fairly confident that our theories are on the right track. But we're
not all the way there yet."
years ago, Professor of Earth and Space Sciences Margaret Kivelson
wrote a scientific paper speculating that planets might not be the
only celestial bodies to exert magnetic spheres of influence. The
paper was skeptically received, and even Kivelson considered the
hypothesis mostly an "intellectual exercise."
Imagine Kivelson's surprise two years ago when the Galileo spacecraft
flew by Ganymede, Jupiter's largest moon, and sent back experimental
data proving she'd been right all along. The experiments, which
Kivelson designed, not only provided the first indisputable evidence
that a moon can generate an internal magnetic field; it caused scientists
to rethink the conditions under which such fields can occur. The
internal processes that enable some planets -- and, which the Ganymede
evidence, some moons -- to create magnetic fields has long served
as one of the greatest puzzels of the natural sciences. The Ganymede
findings were so startling because the moon appears as an icy, geologically
inactive mass, a state inconsistent with assumed magnetic-field
a mystery," Kivelson says, referring to the question of how currents
might come out of the snowball-like satellite. "We're fairly confident
that our theories are on the right track. But we're not all the
way there yet." Indeed, researchers are currently re-examining theories
on Ganymede's thermal evolution.
has also been studying the effects Ganymede's magnetic field has
on the surroundings of Jupiter's magnetosphere. Flowing plasma rotating
around Jupiter interacts with Ganymede's field to create a magnetosphere
around Ganymede. "This is itself an interesting object," Kivelson
explains. "It can trap charged particles, and it can protect parts
of Ganymede's surface from being bombarded by the energetic charged
particles in the environment." Kivelson's UCLA colleague, Krishan
Khurana, is pursuing the promising lead that surface properties
- i.e., how the ices have been altered by charged-particle bombardment
in some regions but not others - can confirm the magnetic geometry
of a system. And Ganymede's magnetosphere provides a new model for
testing many theories related to Earth's own magnetic field.
lot of attention is being given to what we call 'space weather'
- how the Earth's electromagnetic properties change in time because
of its interaction with the solar wind," explains Kivelson. "It
is a practical area of study in that all of our satellites are embedded
in this magnetosphere."
to Ganymede, Kivelson has discovered important data on Jupiter's
other moons, as well. The Galileo flyby of Io, a moon with volcanoes,
surrounded by a corona of neutral gases and charged particles that
may have originated from the volcanic sources, showed surprising
significant magnetic perturbations, suggesting it also generates
its own internal magnetic field. And multiple passes by the moons
Europa and Callisto suggested that magnetic changes around these
moons are the result of fields that are inductive, rather than internal.
concluded that the electrical current-carrying path has to be somewhere
quite close to the surface," says Kivelson. The surfaces of both
moons are water ice - not a good electrical conductor - but the
culprit might be liquid saltwater, similar to the oceans on this
planet. "The most plausible interpretation of our results is that
somewhere not very far below the surface, the ice has melted, or
at least come close enough to melting that it's creating an inductive
response roughly comparable to that created by Earth's oceans."
Kivelson hopes to be able to draw more definitive conclusions after
the next scheduled pass by Europa sometime within the next year.
have been a long time coming for Kivelson, who became a physicist
back in the early '50s, when a scant 2 percent of physics Ph.D.s
were awarded to women. She spent more than a decade working in plasma
physics at RAND, the Santa Monica-based think tank, then in 1967
accepted a faculty position with UCLA's Institute of Geophysics
and Planetary Physics. Before long, she was managing a team analyzing
magnetic-field data from two Pioneer spacecraft missions to Jupiter.
In 1976, she first proposed building the magnetometer for Galileo.
A team of UCLA researchers and engineers began to implement Kivelson's
design the following year, but numerous delays would keep Galileo
Earth-bound for 13 years before it finally began its six-year, 2.3
billion-mile journey to Jupiter in 1989.
"It was an
investment of time that I'm not sure I would have made if I had
known it was going to be close to 20 years before I got the first
data," Kivelson says, smiling. "But certainly the payoff has been
incredible." - Dan Gordon