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Explanation: What would it look like to peer into one of the volcanoes currently active on Jupiter's moon Io? The caldera of Tupan Patera, named after a Brazilian thunder god, reveals itself to be a strange and dangerous place, replete with hot black lava, warm red sulfur deposits likely deposited from vented gas, and hilly yellow terrain also high in sulfur. The robot spacecraft Galileo currently orbiting Jupiter provided the above vista late last year when it swooped by the active world. Tupan Patera is actually a volcanic depression, surrounded by cliffs nearly a kilometer high. The width of the depression is about 75 kilometers. As Galileo has filled its mission objectives and is running low on maneuvering fuel, NASA plans to crash the spacecraft into Jupiter during 2003.
These plumes are believed to be produced the same way geysers are on Earth, except that instead of being made of boiling water they consist of hot sulfur dioxide gas escaping from the volcano's interior. As this invisible gas expands into space, it cools and freezes into something like snowflakes, so that the plumes can be seen.
Io as the source of the plasma torus
Io's atmosphere is also the source for the Io plasma torus (LINK). We still do not fully understand how, but Io's atmosphere slowly escapes into space. Once escaped from Io, this gas does not just disappear, but rather each of its molecules merely falls along with Io in orbit around Jupiter. These molecule's orbits do not move far from Io's orbit, which is nearly a circle centered on Jupiter, so all together they look from the outside like a donut-shaped cloud of gas rotating around Jupiter the way that an auto tire rotates on its axle (see animation). Logically, this cloud should be called the Io neutral gas torus, by analogy with the Io plasma torus (LINK) (also see below), but alas it is not.
These escaped sulfur dioxide molecules do not orbit forever. The ultraviolet part of sunlight and bombardment by electrons knocks them apart. Many of the pieces of these molecules are electrically charged atoms, or ions, and more electrons. These new ions and electrons are immediately accelerated to move with Jupiter's rotating magnetic field. Because these ions and electrons, or plasma, come from Io and its accompanying donut-shaped cloud, this plasma cloud is also donut-shaped. Because of its shape, it is called the Io plasma torus (LINK). It also rotates, but with Jupiter (10 hour period) instead of the slower orbital speed (42 hour period). Because Jupiter's magnetic field is tilted, the Io plasma torus rotation has a wobble or shimmy, like a car tire with a bent hub (see animation).