Jupiter's colorful moon Io, the most volcanically active body in the solar system, has unleashed the most powerful eruption ever seen. Earth's largest optical telescope witnessed the blast, which will help scientists understand the forces that churn Io's interior into a hot mush.
Io orbits 350,000 kilometers above Jupiter's clouds, close enough for the giant planet to wrack the moon with fierce tides. Friction from the churning heats Io's insides, creating pools of magma. Magnesium-rich minerals in the magma might power the most intense eruptions, an exotic process that raged on the early Earth but ceased long ago. To fully decipher how this rare volcanism works, researchers need more observations of violent events. The Galileo spacecraft, near the end of its 7-year mission around Jupiter, witnessed a few of these outbursts but can no longer help.
Fortunately, telescopes on Earth now see Io's volcanos by using adaptive optics, in which flexible mirrors erase the blurring of Earth's atmosphere. Astronomer Franck Marchis of the University of California, Berkeley, and his colleagues were monitoring Io at the 10-meter Keck II Telescope on Mauna Kea, Hawaii, when they got lucky: A small hot spot on 20 February 2001 spread into a colossal eruption two nights later. The team's infrared images, described in the November Icarus, show that the eruption covered 1900 square kilometers. It also poured out 78,000 gigawatts of heat--six times more violent than any other Io volcano and thousands of times the heat from any recent eruption on Earth. The team calculated that the lava was at least 1200°C, close to the 1400°C at which molten magnesium should power the fiercest blasts. The center of the eruption--beyond Keck's ability to resolve from Earth--might have reached that temperature, Marchis notes.
The images are "a spectacular development," says planetary scientist John Spencer of Lowell Observatory in Flagstaff, Arizona. "These are better than most of Galileo's data." Spencer notes that Io's biggest eruptions--including this event--have struck at the moon's high latitudes, rather than near its equator. That favors a model in which much of Io's heat rises from deep within the moon and emerges near the poles, rather than spreading uniformly from shallower reservoirs of magma. Ground-based telescopes need to catch more eruptions to be sure, Spencer says.