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.