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Io: Surface
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Io is one of 28 moons of Jupiter. Io is ... one of the largest at 3630 km in diameter, only Ganymede and Callisto are larger. Io is also extremely volcanic, with hundreds of active volcanoes on its surface at any given time. These volcanoes give rise to very hot lava flows as long as 500 km and gas/dust plumes that can reach heights of 400 km. In addition, Io has tremendously high mountains, which are not volcanoes, and tectonic faults that give rise to the mountains and volcanoes, as well as split the mountains apart. Io has a thin atmosphere consisting of sulfur dioxide mainly from plumes erupting from the volcanoes. Sulfur dioxide acts very similar to water on earth.
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Some of Io's volcanic plumes have been measured rising over 300 km above the surface before falling back, the material being ejected from the surface at approximately one kilometer per second. The volcanic eruptions change rapidly; in just four months between the arrivals of Voyager 1 and Voyager 2 some of them stopped and others started up. The deposits surrounding the vents ... changed visibly.
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Io is believed to have a metallic core surrounded by a partially molten rocky mantle. The moon is bright yellowish orange due to the abundance of sulfur on its surface. In its elliptical orbit Io is continually squeezed in and out like an accordion by the strong gravitational pull of Jupiter and the weaker pull of the other Galilean satellites.
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Mountains on Io (generally, structures rising above the surrounding plains) have a variety of morphologies. Plateaus are most common.[6] These structures resemble large, flat-topped mesas with rugged surfaces. Other mountains appear to be tilted crustal blocks, with a shallow slope from the formerly flat surface and a steep slope consisting of formerly sub-surface materials uplifted by compressive stresses. Both types of mountains often have steep scarps along one or more margins. Only a handful of mountains on Io appear to have a volcanic origin. These mountains resemble small shield volcanoes, with steep slopes (6–7°) near a small, central caldera and shallow slopes along their margins.[75] These volcanic mountains are often smaller than the average mountain on Io, averaging only 1 to 2 km (0.6 to 1.2 mi) in height and 40 to 60 km (25 to 37 mi) wide.
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Although Io always points the same side toward Jupiter in its orbit around the giant planet, the large moons Europa and Ganymede perturb Io's orbit into an irregularly elliptical one. Thus, in its widely varying distances from Jupiter, Io is subjected to tremendous tidal forces. These forces cause Io's surface to bulge up and down (or in and out) by as much as 100 meters (330 feet)! Compare these tides on Io's solid surface to the tides on Earth's oceans. On Earth, in the place where tides are highest, the difference between low and high tides is only 18 meters (60 feet), and this is for water, not solid ground!
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Io's surface is radically different from any other body in the solar system. It came as a very big surprise to the Voyager scientists on the first encounter. They had expected to see impact craters like those on the other terrestrial bodies and from their number per unit area to estimate the age of Io's surface. But there are very few, if any, impact craters on Io (left). Therefore, the surface is very young.
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