picture of the day
Ganymede, the third moon from
Jupiter and the largest planetary satellite in the solar system
Aug 15, 2007
Ganymede Bears Witness
The solar system's
largest moon shows signs of what could be electric scarring.
Is plasma discharge responsible for the topography we see?
its moons have been the destination of several
remote observation platforms launched over the last three
decades. Beginning with
Pioneer 10 in 1973 and including the most recent visit by
New Horizons this year, seven different camera packages have
flown past the planet and many of its moons. Of all the moons,
Ganymede is possibly the most exotic, with a
wild mix of topography,
Ganymede is unique among moons in that it has a
magnetic field surrounding it, something even Mars does
not possess. In December 1995, the
Galileo spacecraft entered orbit around Jupiter. During
a flyby of Ganymede at an altitude of only 838 kilometers,
Galileo discovered a dipole magnetic field very much like
the one surrounding Earth. As with Io, the signature of
Ganymede's flux tube, the electric current that connects it
with Jupiter, can be seen in the
aurora at the poles.
With a mean diameter of 5262 kilometers,
Ganymede is the largest moon orbiting any planet and is
the fourth largest rocky object after the planet Mars. The
magnetic field is supposedly being created by the moon's
core in a "dynamo" of sorts - once again like the Earth's
core is supposed to be generating its magnetic field. There
is an ambiguity, however. Ganymede's core is too hot to hold
on to permanent magnetism. But Ganymede is so small that,
according to conventional astro-geology, it should have
cooled off billions of years ago and should not have a
liquid core in the first place.
ad hoc explanation that NASA scientists have announced
creates its own conundrum, though.
moon once may have been much closer to Jupiter, so it was
alternately compressed and stretched by the tidal forces of
the planet's gravitational field. The constant kneading of
the moon kept its core liquid for much longer than if it had
formed in its present orbit. If that were the case, then
what forced an object bigger than the planet Mercury to move
into a new orbit? Was any thought given to a mechanism for
moving several quintillion tons of rock and ice a few
thousand kilometers against the force of Jupiter's gravity?
course, the most obvious aspect of
Ganymede's bizarre nature is its surface and the
manifold examples of apparent electric discharge machining (EDM).
Thunderbolts Picture of the Day articles presented
evidence for the hypothesis by showing how crater chains
form on Ganymede and other planets and moons. Another likely
example of EDM is the huge
circular structure dominating an entire hemisphere.
Within the darkened circle, several bright craters are
arrayed in a spiral shape that gradually decreases in
diameter toward the center. Some craters have
rays extending outward for several kilometers in all
directions. Several have one or more
nested concentrically within the other. Such features
require a chain of unlikely coincidences if mechanical
impacts are to explain them, but EDM creates such scars
Could the magnetic field of
Ganymede be related to the electrical phenomena that
have scarred and transmogrified it? If Ganymede was indeed
closer to Jupiter at some point in its past and was then
wrenched from orbit and thrown thousands of kilometers
further out from the tidal grasp of its parent, could the
force that was responsible for that event have been
electrical in nature? Was the moon gripped by an
electrodynamic field large enough to imprint its core with
permanent magnetism? What effect does the electrical
connection with Jupiter have on Ganymede today?
NASA plans more
missions to the moons of Jupiter in the next ten years. As
more data is returned from a growing number of deep space
probes, perhaps the evidence for electric effects will help
to increase awareness for electricity in space.
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