Lessons from a Flyby

Voyager came and Voyager went, leaving in its wake the planet Neptune and, on Earth, many happy scientists. The little spacecraft performed beautifully, and--as hoped--called home with bountiful news about Neptune, its moons, and planetary science in general.

One valuable lesson is that a lot of what we thought we knew before the flyby is wrong. This is such a standard occurrence in research involving new technology and new data that scientists don't see it as a setback. Of course, individuals who must surrender a beautiful theory because of an ugly fact have cause to be unhappy, but they too accept that the trip back to the metaphorical drawing board is the most well-beaten path in science.

Some of the scientists challenged to rethink by Voyager information are specialists in planetary magnetic fields. It's a lively study subject; no one really understands in detail even how Earth's interior generates the terrestrial magnetic field, so there's room for argument. Yet there was fair confidence about the general pattern, and no one had real reason to suspect that the terrestrial pattern wouldn't carry over to other planets.

Within the earth, a molten iron core serves as a flowing magnetic conductor. It generates a magnetic field that is dipolar at the planet's surface--the field resembles the field around a bar magnet, with lobes at south and north magnetic poles. The earth's field is inclined only a little from its axis of rotation; that is, the geomagnetic poles are displaced by only about 11 degrees from the geographic ones. A nicely logical theory explained this: interaction of the magnetic field and the flows that generate it should tend to align the magnetic and rotational poles.

As better technology let science reach into space, the theory held up well. Mercury, Jupiter, and Saturn all had magnetic fields close to the earthly pattern, with well-aligned rotational and magnetic poles.

When the second Voyager closed on Uranus in 1986, its instruments revealed that this was a magnetically different planet indeed. Not only was the magnetic field tilted 60 degrees from the rotational axis, its center was offset from the center of the planet by a distance equivalent to about a third of the planet's radius.

Yet this wasn't a theory-killing discovery. Uranus is unique in the solar system because its poles are in what would be the equatorial plane of most planets. Viewed from Earth, the orbits of its satellites draw a bull's eye. Whatever caused its weird orientation might also have caused its magnetic weirdness. The most popular theory explaining these peculiarities was that Uranus had taken a cosmic but off-center blow from another planet-sized body early in the formative years of the solar system. Possibly that catastrophe permanency disrupted any flows generating a magnetic field.

Another suggestion came from Earth's geologic record, which shows that the magnetic field has reversed itself a few times. It was improbable, but conceivable, that Voyager 2 passed Uranus just as the planet's magnetic field was in the throes of a reversal. The theorists believe the dipole field fades during a reversal, reappearing later at full power with its north and south poles exchanged. During the time of fading, its inner complexities might be revealed and might confuse judgments about its actual behavior.

The planetary scientists had three peaceful years to debate the pros and cons of ideas explaining away the unique magnetic field of Uranus. Now they have new facts from Neptune, and the peaceful debates have passed. It's a free-for-all.

Neptune's magnetic field is severely tilted also, by about 50 degrees. Its dipole field is also offset from the planetary center, and is even closer to the surface than is the field on Uranus. But there is no sign that Neptune ever suffered a cataclysmic impact, and no one can quite believe that blind chance would have connived with Voyager's flyby to permit catching two planets during field reversals.

Once more, the drawing boards have been cleared, and the new theories are springing up exuberantly.