Jupiter's moon Io is truly a tumultuous place: sulfurous gases spew from its many volcanoes, scorching lava inundates its surface, and enormous mountains shoot up from its crust. Indeed, many of the mountains on Iosome reaching as high as 55,000 feetwould tower over Earth's Mount Everest. For some time, scientists have wondered how such mighty peaks could form in this environment. The heat released from Iowhich is 25 to 30 times greater per square foot than on our planetshould make for soft, squishy and relatively smooth terrain.

In the February issue of Geology, William McKinnon of Washington University and his colleagues suggest a new model for how the moon's mountains form, based on a new analysis of images taken by the Galileo and Voyager spacecrafts. "Two things work in concert," McKinnon says. "These are compressive stress, due to the general movement or sinking of the crust closer to Io's center, and thermal stress, which is generated when regions of cool crust suddenly become heated." The combination breaks up the crust, producing the irregular ranges. "Heat is actually trying to come out from deep in the interior of Io, but the crust is subsiding, or sinking, as new layers of lava are laid down," he adds. "For this heat, it's like tying to run up a down escalator."