A comparison of Planetary Magnetic Fields

Graeme Ing, 2004 Project 52, HET602A

1. Introduction

Until the mid 1950’s, magnetism on Earth was considered a useful but coincidental natural force. Little thought was given to the possibility of similar magnetic fields around the other planets in the solar system.

In 1955 Franklin and Burke, operating a radio telescope affiliated to the Carnegie Institution, happened across a distinct and very strong radio signal in the sky. This radio object moved, unlike the stellar objects they were used to observing, and they identified the source as Jupiter. They speculated that the strong radio signal was due to disturbances within the atmosphere of Jupiter. (WebStern) Without understanding the true nature of their discovery, they had obtained the first evidence of another planetary magnetic field.

When the era of artificial satellites dawned in 1957 and 1958, the early Sputniks and Explorers discovered that the Earth’s magnetic field extended into space, surrounding the entire planet. After Explorer 1 discovered the pockets of intense radiation known as the Van Allen Belts, (WebExplorer) Drake went back to analyze the radio activity of Jupiter, speculating that it was due to electrons trapped in an intense magnetic field, thousands of times stronger than the Earth’s. (WebStern)

Since then, spacecraft have visited every planet except Pluto and we have discovered each to have (or had in the past) a magnetic field of some description. In this paper, we will first investigate the process by which these planet-wide magnetic fields are generated – the self-exciting dynamo. Armed with this knowledge, we will visit each planet in turn and discuss the properties of its magnetic field. Finally, we shall offer a convenient set of criteria for grouping planetary magnetic fields into three categories, and offer a concise tabular summary.

2. The Planetary Dynamo

Planetary magnetic fields are not the result of a huge mass of magnetized material inside each planet. Instead, they are generated by the principle of electro-magnetism. Geophysicists refer to this process as a self-exciting dynamo. A dynamo is a generator that converts motion (kinetic energy) into an electrical current. (WebDictionary) Ampere’s Law states that the flow of an electrical current will generate a magnetic field. This can be demonstrated by wrapping a coil of wire around a block of non-magnetized metal and passing a current through the wire. The metal will become magnetized and capable of attracting other metals. Therefore, the basic principle of a planetary dynamo is to convert motion into an electrical current and thence into a magnetic field. The term “self-exciting” refers to the dynamo being self-sustaining.

We will use our Earth as a model to explain exactly how this process is believed to work. The Earth has a solid, dense core of nickel and iron. Surrounding this is an outer core of similar material but in a molten form, since it is under insufficient pressure to solidify it. Between the outer core and the crust is a huge mantle of semi-liquid, plastic silicate material. Powerful convection currents stir up the liquid outer core. These convection currents are caused by a) heating - the average temperature of the outer core is 4,500ºC (Freedman & Kaufmann 2001) – and b) angular momentum imparted by the Earth’s swift rotation. Since iron and nickel are good conductors, as the liquid moves about, electrons jump between colliding atoms to create a flow of electricity. These electrical currents are strong enough to induce a magnetic field that encompasses the Earth. (WebAstronomyNotes) As long as the planet rotates and the liquid outer core does not solidify, the dynamo and hence the magnetic field is self-sustaining. (See Figure 1)

Figure 1: The planetary dynamo

Thus the pre-requisites of a planetary dynamo are: Conductive material, a liquid medium susceptible to convective currents and either heating or rotation to create rapid currents.

The strength of a planetary magnetic field is calculated by multiplying the strength of the field at the surface equator by the cube of the planetary radius. This is known as the magnetic moment of the field. (WebRussel8) The accepted unit for magnetic moment is the nano-Tesla (nT), where a Tesla is the metric unit of magnetic flux density. (WebWiki) In physical terms, the field strength is a function of the amount and conductivity of the material within the planet’s interior and the rotational velocity of the planet. Using the inverse cube law, the magnetic strength decays as a factor of the cube of the planetary radius, as distance from the planet increases.

One feature of planetary magnetic fields is that the magnetic axis is rarely aligned with the rotational axis. In fact, Saturn is the only planet with coincident axes.

3. Composition of a Magnetosphere

The magnetic field surrounding a planet is termed the Magnetosphere. In this section, we will identify the more common components of the magnetosphere. (WebWindows) (See Figure 2)

Figure 2: Earth’s magnetosphere

The Bow Shock is that portion of the magnetic field upon which the supersonic solar wind impacts. As the plasma particles of the solar wind push against the magnetic field of the planet, they are slowed to subsonic speeds, compressing the magnetosphere on the sunward side, often by the distance of several planetary radii. The bow shock is why a magnetosphere is never uniform.

The magnetopause is the boundary between the edge of the magnetosphere and the Interplanetary Magnetic Field (IMF). The IMF at the magnetopause is the combined force of the solar wind flowing past the planet unheeded by the magnetosphere; and the solar wind deflected around the edge of the magnetosphere from the bow shock. The area between the bow shock and the magnetopause is known as the magnetosheath.

After flowing around the magnetopause, the deflected solar wind trails out behind the planet in a tapering cone known as the magnetotail. This tail often stretches for millions or tens of millions of kilometers (km). The bow shock and magnetotail are not aligned with the orbital motion of the planet, but face towards and away from the Sun respectfully, perpendicular to the orbit.

Sometimes there are areas within a magnetosphere where ionized particles become trapped, even if temporarily. These usually stretch out to become toroidal belts around the planet and are the source of intense radiation. Above Earth, these are known as the Van Allen Belts, discovered in 1958. (WebExplorer)

4. The Planetary Magnetic Fields of our Solar System

4.1 Mercury

Before Mariner 10’s arrival at Mercury in 1974, Mercury was believed too small to sustain a dynamo-driven magnetic field. At a diameter of 4880 km Mercury is not much larger than Earth’s Moon (3476 km), and because of its small size it was thought that its core was solid. Smaller bodies lose heat faster than larger ones. (WebAstronomyNotes) Mercury’s 59-day rotation was considered too slow to sustain a dynamo. (WebEarthSci1)

Mariner 10 discovered a weak field, approximately 1% the strength of Earth’s. It also gathered data that provided a greater insight into the internal makeup of Mercury. The core is larger than expected, perhaps 70% to 80% of the planet’s mass. (WebPSRD) In contrast, Earth’s core comprises approximately 15% of Earth’s mass. (WebUSGS) The radius of Mercury’s solid inner core is estimated at 1500 km, surrounded by a slim molten outer core 300 km in radius. (WebThomas) Mercury’s density is similar to Earth’s (5430 kg/m³), with the inner core likely composed of iron and nickel, and the molten outer core of iron with sulphur impurities. Mercury’s core is like Earth in miniature. The impurities moving within the molten ferric outer core fulfill the requirements of a planetary dynamo. It is probably only the large size of Mercury’s core that allows the weak dynamo to perpetuate, explaining the extremely weak magnetic field.

Mariner 10 made three flybys of Mercury in 1974 and 1975 and measured a well-defined but small magnetosphere, about 5% the volume of Earth’s. (WebBepi) Mariner measured field strengths from 100nT to 400nT with an average of 220 to 250nT, (WebRussel1) and recorded dramatic changes within the magnetosphere every few minutes. A very distinct bow shock prevents solar particles from reaching the surface. Due to the strength of the solar wind at Mercury, the bow shock is severely compressed and merely 3660 km from the planet. (WebThomas; WebRussel1) The tail was found to stretch about 24400 km.

If the molten outer core of Mercury is as thin as we suspect, then it is possible that it will solidify entirely within the next billion years as the planet continues to cool. If this happens then Mercury will be unable to sustain its dynamo and its magnetic field will fail.

4.2 Venus

As a “sister planet”, Venus has a very similar size, mass and volume to Earth. It is natural to expect it to have a similar magnetic field system, but it does not. There is much speculation about whether Venus has a part solid, part molten core, or an entirely solid core. It is also possible that it has a fully molten core, due to internal pressure and temperature effects. The internal pressure of Venus is less than Earth’s (perhaps because its mass is less), and probably insufficient to compress the iron-nickel material into a solid. (WebThomas) A uniformly molten core may not be conducive to the flow of convection currents. Furthermore, Venus’s rotation of 243 days is extremely slow and probably unable to impart enough angular velocity into the liquid core to create sufficient convection. Either or both of these factors would prevent a dynamo from starting. If it does have one, we have been unable to measure it to date.

During the 1960’s and 1970’s, Mariner and Venera probes (starting with Mariner 5 in 1967), detected some form of magnetic activity around the planet. A weak bow shock was discovered at the upper interface of the Venusian ionosphere. In the absence of a magnetosphere, the solar wind is able to impact the upper atmosphere. There, it ionizes the upper layers of the ionosphere to create a conductive layer, through which the solar particles flow around the planet and stream out behind the planet. (WebRussel2) This interaction between the solar wind and atmosphere is known as an “induced magnetosphere”. Venus’s magnetic field is about 25,000 times weaker than Earth’s. (WebEarthSci2) Beginning in 1978 the Pioneer Venus mission made field measurements over several years and measured the average field strength at 5nT, with tiny fluctuations as high as 60nT. These fluctuations varied with the solar cycle, being larger and stronger during solar maximum. (WebFortes) In 1974, Mariner 10 was the first spacecraft to fly through Venus’s tail. (WebMariner10) It determined that the tail was not a true magnetotail, but instead a stream of ionized gas stripped from the ionosphere by the solar wind in much the same way a comet sheds material as it nears the sun. (See Figure 3)

Figure 3: Interaction of the solar wind with the ionosphere

If Venus’ core is entirely molten, it is likely that in a few hundred million years it will cool enough to become solid in the center. It may then develop a self-sustaining dynamo to equal Earth’s; unless the planet’s slow rotation remains a gating factor.

4.3 Earth

Earth is of course the model against which we make our comparisons. Earth’s inner core is 1300 km in radius and composed of solid iron and nickel. An outer core of radius 2200 km, composed of liquid iron and nickel, surrounds it. The total core volume is approximately 15% of the Earth. (WebUSGS) Since it is our model, it is not surprising that Earth meets all the criteria for a healthy planetary dynamo: Earth rotates faster than the other terrestrial planets, driving fast convective activity within a molten outer core comprised of good conducting material.

Earth’s magnetosphere was discovered in 1957-1958, and since then hundreds of spacecraft and artificial satellites have studied Earth’s magnetic field. (WebSevem) Since the pressure of the solar wind at Earth is substantially less than at Mercury, Earth’s bow shock lies approximately 100,000 km from Earth’s centre. (WebRussel3) In contrast, the magnetotail stretches for over 6 million km downstream. The magnetic axis is tilted 11.3º from the rotational axis.

4.4 Mars

When Mariner 4 made a Mars flyby in 1965, it failed to detect the Earth-like magnetic field that scientists were expecting. With a radius of 3400 km, Mars is smaller than the Earth and much less dense at 3934 kg/m³, but is otherwise believed to have followed a similar geological development to Earth and Venus. Mariner 4 did discover a small bow shock about 4600 km from Mars’ centre. Several Soviet spacecraft analyzed the magnetic field at Mars, notably the MARS series in the early 1970’s. From this data, scientists speculated that Mars had an induced magnetosphere, similar to Venus. In 1975-1976, the Viking Landers and orbiters confirmed this with measurements of the Martian ionosphere that evidenced ionization by the solar wind, similar to that found at Venus. (WebRussel4)

Theories for the lack of a Martian dynamo argue for either: a) a fully molten core or, b) a fully solid one. The two diametric arguments are as follows:

a) Mars may have more sulphur in its core than either Venus or Earth, since sulphur is a more volatile element that exists in greater quantities further from the Sun. If so, the sulphur would greatly lower the melting point of the iron-nickel core, preventing solidification of the core. (WebThomas) The validity of this theory can only be proven by substantial seismic interrogation of Mars to determine the makeup of its core. In this scenario, Mars lacks a dynamo for the same reason as Venus, even though Mars has a fast rotation period like Earth.

b) Alternatively, the Martian core may have solidified billions of years ago. A smaller object loses heat faster than a large one because it has a higher ratio of surface area to volume. Mars may have radiated most of its heat, leaving its core totally solid. (WebAstronomyNotes) Whilst it is clear that Mars has had a volcanically active past, there is little evidence for geologically recent volcanism. The last major volcanic activity was probably about a billion years ago. (WebVolcano) In this, the favoured scenario, Mars lacks a liquid interior in which convection currents can develop.

In 1997, Mars Global Surveyor (MGS) entered Mars orbit. Carrying a sensitive Magnetometer/Electron Reflectometer (MAGER) scientific package, part of MGS’ mission was to look for a Martian magnetic field, and it found one. It measured 1/8000^th the strength of Earth’s field with a similar polarity. (WebPlanetary) Initial speculation was that Mars had a very weak dynamo, but recent evidence suggests that MGS detected the remnants of a long-dead dynamo. The Martian crust is irregularly magnetized, with some rocks holding a greater magnetic field than others do. Brain, at the University of Colorado suggests that Mars once had a powerful dynamo that failed when the Martian core solidified, and the rocks have retained a residual magnetism. (WebSpaceflightNow1) The northern hemisphere has retained a lower intensity field than the southern, which is consistent with the northern being younger, and more recently resurfaced by geological activity.

4.5 Jupiter

Jupiter sustains the largest magnetic field of all the planets. It stretches laterally for approximately 14 million km with a bow shock a long way from the planet, at 8 million km. The enormous magnetotail stretches 750 million km, out past the orbit of Saturn. (Hey 2002) The overall volume of the magnetosphere is greater than the volume of the Sun. Its strength is about 20000 times that of Earth’s. (WebStern; WebUlysses)

Jupiter’s rocky core is believed to be equal in size to the entire Earth, and yet this core is not the origin of Jupiter’s huge magnetic field. Over 55% of Jupiter’s volume (up to 75% of the planet’s radius) is a mantle of hydrogen and helium, accreted from the solar nebula at the formation of the solar system. (WebThomas) The enormous pressure within the planet has compressed the hydrogen until the atoms are in very close proximity; close enough for electrons to jump between the atoms and allow electrical currents to flow. In this super-compressed state, hydrogen is known as liquid metallic hydrogen because it shares the conductive properties of metal. (Freedman & Kaufmann 2001) Jupiter thus has a voluminous liquid mantle wrapped around the large solid core. It also possesses the fastest rotational period of the planets, producing a “day” of less than 10 hours. Such fast rotation propagates strong convection currents within the enormous mass of liquid hydrogen, leading to the generation of Jupiter’s powerful dynamo.

Figure 4: Io Torus and flux tube

Jupiter’s four largest moons orbit inside its magnetosphere, creating a variety of intriguing interactions. Voyager 1 was the first probe to observe these interactions in 1979 when it discovered highly charged gases (helium, sulphur and oxygen) moving at high speed within the magnetosphere in the form of plasma. (Freedman & Kaufmann 2001) Plasma is formed when intense heat (or other form of energy) strips electrons from gas molecules, producing a highly charged form of matter that is easily influenced by magnetic fields. (WebPlasma) Galileo in 1995 and recently Cassini, identified the moon Io as the source of the plasma. Io is extremely volcanically active, spewing material into space which, after becoming ionized by the intense energy of the Jovian magnetosphere, forms a plasma trail behind Io in its orbit. Over time, this has created a ring of plasma around Jupiter, called the Io Torus. (WebSpaceflightNow2) (See Figure 4) Using its spectroscope, Galileo identified the major components of the torus as sulfur and oxygen (probably originating as sulphur dioxide), confirming Io as the culprit. (WebHyperPhysics) As Io orbits around Jupiter every 1.7 days, the planet’s magnetic field induces a strong electrical current flowing between Io and Jupiter, in the same way that swiping a credit card “reads” the magnetic strip on the card by the induced electric current. This current flows from Io to Jupiter’s north and south magnetic poles. This “Io Flux Tube” is estimated to carry a current of 2¹² Watts, greater than all the power generated on Earth. (WebCSep)

Data from Galileo suggested that both Io and Ganymede have their own, internally generated magnetic fields, each of which carves a cavity in Jupiter’s magnetosphere as the moons orbit the planet. (WebGalileo) Weak magnetic fields have also been detected around Europa and Callisto. Whilst Io and Ganymede are believed to operate their own micro dynamos due to heat convection within a molten interior, Europa and Callisto are more likely to have an induced field, created by electricity induced in liquid oceans beneath their icy surface as they move through Jupiter’s magnetosphere.

4.6 Saturn

As the second largest planet, it is not surprising that Saturn possesses the second largest magnetic field, though its strength is about 1/36^th that of Jupiter, but still almost 600 times more powerful than Earth’s field. (WebEarthSci3) The internal structure of Saturn is similar to that of Jupiter, but in different proportions. Saturn’s rocky inner core is thought to be much larger, comprising 13% of Saturn’s mass, compared to Jupiter’s core of 2.6% its total mass. (WebMira) Saturn’s smaller size and gravity creates less internal pressure, so that less of the planet’s hydrogen mantle is compressed into liquid metallic hydrogen. Saturn has a fast rotation period of 10 hours and 39 minutes, (WebRussel5) and so like Jupiter, powerful convective currents are created within the highly conductive mantle, leading to the formation of a strong magnetic field.

The existence of a magnetic field around Saturn was not confirmed until Pioneer 11 made a flyby in 1979, followed by Voyagers 1 and 2 in 1980/1981. The Voyagers discovered a bow shock at a distance of about 2 million km from Saturn’s centre, the magnetopause at about 1.2 million km, and determined that the overall dimension of the magnetosphere was approximately 3 million km, or about 20% the size of Jupiter’s. Unusually Saturn’s magnetic dipole axis was found to be within 0.1º of the rotational axis, the only planet to have coincident axes. (WebThomas; WebRussel5) The magnetotail has yet to be measured and accurate data is expected as part of the Cassini mission of 2004/2005.

Saturn’s magnetic field is not as active with charged particles as Jupiter’s “noisy” field. The large ring system and inner moons are responsible for collecting and holding most of the particles moving within the magnetosphere, so most of the inner regions are free of such particles.

Titan orbits at the fluctuating edge of the magnetopause. Most of the time the magnetopause shields Titan from the solar wind, but at times of high solar activity, the increased pressure of the solar wind compresses the magnetopause. At these times, Titan becomes exposed to the full force of the solar wind, which strips and ionizes particles from the moon’s dense atmosphere, creating a torus at Titan’s orbit very similar to the Io Torus. (WebRussel5)

4.7 Uranus

Voyager 2 made a flyby of Uranus in 1986, the only spacecraft to do so, and discovered that Uranus has a bizarre magnetic field. The magnetic axis is offset from the rotational axis by 59º, aligning the magnetic poles close to the planet’s equator. However, the planet’s rotational axis is itself inclined at 98º to the vertical, so that Uranus lies on its side with one pole pointing toward the Sun. The magnetic pole thus finishes up being almost perpendicular to the ecliptic. (WebVoyager) (See Figure 5)

Figure 5: Rotational and magnetic axes of Uranus

Voyager went on to discover other differences between Uranus’ magnetosphere and the inferior - closer to the sun - planets. The magnetic axis does not pass through the centre of the planet as it does in most planets. (Earth’s magnetic axis passes within 500 km of the centre of the core.) Instead it is offset by approximately 30% of the planetary radius or 8000 km, and inconclusive evidence suggests the existence of two axes rather than one, sandwiching the core between them. Over the course of the 17-hour Uranian day, the magnetic axis describes a swift, large circle around the planet, similar to the imaginary circle scribed by the spindle of an unstable spinning top. As Uranus moves along its orbit, this creates a spiraling magnetotail, never imagined until Voyager’s discovery. Each loop in the spiral spans approximately 70 million km. (WebThomas)

To explain the unusual magnetic field of Uranus, scientists speculated that the planet might be undergoing a slow pole reversal, which is known to have happened on Earth and Mars, (WebHarvard) but further data analysis pointed to the planet’s interior as the cause of the displaced magnetic axes. We expect Uranus to have a solid rocky core, approximately one Earth mass, surrounded by a liquid mantle of ammonia, water and methane; covered with a gaseous atmosphere of helium, hydrogen and methane. The liquid mantle extends outward to approximately a third of the planet’s radius, but unlike the larger Jovian planets, Uranus’ mantle is uniformly consistent across its volume. It is probably too stable, with insufficient internal heat for convection currents strong enough to sustain a Jupiter-like dynamo. Scientists suspected the existence of a thin crust-like layer of metallic liquid slush, sandwiched between the top of the mantle and the atmosphere; (WebSpaceDaily) but was there enough conductive material and enough convection to support a dynamo.

Stanley and Bloxham of Harvard created such a model of the interior and discovered that it would indeed lead to the magnetic phenomena measured by Voyager. Uranus’ fast rotation could cause convection currents in the thin layer of metallic liquid slush. This would explain the offset magnetic axis, closer to the surface than the core, as well as the existence of two magnetic axes, one on either side of the mantle. (WebSolarViews; WebDiscovery)

4.8 Neptune

Like Uranus, the only information we have about Neptune’s magnetic field comes from Voyager 2 during its flyby in 1989. Although Neptune is almost identical in size to Uranus, its magnetic field is half that of Uranus, but still approximately 25 times as strong as Earth’s. (WebThomas)

Neptune rotates with an inclination of 29º to the vertical, and the magnetic axis is inclined a further 47º. (WebRussel7) This highly inclined magnetic axis leads to yet another unique planetary magnetic field. For part of the Neptunian day of 16 hours, the magnetic South Pole points directly toward the Sun and into the solar wind, but later in the day it is orientated almost perpendicular to the solar wind. Neptune’s magnetic field varies greatly in its shape because of this.

The interior of Neptune is considered similar if not identical to Uranus, and so it is likely that the Neptunian dynamo is generated in a similar layer of liquid metallic slush above the large methane and ammonia mantle. Further evidence of this is Voyager’s measurement of a magnetic axis displaced from the planetary core by nearly 14,000 km or half the radius of the planet. (WebThomas)

4.9 Pluto and beyond

Considering its distance from the Sun (an average of 6 billion km), it is not surprising that we know very little about Pluto and its twin, Charon. To date, no spacecraft have visited Pluto. We speculate that Pluto has a solid interior of rock or frozen ices, with little chance of interior heat. Further, Pluto has a slow rotation of 6 ½ days. These conditions are not conducive to the formation of a planetary dynamo and Pluto almost certainly has no magnetic field.

The Sun’s magnetic field (Interplanetary Magnetic Field or IMF) extends outside the orbit of Pluto, so it is possible that Pluto will have an extremely weak induced magnetic field if there was sufficient atmosphere surrounding Pluto for the solar wind to strip and ionize. We see similar induced magnetic fields as comets move through the IMF. (WebWindows1) Similarly, we do not expect to find a measurable magnetic field around other planetary bodies recently discovered in the Kuiper Belt far beyond Pluto.

5. Grouping Planetary Magnetic Fields

It is possible to group the planetary magnetic fields within our solar system into three distinct groups, classified by the origin of their dynamos. These three groups are: (WebThomas)

Terrestrial - Mercury, Venus, Earth, Mars (and possibly Pluto)

Large Gas Giants - Jupiter and Saturn

Icy Giants - Uranus and Neptune

The Terrestrial group consists of the non-Jovian planets that possess cores of iron and nickel. These cores exist in a solid, molten or part-solid form. In the case of Earth and Mercury, the inner core is solid and surrounded by a molten outer core. As discussed earlier, convection currents within these liquid outer cores allow the formation of a planetary dynamo, generating a magnetic field to encompass these planets. Venus is believed to have an entirely molten core, which in combination with its slow rotational velocity prevents it developing a dynamo. Over time, its inner core may solidify and provide suitable conditions for a dynamo to form. In contrast, we believe Mars’ core solidified long ago, which also prohibits dynamo development. Evidence suggests that it had one is the past, when it probably possessed a liquid outer core. The dynamos of the Terrestrial planets are created deep within their molten outer cores. Due to the small size of these planets, this group possesses the weakest magnetic fields.

The Large Gas Giant group has as its members Jupiter and Saturn, the largest Jovian planets. Both planets possess a large amount of hydrogen, compressed by incredible internal pressures to a liquid form referred to as liquid metallic hydrogen. The enormous volume of this material is stirred up by strong convection currents driven by internal heat and the fast rotation of the two planets. These ideal conditions allow this group to have the largest and most powerful magnetic fields.

The Icy Giants group includes Uranus and Neptune. Though Jovian planets, they both lack the liquid metallic hydrogen of their larger cousins. Instead, they possess a mantle of methane and ammonia, probably capped with a thin layer of conductive slushy ices. This slushy ice layer is understood to be the source of the planetary dynamos, causing the magnetic axis to be grossly offset from the centre of the planet, and somehow leading to the large inclination of the axes to the vertical. The fast rotation and size of both Uranus and Neptune allows them to sustain a medium strength magnetic field, larger than the Terrestrial group but smaller than the Large Gas Giant group.

6. Conclusion

The table below summarizes the key points of each planet’s magnetic field.

Planet	Mass (Earths)	Rotation (hours)	Magnetic Moment (Earths)	Axial Tilt and polarity	Group
Mercury	0.06	58.6 days	1/2500^th	?	Terrestrial
Venus	0.82	243 days	1/25000^th	n/a	Terrestrial
Earth	1.0	24.0	1	+ 11.3º	Terrestrial
Mars	0.11	24.6	1/8000^th	n/a	Terrestrial
Jupiter	317.80	9.8	20000	- 9.6º	Large Gas Giant
Saturn	95.16	10.2	600	+ 0.1º	Large Gas Giant
Uranus	14.53	17.2	50	- 58.6º	Icy Giant
Neptune	17.15	16.1	25	- 47.0º	Icy Giant
Pluto	0.002	6.4 days	?	?	Terrestrial?

The study of planetary magnetic fields is a regular inclusion in the mission profile for interplanetary probes. Almost every probe launched from Earth in the past, and probably for the near future, has carried a magnetometer, a sensitive device for measuring magnetic fields. In 50 years of space exploration, the discipline of planetary magnetic fields has grown from nothing to a considerable knowledge of dynamo theory and the magnetic properties of our neighbouring planets. However, there are still numerous unanswered questions and much research remaining.

Whilst useful to group planets into the three groups: Terrestrial, Large Gas Giants and Icy Giants; each planet possesses unique qualities to its magnetic field. There is no convenient one rule that encapsulates every planetary magnetic field that we have investigated. This makes it very difficult to state which planet, if any, possesses the magnetic field typical of planets throughout the universe.

To quote Solomon, the director of the Department of Terrestrial Magnetism at Carnegie Institution: “We do not have a general theory of magnetic fields that will apply with equal validity to all Earth-like planets”. (WebEOS)

Appendix A – Table of Figures

Figure 1: The planetary dynamo.

Courtesy of Astronomy Notes. (See References)

Figure 2: Earth’s magnetosphere.

Courtesy of NASA, (re-annotated by Ing, G.)

http://science.nasa.gov/ssl/pad/sppb/edu/magnetosphere/bullets.html

Figure 3: Interaction of the solar wind with the ionosphere.

Courtesy of Luhmann, J. G., Russell, C. T.

Figure 4: Io Torus and flux tube.

Courtesy of Spencer, J., (annotated by Ing, G.)

http://www.boulder.swri.edu/~spencer/digipics.html

Figure 5: Rotational and magnetic axes of Uranus.