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Mercury, November/December 2000 Table of Contents

A giant torus of charged particles surrounding Jupiter gives astronomers a chance to study a nebula up close.

by Duane H. Pontius, Jr., Birmingham-Southern College

"Astronomers are accustomed to a hands-off relationship with their subjects. Even the most lovingly studied celestial objects tend to remain remote, separated from our scientific instruments by immense stretches of space and time. Apart from a few small rocks in our own solar system and the lone star at its center, the multifarious denizens of the sky remain aloof."

This statement certainly holds true for the vast clouds of gas and dust that astronomers call "nebulae." The closest nebulae lie hundreds or even thousands of light-years away. But there is one exception, a nebula that resides right here in our own solar system: the Io plasma torus.

In this article, Duane Pontius traces the nebula's origin to Jupiter's volcanic moon Io. Powerful volcanic eruptions lift material into space, where it gets trapped by Jupiter's magnetic field. The material gets whipped around by Jupiter's rotation, forming a donut-shaped cloud of charged particles. Pontius describes various ground-based and spacecraft observations of the Io torus and the complex interactions that take place within the torus. At the end of the article, Pontius explores how studying the Io plasma torus will help astronomers understand what's happening in more distant realms.