Io and Its Volcanoes
It was thought that Io would be like our Moon: geologically dead, with little internal heat to power tectonic movement or volcanic eruptions. Io's surface was expected to be heavily cratered due to lack of volcanic activity. However, the Voyager l spacecraft sent back pictures of Io on March 5, 1979 showing that it has no impact craters at all. Io's surface is marked with irregular shaped pits and is blotched with color, giving it an appearance unlike any other in the Solar System. (Freeman, Kaufmann III , 317). Scientists concluded that Io's surface is the result of intense volcanic activity.
Io's internal heat is caused by Jupiter's tidal forces. Tidal forces are differences in the gravitational pull on different parts of a planet or satellite (Freeman, 318). These forces tend to change on deform the shape of a planet or satellite. Jupiter's tidal forces distort the shape of Io as it orbits around the planet in an elliptical path influenced by Europa and Ganymede gravitational forces. Io's speed varies as it moves around its orbit. Because the orbital and rotational periods are not synchronized, strongly variable tidal stresses are exerted on Io. These varying tidal stresses alternately squeeze and flex Io, similar to the way a ball of clay or bread dough is squeezed and flexed in the palm of a hand. The squeezing and flexing of the clay or dough over a period of time will cause them to heat. Tidal heating adds energy at the rate of about l0 1 4 watts, equivalent to 24 tons of TNT exploding energy second. (Freeman and KaufmannIII, 318). Freeman and Kaufmann show that, from Io's interior energy, 2.5 watts of power reaches each square meter of Io's surface. In comparison, the average global heat flow through the Earth's crust is 0.06 watts per square meter. They further stated that only in volcanically active areas on Earth are there heat flows that are somewhat comparable to Io's average.
The Galileo spacecraft sent back detailed images of plumes produced by erupting volcanoes on Io. Some of these plumes reached heights of 70 to 280 km above the surface. These extreme heights can only be reached by materials that are ejected from volcanic vents at speeds between 300 and 1000 m/s (700 to 2200 mi/h). The most violent terrestrial volcanoes, like Vesuvius, Thera, and Mount Saint Helens, have eruption speeds of only around 100 m/s (220 mi/hr). Scientists feel that volcanoes on Io must be different from those on Earth.
Io's volcanism is driven much by sulfur and sulfur dioxide, as shown by, images of Io's volcanic plumes. The plumes are more like geysers. In a geyser on Earth, water seeps down to the rocks heated by the radioactive process, converts to steam and explodes violently through vents. In the case of Io, sulfur dioxide, a solid at the frigid temperatures on Io's surface, becomes molten liquid sulfur dioxide a few kilometers beneath the surface. This liquid sulfur dioxide becomes with a high-pressure gas capable of colossal eruption with velocities up to 1000m/s. (Freeman and Kaufmann III, 319)
Sulfur and sulfur dioxide help explain the noticeable coloration of Io. Ejecta from the volcanic plumes eventually fall back to the surface. The bright yellow is sulfur and is the dominant color of Io's surface. If the sulfur is heated and cooled suddenly, the sulfur would take on a range of colors from orange to red to black, as in the case of a volcanic eruption through a vent. The sulfur would be heated, ejected through a vent and would be cooled rapidly. These colors are found around volcanic vents.
Sulfur dioxide on Io leaves a white deposit after a volcanic eruption. When hot sulfur dioxide gas is released through a volcanic vent into the cold frigid atmosphere, the sulfur dioxide crystallizes into white snowflakes and falls to the surface of Io as "Snow" rain. (Freeman and Kaufmann III, 319). On Earth, sulfur dioxide is ejected through vents and fall back to the surface of the Earth in the form of acrid gas.
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