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Jovian Cauldron: Io's Volcanoes Revealed in Sharp Detail
By Robert Myers
Multimedia Producer
posted: 07:15 pm ET
26 October 2000
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io_sulphur_snow_001027 Jupiter's moon Io continues to surprise scientists with its degree of volcanism. The latest images from NASA's Galileo probe show astounding volcanic structures and landscapes blanketed with sulfuric "snow."
| Small white diffuse halos surrounding the darkest lava flows are probably sulfurdioxide-rich snows and frosts that have been vaporized by the hot lava. |
"We see this volatile material everywhere on Io where we've had a close-up look," Alfred McEwen, a planetary scientist at the University of Arizona, said of the snow. The material is apparently made up mostly of sulfur dioxide, which exists as a gas at Earth-like temperatures. "It looks like [the snow] is sublimating or eroding away by some means," he said. "We'd like to know where it's coming from, how the surface layer is being resupplied." Io's volcanoes seem to be part of the system that distributes the sulfuric snow across the planet. The edges of some lava fields have a bluish haze, where the heat would change the solid snow back into a gas. ~ The haze is probably the condensation of the gaseous sulfur back into a solid. In some locations, the material apparently condenses into particles or crystals of sulfur flakes, and falls from volcanic plumes to the surface. 
| Galileo scientists are now studying whether heating of the volatile, sulfur dioxide-rich plains by encroaching hot lava might account for the persistent plume activity observed near Prometheus. |
"We see the bright volatiles being re-deposited up to about a kilometer (0.6 mile) away," said Moses Milazzo, also of the University of Arizona. Yet the snow blanket appears to be fairly uniform across the surface of the moon, and is not concentrated at the poles like the ice caps on Earth or Mars. Researchers are still unclear on why this is. ~ The structure of Io's volcanoes are also puzzling scientists, as the planet seems to break many of the rules by which volcanism is understood. Most interesting are the appearance of volcanic calderas on Io's mountain tops. Calderas are the pit-like scars of shallow lava eruptions. Unlike most volcanoes which build mountains, a caldera forms when a wide area of land collapses after being undercut by a wide pool of magma. 
| Planetary geologists aren't sure whether Io's paterae form over magma chambers or if they result from fractures and movements in the crust, and the lava subsequently follows the fractures. |
Io however seems to have no qualms about mixing the two volcanic "styles." The calderas, which are also less circular than similar structures on Earth, seem to be linked to cracks in the moon's crust. In one area, a string of the structures follows a line across the surface, taking a right-angle bend. The more traditional volcanoes of Io are still unearthly in their heat. The volcano Pele, named for the Polynesian fire goddess, reaches temperatures of about 2,700 degrees Fahrenheit (1,500 degrees Celsius) -- far hotter than the 2,192 degrees Fahrenheit (1,200 degrees Celsius) of a comparable Earth volcano. "One of the most interesting questions about Io is: do all Io's volcanoes erupt such hot lavas, or are most volcanoes similar to basaltic volcanoes on Earth?" asked Rosaly Lopes-Gautier, of JPL. In fact, the entire surface of Io might be one immense cooling field of lava, given the amount of heat the moon puts out. Even though the average surface temperature ranges from minus 297 to minus 288 degrees Fahrenheit (90 to 95 degrees Kelvin), that is far hotter than it should be if Io only absorbed its heat from the distant Sun.
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