Think your neck of the woods gets cold at night? On Io, one of Jupiter’s largest moons, the atmosphere turns to frost and collapses every time the orb passes into the planet’s shadow—for about 2 hours during each of the moon’s 42-hour orbits. Now, for the first time, scientists have observed the collapse—and recovery—of its atmosphere using far-infrared wavelengths. Io’s atmosphere (the reddish haze in the artist’s sketch, above), which is largely made of sulfur dioxide, originates from volcanic plumes scattered around the moon (depicted as blue fountains, above). Researchers had previously suggested that a substantial fraction of the tenuous shroud, which has a surface pressure about one-billionth that of Earth’s atmosphere at sea level, turns to frost for the 2 hours that Io lies in Jupiter’s shadow. But observations to prove the notion have been hard to come by, because the atmosphere can’t readily be seen at visible wavelengths when the moon is eclipsed. Now, astronomers have used a large ground-based telescope to observe the volcanic moon in far-infrared wavelengths, where certain ranges of radiation from the moon’s surface get absorbed by the sulfur dioxide in the atmosphere. As Io passes behind Jupiter, its surface quickly cools from 127 K (–146°C) to about 105 K (–168°C), a sudden chill that’s sufficient to cause about 80% of the atmosphere to turn into frost, the researchers report online today in the Journal of Geophysical Research: Planets . Then, when the eclipse is over and sunshine returns, the sulfur dioxide frost evaporates back into a gas. The new findings may help scientists better understand the rest of the Jupiter system, where about half the sulfur dioxide spewed from Io’s volcanoes eventually drifts to circulate around the planet.