipped about 15 degrees from the north geographic pole of Earth, which is located at the planet's spin axis.

Before the exploration of Earth's magnetic field in the late 1950's, scientists thought that it was a simple dipole field. The satellites proved them wrong; the field interacts with electrically charged particles from the sun--the solar wind--and these particles distort the shape of the field. Moving charged particles produce a magnetic field. It is the interaction of these two fields that causes the distortion. The magnetic field is pushed in on the side pointing toward the sun and is stretched out into a long tail on the side away from the sun. This field is then known as the magnetosphere. Other planets, such as Jupiter, have these complex magnetic fields. Satellites are used to plot the limits of the magnetospheres of the various planets. This information enables scientists to determine the influence of the solar wind on the objects of the solar system. For example, as comets approach the sun, they develop one or two tails. One of these is caused by the solar wind pushing atoms away from the comet's nucleus.

The interaction of the electrically charged particles from the sun and Earth's magnetic field produces the northern lights, or Aurora Borealis, that are seen near the magnetic pole in the Northern Hemisphere. The southern lights, or Aurora Australis, are seen near the magnetic pole on the Southern Hemisphere. As the particles move, they spiral along the field lines, which bend toward Earth's surface near the poles. The particles interact with the atoms in the upper layers of Earth's atmosphere, which causes the atoms to radiate light that is seen as the northern lights. If the sun is very active in emitting particles, more particles strike the atmosphere, and the lights are seen farther south than normal. In the United States, they have been seen as far south as northern Florida. These particles also interfere with the transmission of radio signals by disturbing the layers of ionized atoms that reflect radio waves over long distances. Some of the solar wind particles, particularly electrons and protons, become temporarily trapped in Earth's magnetic field by spiraling along the field lines. These particles form the Van Allen radiation belts that surround Earth.

Field intensity varies dramatically from planet to planet. Magnetic fields are produced by moving electrical charges. Magnetic fields are also found around permanently magnetized materials such as iron. These fields are produced by the movement of electrons in the atoms that make up the magnetized material. Thus, moving electrically charged particles produce all magnetic fields. Earth's magnetic field actually has two sources: interior and exterior. Electrical currents flowing through the upper atmosphere produce a small amount but the majority comes from a source in the interior. Earth could have an electrical current flowing in its interior that produces the field, but the field moves over Earth's surface. A simple current could not account for that. Also, the current would decrease over time as the current heats Earth's interior, such as an electrical current passing through the wire in a toaster. Earth contains various permanently magnetized minerals, such as magnetite, an iron oxide that could provide the magnetic field.

Several problems arise, however. Earth's interior is too hot for a permanent magnet to stay magnetized. When a permanently magnetized material is heated above its Curie temperature, the material loses its permanent magnetism. Most of Earth's interior is above the Curie temperature of all permanently magnetized materials. In addition, Earth's magnetic poles, where the magnetic field enters the Earth perpendicular to the surface, are not stationary; they move slowly over the surface. A bar magnet in Earth's interior could not move in such a manner to explain the observed movement.

There is also evidence that the poles of the field change position. This condition is called reverse polarity. Scientists believe that Earth's magnetic field has flipped from normal to reverse and back again many times in the past 100 million years. The last change, from reverse to normal, occurred about 700,000 years ago. These facts rule out a permanent magnet as the source; however, a permanent magnet is the likely source of the magnetic field of the Moon, Mercury, and Mars, since their magnetic fields are not as strong as that of Earth. They are "dead" planets, whose interiors have largely cooled below the Curie temperature of permanent magnets.

If a conductor moves through a magnetic field, an electric current is produced. That current, which is a moving electrical charge, produces a magnetic field. This magnetic field can reinforce the original magnetic field. This process, known as the dynamo effect, continues until the field reaches a steady state condition and becomes stable. The dynamo effect is used to produce electricity in power plants and is the source of the intermediate and high-strength fields of the planets. Scientists study the dynamo effect in the Earth trying to determine how it produces the magnetic field and the location of the moving conductor. Earthquake studies indicate that Earth's interior is composed of several layers. A thin--70 to 100 kilometers--layer called the crust is composed of rocky material. Under the crust is a layer called the mantle--2,900 kilometers thick--which is composed of rock. Below the mantle, the outer core is 2,300 kilometers thick. Finally, the inner core is 1,200 kilometers thick, giving Earth a radius of approximately 6,400 kilometers.

Earth has an average density of 5.5 times that of water. The rock of the crust and mantle average between 3 and 3.5. Therefore, the core must be denser than 5.5. Iron is the element that is most common and is the right density. It is probably mixed with nickel and small amounts of other elements. Based on earthquake studies, it has been determined that the outer core is liquid, which means that it can move. Earth's interior is hot, which causes convection currents--such as those in boiling water--to circulate in the outer core. The metal of the outer core, a conductor, moves as a result of convection, through a magnetic field. There is always a stray magnetic field that exists, such as the sun's. A current is produced in iron, which produces a magnetic field that reinforces the original field. This increasing of the field continues until the steady state is achieved. A field produced in this manner can shift position as Earth's magnetic field shifts. It is not limited by the high temperature in Earth's interior; in fact, it needs heat as a source of energy to power the dynamo. The dynamo action is also capable of changing polarity.

The dynamo effect is the best theory at this time for explaining Earth's magnetic field.

Venus and Earth are about the same size, which would lead a scientist to predict that they should have similar magnetic fields. Yet, Venus has a much lower strength field--1/10,000--than the Earth. Scientists have reasoned that Venus' slower rotation--once in 243 days--produces much less convection of its metallic core, which inhibits the dynamo action.

Another possibility is that the core is not molten like Earth's. Magnetized minerals cannot be the source of its field because the surface is about 750 Kelvins, which is hot enough to melt lead.

Jupiter and Saturn have a strong field. These planets are larger than Earth, and they have much lower densities. Scientists conclude that Jupiter and Saturn are composed mostly of lighter elements such as hydrogen and helium, with small rocky and metallic cores. The core is too small to be the source of the field. The gases are under greater and greater pressure as one moves deeper into the planet. This pressure liquefies the gases, which results in deep oceans of liquid hydrogen. Under enough pressure, hydrogen becomes liquid, metallic hydrogen.

Calculations show that both planets should have thick layers of this material. Scientists believe that this material is the field's source: The metallic hydrogen is the conductor, and the planets' internal heat produces the necessary motion for the dynamo effect.

Uranus and Neptune have lower intensity fields. They are similar to Jupiter and Saturn with low density and higher light element compositions. They are smaller, however, and this produces lower pressures in the interior. As a result, they do not have layers of metallic hydrogen, and therefore no dynamo effect, and no high-intensity field. Their fields are probably caused by the permanent magnetism of their small metallic cores.

Applications

The solar system has existed for 4.6 billion years. Scientists have attempted to reconstruct the history of the solar system by looking for events that were recorded in the materials that compose the objects of the solar system. Not all the events were recorded in the planets' rocks, or preserved for analysis, but most of the major ones were preserved. Scientists have developed techniques for extracting data from rocks, and they have interpreted that data to reveal information about each planet's history.

Earth has a strange characteristic: Some of its edges at the continental shelf boundary fit together like a jigsaw puzzle. The eastern edge of the South American continent fits very well into the indentation of the west coast of Africa. Eastern North America matches the bulge of northwestern Africa. This fit indicates that these continents were once closer together. Scientists in the early twentieth century gathered a large body of evidence indicating that all the continents formed a supercontinent hundreds of millions years ago, and have moved to their present positions. The scientific community did not accept this theory of continental drift because they could not see how a body that large could move; there was no mechanism to make the continents move. An unexpected area of science provided the evidence that the continents had moved. In the mid-1900's, geophysicists began to study the magnetic properties of rocks. They found that some rocks were reliable recorders of Earth's magnetic field in a past time. When the rock formed, the magnetic particles in the rock faithfully recorded the magnetic field present at that time. The magnetic pole that was recorded seemed to wander off the face of Earth as rocks of different ages were sampled. The pole may have actually moved relative to the continent or the continent may have moved relative to a stationary pole. By checking the pole movement of rocks from other continents, the scientists discovered that the continents moved. They had moved in a manner that indicated that North America was close to northern Africa 200 million years ago and that southwestern Africa was near South America.

The continents ride the plates that compose the outer layer of Earth's surface. These plates move and carry the continents with them. The theory of plate tectonics is the unifying theory of the geological sciences and can explain much about the origin of Earth. Earthquakes are caused by the movement of one plate boundary past another; volcanoes occur when molten rock from a plate diving under another plate rises and flows onto the surface.

The magnetic field of a planet provides insight into its interior. It is known already that it is too hot in Earth's interior for its field to be caused by a permanent magnet. A molten metallic core generates the magnetic field by a dynamo effect. The Moon, Mercury, and Mars have a low-level field because they are "dead" worlds, with little internal activity that could produce a magnetic field. They have only the remains of a field stored in their magnetized rock.

Jupiter and Saturn have a relatively small metallic core, which could not provide the planets' high-level field. The key lies in the high pressures of the interior that compress liquid hydrogen into its metallic form; this material provides the necessary dynamo action that leads to a large magnetic field.

As one looks at the magnetic fields of the planets, one can see that there is a variety of orientations with respect to the spin axis of the planet. The Moon, Venus, Mercury, and Mars are of no concern, since their fields are weak. Earth's field is tipped 15 degrees from the geographic North Pole. The recording of the field position of several million years leads one to conclude that, on the average, the pole is located at the geographic pole. The difference between the field and the spin axis is believed to be caused by the change in the outer core's rotation. The fields of Jupiter and Saturn are not tipped very much, which is not surprising since little will change the rotation of these large planets. Uranus and Neptune have fields about the same strength as Earth's, but they are tipped at large angles. Uranus is an unusual planet; its axis of rotation is tilted by 98 degrees. It appears to be "rolling" around the sun on its side. The field is tipped 60 degrees to the spin axis. The possibility exists that Uranus was hit by a massive object billions of years ago, which knocked it on its side; however, the core where the field originates was less affected by the collision. Neptune's field is inclined by 50 degrees. The difference may be caused by a rare event that may have happened billions of years ago: Pluto was once Neptune's satellite.

This possibility would explain the strange orbits of Neptune's two larger moons--Triton and Nereid--and the odd orbital behavior of Pluto. Pluto has the most elliptical orbit of the planets; from 1979 to 1999, Pluto will be closer to the sun than Neptune.

Context

The beginning of humanity's knowledge of magnetism is unknown since nothing special is required to observe it; chunks of magnetite occur naturally. If a chunk is picked up; one can see smaller fragments clinging to it. The Greeks knew of lodestone (magnetite) by 600 B.C.

The use of small magnets as compasses dates to the first century B.C. in China. The Chinese used a piece of lodestone cut in the shape of a spoon that could rotate under the influence of Earth's magnetic field.

William Gilbert (1544-1603) wrote DE MAGNETE (1600), in which he concluded that Earth acts as a magnetized sphere of lodestone. With the development of batteries in the early 1800's, scientists could control electrical current. Up to this time, scientists thought that magnetism and electricity were two separate entities. In 1820, Hans Christian Orsted (1777-1851) discovered that the flow of electrons produces a magnetic field. Other scientists discovered that a moving conductor generates an electrical current. These developments led to the invention of electrical generators and motors. In the late nineteenth century, James Clerk Maxwell (1831-1879) discovered that electricity and magnetism are interrelated and are responsible for all the electromagnetic spectrum from radio waves to γ rays.

Scientists began to study the magnetism of rocks in the early 1900's. At first, they could study only the stronger rocks such as the igneous rocks called basalts. As more sensitive magnetometers, which measure magnetic field strength, were designed, rocks with weaker fields were studied. This development led to the discovery in the early 1960's that rocks recorded Earth's ancient magnetic field and that the continents had moved. The theory of plate tectonics was developed. One area that geophysicists are studying is the strange terrain that is found along the west coast of North America. It is composed of hundreds of sections of continent that were moved together. Geophysicists hope to discover how these areas were formed and where the original sections of this continent came from.

In the late 1950's, the United States sent satellites into space to record the magnetic field, radiation levels, and other characteristics near Earth. On an Explorer satellite, the radiation counter rose to a peak level, dropped to zero, and went back to a peak as it traveled away from Earth. James A. Van Allen interpreted this reading as an indication of high-level radiation belts surrounding Earth, which were named the Van Allen radiation belts.

Scientists continue to send spacecraft to the planets and the Moon to discover their magnetic characteristics. Orbiter probes will be sent to other planets to engage in the long-term investigation of the planetary magnetic fields. Thus, scientists will refine their knowledge of the sources of magnetic fields and the interiors of the planets.

Principal terms

DIPOLE FIELD: the field configuration caused by two electrically charged particles or two magnetic poles

MAGNETIC FIELD: the physical phenomenon produced by a moving electrical charge

MAGNETIC POLE: in a bar magnet, one of its magnetized ends; all magnets have at least two poles--north and south

MAGNETOSPHERE: the complex magnetic field associated with a planet

VAN ALLEN RADIATION BELTS: belts of electrons and protons trapped within Earth's magnetic field

Bibliography

Berry, Richard. "Neptune Revealed." ASTRONOMY 17 (December, 1989): 22-34. This is a follow-up article of the Voyager flyby of Neptune (cited below). Contains many photographs of Neptune, its moons, and its ring. For the general reader.

Berry, Richard. "Triumph at Neptune." ASTRONOMY 17 (November, 1989): 20-28. Written shortly after the Voyager flyby of Neptune, this article presents information known about Neptune at that time. Contains many photographs of Neptune, its moons, and its ring.

Flower, Phillip. UNDERSTANDING THE UNIVERSE. St. Paul, Minn.: West, 1990. This astronomy textbook is a very good reference for understanding the various characteristics of the planets and the stars. Contains many color diagrams and tables. Discusses the information from the Voyager flyby of Neptune, such as its magnetic field.

Hamblin, W. Kenneth, and Eric Christiansen. EXPLORING THE PLANETS. New York: Macmillan, 1990. Covers the geology of the various objects of the solar system; the last chapter provides a comparison of the planetary geology. Contains line drawings and black-and-white photographs. The chapters include review questions. Contains a glossary.

Moore, Patrick. ATLAS OF THE SOLAR SYSTEM. New York: Crown, 1990. An authoritative text with many diagrams, figures, photographs, and tables. For the interested reader.

Van Allen radiation belts

Earth's magnetic field has reversed during its history

Venus' interaction with the solar wind

Earth's magnetosphere

Charges and Currents

Conductors and Resistors

The Measurement of Magnetic Fields

Motors and Generators

Magnetic Properties of Solids