The Earth's own magnetic field is probably created in the Earth's core, believed to contain molten iron, by a "fluid dynamo". These are rather special conditions and scientists who studied the Earth's dynamo in the 1950s and 1960s must have wondered whether the Earth's planetary magnetism was unique.|
We now know better: space probes have found that Jupiter, Saturn, Uranus and Neptune all have magnetic fields, as does tiny Mercury. The Moon has patches of magnetized rocks and might have had a field when those rocks formed long ago, abd Venus seems non-magnetic. Mars was a mystery until September 1997, when Mars Global Surveyor found it to be magnetized in patches, like the moon but several times more strongly.
The sources of the planetary fields seem quite different from the Earth's and one can only speculate about their origin. For instance, does the magnetism of Jupiter and Saturn originate in cores of metallic hydrogen, a form which can exist under the enormous pressures at their centers? And since Uranus and Neptune do not generate enough pressure for hydrogen to become metallic--are their internal currents carried by ions dissolved in water or methane ices?
The magnetization of Mercury, Mars and the Moon must belong to a different class (see "Mercury: the Forgotten Planet" by R.M.Nelson, Scientific American, November 1997, p. 56). In particular, the moon, and perhaps Mars, may contain permanently magnetized rocks, from lavas which poured out in the distant past, when the parent body was magnetized, and became weakly magnetized themselves (this process does happen on Earth). All this, though, is speculation: we really do not yet know
|Jupiter is the largest planet of the solar system and has the most powerful magnetic field, also the largest radiation belt; radio emissions from its belt were first detected by radio astronomers in 1955. Jupiter's magnetosphere was explored by the space probes Pioneer 10 and 11, Voyager 1 and 2, and Ulysses, and the space probe Galileo has been orbiting there since late 1995.|
Jupiter's radiation belt is quite intense, and just one pass through its denser part by Pioneer 10 in 1973 was enough to cause some radiation damage, luckily rather minor.|
The magnetospheres of the giant planets differ from the Earth's in at least four ways. First, they are much bigger, not only because the planetary magnets are stronger but also because the solar wind weakens as it moves away from the Sun and spreads out. Both of these factors cause the solar wind to be stopped further away from the planet than is the case with Earth.
Picture of a head-on magnetosphere.|
| The real surprise came with Uranus, whose rotation axis is nearly parallel to its orbital plane. At the time of the 1986 fly-by of Voyager 2, that axis pointed almost exactly at the Sun. Based on their experience with Earth, Jupiter and Saturn, scientists expected the magnetic axis of Uranus to be close to its rotation axis, and to also point more or less sunward. They therefore expected a completely different magnetosphere, a "head-on" magnetosphere which met the solar wind not with a "hard" nose of ordered magnetic field lines (as the Earth does), but with its "soft" cusp region (picture below). Earth never attains this position.|
But it wasn't to be. As Voyager 2 found, the magnetic axis of Uranus was actually steeply inclined to its rotation axis, at nearly 60 degrees, causing it to spin around like the axis of a top that is about to topple. As a result, the direction of the magnetic axis in space varied constantly and rapidly, but it never pointed towards the Sun--though it might do so, briefly, in other parts of the planet's orbit. Neptune was somewhat similar, with its magnetic axis angled by 47 degrees to its rotation axis.|
All this suggests that not only isn't the Earth's magnetosphere unique, but different kinds of magnetospheres are possible, and some of them can be found in our solar system. Not only do we have in our magnetosphere a natural laboratory for studying cosmic plasmas, but different examples of such plasmas are also accessible (though not easily), to be studied perhaps by future generations. We are indeed fortunate!
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