The first detailed imagery of the Galilean satellites of Jupiter was by the Voyager I mission in 1979. Outermost Callisto turned out to be what everyone expected, a dark, heavily cratered icy ball.
And that was about the last thing anyone saw in the outer Solar System that looked like we expected.
Ganymede, which nudged out Saturn's satellite Titan for the title of largest satellite in the Solar System by just a few kilometers, turned out to be the only body apart from Earth to show evidence of large scale horizontal crustal movements. Europa turned out to be criss-crossed by cracks and have almost no craters, but Io was the most surprising of all. It had no craters at all, a bizarre colored surface that many likened to a pizza, and the only active volcanism seen anywhere but Earth.
The Galilean satellites were the first new objects in the Solar System ever discovered. Galileo, no dummy, wanted to call them the "Medicean stars," hoping to gain favor and patronage from the powerful Medici family, but nobody outside of Florence agreed. As satellites, they should be named after figures associated with Jupiter but subordinate to him. If you know your mythology, right away you know that rules out his wife Juno (Greek: Hera. She did get an asteroid in her honor). Jupiter's favorite hobby, after ruling Olympus and hurling thunderbolts, was having love affairs, and Juno's favorite hobby was breaking them up and generally inflicting awful vengeance on the lovers. The four Galilean satellites were named for lovers of Jupiter. Ganymede was a boy. The Greeks and Romans were okay with that. The number of small satellites eventually became too great for even Jupiter's prodigious libido, so female mythological names in general are now used.
Features on Io are named for figures from various world fire and underworld myths. Mountains and other geographic features are also named after places in the myth of Io, who was forced to flee from a gadfly sent by spoilsport Juno (Hera) and apparently collected a lot of passport stanps in the process.
When Voyager passed Io, its geology turned out to be remarkable in a number of respects. Io completely lacked craters. It had a bizarre surface covered with white, yellow and red materials. And it had active volcanism.
Io is about the size of earth's moon, which long ago ceased significant volcanism. Where would Io be getting its heat? Stan Peale, Patrick Cassen, and R. T. Reynolds had published a paper in Science barely a week earlier describing how tidal friction with Jupiter could cause melting in Io. They even speculated on active volcanism. (The timing is fortuitous since it generally takes at least a year to get a scientific paper published, so Peale and his colleagues had written their ideas up at least a year earlier.)
Quiet lava flows are one thing, but many of Io's volcanoes were spewing material a couple of hundred kilometers high. What could be powering explosive volcanism? On earth, explosive volcanism is mostly powered by water vapor; volcanoes are gigantic natural steam explosions. But Io's gravity is so small that any erupted water vapor would have escaped completely over the age of the solar system. So to explain Io's explosive eruptions, we need something abundant in the universe, volatile, and heavy enough for Io's gravity to retain it. It should ideally also be capable of explaining the weird surface colors. Sulfur fills the bill on all counts. Some planetary geologists believe that all of Io's sulfur has been erupted onto the surface, others that only a small amount has. So the sulfur coating on Io could be anywhere from centimeters to kilometers thick.
Some of the eruptions seem to be more like geysers than true volcanoes. If Earth lacked an atmosphere, Old Faithful could spray water kilometers high, and if Earth had the weak gravity of Io, hundreds of kilometers. Nevertheless, much of Io's volcanism is believed to be silicate volcanism. First, temperatures of some flows fall in the silicate lava range and are far above the vaporization point of sulfur. Second, there are cliffs and caldera rims on Io that are too high and steep to be supported by solid sulfur. It is estimated that Io is resurfaced every million years or so, meaning its remote past is hidden.
Created 11 April 2014, Last Update
26 March 2015
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