The eruption took place in February 2001, though image analysis was only recently completed by a team of University of California, Berkeley, astronomers. The group was co-led by postdoctoral research associate Franck Marchis and Imke de Pater, professor of astronomy and of earth and planetary science.

Their results are published in the November issue of the planetary sciences journal Icarus.

"It is clear that this eruption is the most energetic ever seen, both on Io and on Earth," Marchis said. "With the end of the Galileo mission, ground-based telescopes equipped with adaptive optics systems are the best tools for monitoring volcanic activity of Io. It is clear that future monitoring of Io's volcanism lies in the hand of terrestrial observers."

Adaptive optics employs a technique to remove the twinkle from stars, by flexing segmented mirrors fast enough to stabilize and focus the bouncing image created by turbulent air in the atmosphere.

Io, one of four large Jovian moons, is highly volcanic with high-temperature eruptions similar to those common on Earth, indicating a similar silicon-rich composition. The 2001 Io eruption was very close to Surt, the site of a large eruption in 1979 that took place between the Voyager 1 and Voyager 2 flybys.

"The Surt eruption appears to cover an area of 1,900 square kilometers, which is larger than the city of Los Angeles and even larger than the entire city of London," Marchis said. "The total amount of energy being released by the eruption is amazingly high, with the thermal output from this one eruption almost matching the total amount of energy emitted by all of the rest of Io, other volcanoes included."

The area covered by the Io lava is considerably larger than the entire cone of one of Earth's most active volcanoes, Etna in Italy, and far larger than Etna's most recent eruption in 1992.

"This eruption is truly massive," said Ashley Davies, PhD, a scientist at NASA's Jet Propulsion Laboratory who aided in modeling the eruption. "The observed energy indicates the presence of a vigorous, high-temperature volcanic eruption. The kind of eruption to produce this thermal signature has incandescent fire fountains of molten lava which are kilometers high, propelled at great speed out of the ground by expanding gases, accompanied by extensive lava flows on the surface."

Io's volcanism has been monitored for the last eight years by the Galileo spacecraft and now, with the advent of adaptive optics systems, by Earth-bound astronomers. Ground-based observations with an adaptive optics system, which produces very high-resolution images, provide a competitive alternative to the limited temporal and spatial coverage of Io by space missions, Marchis said. The spatial resolution is 105 kilometers (66 miles) per pixel, comparable to many infrared observations obtained by the Galileo Near Infrared Mapping Spectrometer from orbit around Jupiter.

"We were lucky to detect the beginning of an outburst eruption," de Pater said. "Thanks to the high-resolution capabilities of the adaptive optics system, it was possible to pin-point the location of the eruption, and the wavelength coverage allowed us to apply constraints to the nature of the eruption."

With a specialized infrared camera, the Keck telescope captured images of Io on two days, Feb. 20 and 22, at three different wavelengths. On the first day, Io was mostly quiet, with visible surface features such as dark calderas and relatively bright areas rich in sulfur dioxide frost. Two days later, however, what seemed a small hot spot on the surface had become a large bright eruption.

"We observed the same side of the satellite and were amazed to see a very bright eruption that had suddenly appeared," Marchis said. The UC Berkeley team quickly obtained data before Io entered the shadow of Jupiter.

The data were analyzed using advanced image processing techniques and a package called MISTRAL, developed by France's Office National d'Etudes et de Recherche Aerospatiales (ONERA) MISTRAL yields clear images of a quality comparable to observations taken above the Earth's atmosphere. The data showed that the temperature of the erupting lava was about 1,500 Kelvin, similar to that commonly seen on Earth at locations such as the Hawaiian volcanoes.

The investigative team consists of Marchis and de Pater of UC Berkeley, Davies of the Jet Propulsion Laboratory, UC Berkeley graduate student Henry G. Roe, Thierry Fusco of ONERA, David Le Mignant of the W. M. Keck Observatory, Pascal Descamps of the Institut de Mécanique Céleste, Bruce A. Macintosh of Lawrence Livermore National Laboratory and Renée Prangé of the Institut d'Astrophysique Spatiale.

This observational study of Io was supported by the France-Berkeley Fund, the National Science Foundation and the Technology Center for Adaptive Optics managed by UC Santa Cruz.

The W. M. Keck Observatory, located at the summit of Mauna Kea, provides astronomers access to two 10-meter optical telescopes, the world's largest. Each telescope features a revolutionary primary mirror composed of 36 hexagonal segments that work in concert as a single piece of reflective glass to provide unprecedented power and precision. Both Keck telescopes are equipped with adaptive optics.