#32.     Magnetospheres other than Ours

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

Differences in Scale

The speed of the solar wind however remains the same, about 400 km/sec. As a result, the wind needs a much longer time to traverse the length of the magnetosphere.

With the Earth's magnetosphere, it takes the solar wind about one hour to advance from the "nose" to the distant tail regions where ISEE-3 and Geotail have probed it, some 200 RE downstream. During that one hour the Earth rotates by a rather small angle, 15 degrees, and if "open" field lines in the lobes connect it to the solar wind, those lines might become twisted by about 15 degrees.

If Jupiter's magnetosphere has the same proportions, the solar wind would need 2-3 days to cover the corresponding distance (equal to about half the Earth-Sun distance!), during which the planet might have rotated 5-7 times around its axis. One might therefore expect the lobes of Jupiter's magnetotail (and Saturn's, too) to be severely twisted, and the Galileo mission might be the first opportunity to examine this point. All other probes sent to Jupiter used the planet as a pivot to gain extra speed, the way "Wind" used the moon, and the orbits required for this maneuver kept them out of the lobes.

Satellites

But they do more than that. Saturn seems to have an inner belt like the Earth's, and calculations suggest it is produced by cosmic ray neutrons ejected from the planet's rings. Jupiter's magnetosphere is heavily loaded with sulfur ions, believed to originate in the sulfur volcanoes of the satellite Io. This may also be the source of the sodium cloud around the planet, studied by telescopes from Earth.

Rotation

The planets with the largest magnetospheres, Jupiter and Saturn, rotate rapidly (periods of about 10 hours), and data from space probes has suggested that the plasma surrounding them participates in that rotation to a much greater extent than the Earth's, perhaps up to the "nose" itself. How then do intense radiation belts arise? Perhaps very powerful magnetic storms overcome the rotation and inject them deep into the magnetosphere, or perhaps the process differs from what occurs near Earth. Again, Galileo might tell.

Tilt

Jupiter's magnetic axis is inclined to its rotation axis by about the same amount as the Earth's. Its magnetic north-south polarity is the opposite of the Earth's--but it's worth noting that fossil magnetic records, in sea-floor rocks, indicate that the Earth's polarity has reversed many times in the distant past. Saturn's magnetic axis seems exactly aligned with its rotation axis, within the errors of the observations, and that has bothered some theorists, since a 1931 theorem by Thomas Cowling stated that a planetary dynamo field cannot be axially symmetric. However, since the magnetic fields of irregularities die out quickly with distance, it can be that observations closer to the planet might find an asymmetry.

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!