Jupiter's volcanic moon Io has a thin atmosphere that collapses in the shadow of the planet condensing as ice, say NASA-funded researchers, revealing the freezing effects of its shadow during daily eclipses on the moon's volcanic gases. Io is the most volcanically-active object in the solar system. "This is the first time scientists have observed this remarkable phenomenon directly, improving our understanding of this geologically active moon," said Constantine Tsang, scientist at the Southwest Research Institute in Boulder, Colorado. The volcanoes are caused by tidal heating, the result of gravitational forces from Jupiter and other moons. These forces result in geological activity, most notably volcanoes that emit umbrella-like plumes of sulphur dioxide gas that can extend up to 480 km above Io and produce extensive basaltic lava fields that can flow for hundreds of miles. The new study documents atmospheric changes on Io as the giant planet casts its shadow over the moon's surface during daily eclipses. "Io's atmosphere is in a constant state of collapse and repair and shows that a large fraction of the atmosphere is supported by sublimation of SO2 ice," added study co-author John Spencer. Though Io's hyperactive volcanoes are the ultimate source of the SO2, sunlight controls the atmospheric pressure on a daily basis by controlling the temperature of the ice on the surface. "We've long suspected this, but can finally watch it happen," Spencer noted. The study, published in the Journal of Geophysical Research, was funded by NASA's Solar System Workings and Solar System Observations programmes.