After having its mission extended for two years to observe Jupiter's moons, time is finally running out for the intrepid Galileo spacecraft. So on October 10, tense mission scientists were ready to take some calculated risks. To obtain the closest images yet of Io, Jupiter's turbulent moon, they directed Galileo to make a flyby just 608 kilometers above the surface--an area of intense radiation that might destroy the craft's sensitive electronics.
Except for a few minor holes burned in its memory, Galileo fortunately came through unscathed. And it returned some of the sharpest images yet of the most volcanically active body in the solar system. The new pictures, which were released by NASA on November 19, reveal a fiery landscape swept with colossal lava flows, vast lava lakes, towering and collapsing mountains and more than 100 active volcanoes. "Io makes Dante's Inferno seem like another day in paradise," quipped imaging team member Alfred McEwen of the Planetary Image Research Laboratory at the University of Arizona.
Indeed, Io is hardly the kind of world that anyone would want to visit for long. One snapshot looking down into the vast caldera of Pele, one of Io's most active volcanoes, displays a glowing hot spot that has remained remarkably intense. Because most lava flows erupt and ooze outward, researchers speculate that this hot spot marks a lava lake that is constantly exposed to new lava through breaks in its crust. Pele's lava lake is 100 times larger than similar active lakes in Hawaiian volcanoes.
Another shot of Pele, taken at night, reveals a curving line of glowing dots that extend for 9.6 kilometers along the edge of the caldera. Scientists believe this line is liquid lava that becomes exposed as the solidifying crust breaks up along the caldera's walls. Lava solidifies quickly, so these puddles cannot be more than a few minutes old. A photograph of another extremely active volcano, called Prometheus, captures a plume of vaporized sulfur dioxide towering 80 kilometers above an advancing lava flow. Similar plumes were observed at Prometheus by the Voyager spacecraft in 1979.
Io is also marked by ranges of jagged mountains that soar to 15.8 kilometers (52,000 feet). These mountains do not appear to be of volcanic origin, and scientists are not sure how they form. But the new Galileo images provide an explanation of how they age and die. Smooth, rounded mountains that may be older show signs of huge landslides around their margins. These ridges bear a striking resemblance to the rugged terrain surrounding giant Olympus Mons on Mars.
The new data from Io is providing scientists with a window on similar volcanic activity that raged on Earth eons ago. The last comparable lava eruption on Earth occurred 15 million years ago, and it's been more than two billion years since lava as hot as that found on Io (reaching 1,480 degrees Celsius, or 2,700 degrees Fahrenheit) flowed on Earth. "Io is the next best thing to traveling back in time to Earth's earlier years," says Torrence Johnson, Galileo project scientist at NASA's Jet Propulsion Laboratory. "It gives us an opportunity to watch, in action, phenomena long dead in the rest of the solar system."
There may be more to come. On November 25, Galileo's controllers plan a riskier maneuver: Galileo will dip to just 300 kilometers above the surface and may provide researchers with an even closer look at the violent Jovian satellite.