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 Credit: NASA/JPL-Caltech |
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Aug 04, 2004 Jupiter's inner Galilean moon, Io, spews plumes of material into space from several hot spots. They have been called volcanoes. But are they? The hot spots are unexpectedly hot. Probe sensors overloaded after registering temperatures higher than any lava on Earth. And some of the hot spots move over the surface. Plume velocities are unexpectedly high and uniform. The plumes are tall, have an umbrella shape, and deposit material in a ring around the source. They also have a filamentary structure. Io orbits inside a donut-shaped cloud of charged particles that come from the material in the plumes, and a tube of electrical current connects Io with Jupiter's auroras. These discrepancies from Earth volcanoes prompted Thomas Gold (in 1979) and Anthony Peratt and A. J. Dessler (in 1988) to note the similarities of the unexpected features to electrical discharges in plasma. An electrical arc is about as hot as the surface of the Sun. It would easily "blind" a spacecraft's sensors. And an arc often wanders over the surface of a cathode. Arcs accelerate material to fairly high and uniform velocities. This produces uniform trajectories that deposit material a uniform distance from the source, explaining the rings around the "volcanoes." And the forces in the discharge channels pinch the arcs into filaments. Repulsive forces between filaments tend to space them equally, often in pairs, around the plumes. Peratt and Dessler remarked on the similarity of the filamentary umbrella shape to the shape discharged from a laboratory "plasma gun". The cloud of charged particles flowing past Io constitutes an electrical current. Peratt and Dessler calculate that the power it should induce across Io is about equal to the energy of the "volcanoes". The largest planet in the Solar System, with the most active magnetosphere in the Solar System, has its electrical circuits "shorted out" by its inner satellite. The million-Ampere currents flowing through Io's crust make it a unique laboratory for studying the processes of interplanetary-scale electrical discharges. The scars produced will help us to distinguish electrical from impact scars left on other moons, comets, asteroids and planets. |
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