Volcanic eruption on Io breaks the record. (Leapin' Lava!).

Within hours of erupting, the volcano shot fountains of lava high into the atmosphere and dumped molten rock over a region larger than London. The cataclysmic event, which occurred last year on Jupiter's moon Io, ranks as the most powerful volcanic eruption ever recorded in the solar system.

The enormous tidal forces exerted by Jupiter's gravity relentlessly flex Io and heat it up, making it volcanically active. The moon may experience a dozen or so massive eruptions each year. Even so, planetary scientists were surprised when one of Io's volcanoes pulled off the record-breaking eruption. The event, which took place on Feb. 22, 2001, was twice as powerful as any other eruption observed on the Jovian satellite. Researchers report their observations in the November Icarus, a planetary science journal.

Although the Galileo spacecraft has been touting Jupiter, Io, and Jupiter's three other large moons since 1995, the craft has only made short visits to Io's vicinity. So it was an Earth-based telescope, the Keck II on Hawaii's Manna Kea, that spotted the eruption. On Feb. 20, 2001, the moon appeared relatively quiet. But 2 days later, a small hot spot on the surface, near a volcano called Surt, had ballooned in size.

"We were lucky enough to detect the beginning of an outburst," says planetary scientist Imke de Pater of the University of California, Berkeley. She, her Berkeley colleague Franck Marchis, and their collaborators estimate that the heat from the eruption nearly equaled the heat emitted by all the rest of the moon's surface. That estimate, based on near-infrared observations from Keck, represents only a lower limit to the volcano's energy output, Marchis notes.

Surt lies about 45[degrees]N of Io's equator. That jibes with a volcanism model in which the most powerful eruptions on that moon take place at high latitude, says John Spencer of Lowell Observatory in Flagstaff, Ariz. The model suggests that the moon's crust may be thicker there or the lava that emerges may be pastier. Either feature could lead to eruptions that tend to be shorter-lived and more powerful than those near the equator, Spencer says.

Material from the record-breaking eruption appears to cover 1,900 square kilometers, an area about a thousand times the dominion of Italy's Mount Etna, one of Earth's most active volcanoes. The temperature of the lava, about 1,500 kelvins, is similar to that of the hottest volcanoes on Earth.

Astronomers credit the Keck telescope's adaptive optics with its success at pinpointing the Io eruption. In Keck's optics, segments of a mirror flex fast enough to compensate for the blurring caused by turbulence in Earth's atmosphere. With the Galileo mission drawing to an end, "ground-based telescopes equipped with adaptive optics are the best tool for monitoring volcanic activity on Io," says Marchis.