AFTER five years of watching Jupiter鈥檚 volcanic moon Io, scientists have confirmed that it is a violent place. But they were surprised to find that most of the fireworks are confined to just a few places on the moon鈥檚 surface.
Paul Geissler from the US Geological Survey in Flagstaff, Arizona, and his team compared successive photos of Io, taken by the Galileo spacecraft between 1996 and 2001 as it explored Jupiter and its moons. Because there are no impact craters visible on Io, its surface must regenerate very fast, so the scientists expected to see changes all over the moon. But they report in an upcoming issue of Icarus that although they saw 82 separate events, including volcanic plumes, colour changes and sulphur dioxide seepage, the activity was restricted to just 17 per cent of the surface.
However, infrared measurements show that Io is losing heat elsewhere, too. Geissler concludes that its surface is also being replenished by lava quietly turning over within huge lakes, rather than by dramatic volcanic eruptions.
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The scientists also found that the volcanoes on Io come in two types. Some produce relatively small eruptions that cover areas up to about 250 kilometres across with near-circular rings of sulphur dioxide, tinged yellow by silicates. But there are also a few larger plumes forming oval rings some 500 kilometres or so across.