Deciphering planetary magnetism
2 neared Uranus in January 1986, NASA scientists waited for the radio waves that
would be the first evidence of the gas giants predicted magnetic field.
They expected the field to be much like those of Jupiter, Saturn and Earth
a large-scale, stable field, much like a bar magnet, with north and south poles
aligned along the planets axis of rotation. What they discovered instead
was a small-scale fluctuating field with multiple poles and a 60-degree tilt toward
the equator. Three years later, Voyager 2 flew by Neptune and detected a similar
field there. Until now, scientists have been unable to effectively model and explain
this odd behavior.
Neptune, captured on camera here by NASAs Voyager 2, and Uranus both have
unusual magnetic fields that tilt at a 60-degree angle toward their equators.
Scientists are now able to model the processes that shape the planets magnetism.
Image courtesy of NASA/JPL.
In the March 11 issue of Nature, Harvard University researchers Sabine
Stanley and Jeremy Bloxham present the first model to successfully reproduce the
planets magnetic fields. The results also show that despite Uranus
and Neptunes unusual interiors, their magnetic fields nevertheless are generated
by processes similar to those at work on Earth.
The dynamo that creates Earths magnetic field is a thick layer of fluid
iron vigorously convecting around an electrically conducting, solid-iron inner
core. The solid nature of the inner core gives the field long-term stability by
restraining the magnetic fluctuations that occur in the more changeable liquid
outer core. And the strong differential rotation between the inner core and outer
core causes the axis of the magnetic field to align with the rotational axis of
In the early and mid-1990s, planetary scientists studying thermal models and luminosities
of Uranus and Neptune proposed that, save a tiny rocky core, the interiors of
both planets are composed of stratified layers of liquid methane, water and ammonia
often called ices which are electrically conducting. The scientists
hypothesized that a thin layer of such liquid convecting above similar, yet thicker
and more stable, layers might produce the unusual features observed in the magnetic
Bolstering this hypothesis, the new model shows that a dynamo formed by the interaction
of two liquids, as opposed to a solid and a liquid as in Earth, could produce
smaller scale magnetic fields with multiple poles. Additionally, the lack of differential
rotation between the two fluid layers would inhibit the alignment of the magnetic
field with the axis of rotation, thus causing the tilted poles found by Voyager
Stanley and Bloxhams model clearly and impressively demonstrates that
the same fundamental process ... can explain the basic structure of all the dynamo-generated
planetary magnetic fields presently observed in the Solar System, writes
geophysicist Jonathan Aurnou of the University of California, Los Angeles, in
an accompanying Nature commentary.
To create the dynamo, Stanley and Bloxham modeled a planet where one-sixth of
the radius of the dynamo region is a rocky nonconductive core, one-half is a nonconvecting
stably stratified liquid, and the top one-third is a convecting liquid. In addition
to the radius, the researchers could vary other conditions, including how vigorously
the convection is being driven and the stability of the interior. How the model
responds will give researchers more information about the interior structures
of the distant bodies.
The next step is to see how important these parameters are on the fields
you get, Stanley says. If theyre very sensitive to those parameters,
maybe we can tell from that whats really happening in Uranus and Neptune.
The findings may also play a role in future orbiter missions to the giant gas
planets to better measure their magnetic fields, which would help determine the
locations of their dynamos.
We already suspect that the dynamo region in Neptune and Uranus, and possibly
in Jupiter, may be relatively much closer to the planets surface than is
the case for Earth, says William Hubbard, a planetary scientist at the Lunar
and Planetary Laboratory at the University of Arizona in Tucson. Studies
such as the Stanley-Bloxham paper should help us to better understand the utility
of high-degree and high-order magnetic field measurements of giant planets, for
locating the depth of their dynamo region.
Geotimes contributing writer
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