Jupiter's innermost large moon, Io, is one of the most dramatic bodies in the Solar System. It's the most volcanically active thing we've ever seen, and its surface is spotted with pools of liquid sulfur and punctuated by some of the largest mountains in the Solar System. The Jovian environment bombards it with intense radiation, and the giant planet's magnetic field sweeps away material from Io and sends it on to other moons.

Now, researchers have found evidence that the moon's thin atmosphere has its own dramatic behavior, condensing and collapsing to the surface whenever deprived of sunlight. In making this discovery, we've probably also learned where the atmosphere originated.

The atmosphere of Io doesn't make it the sort of place you'd want to draw a deep breath. For one, it's incredibly thin, so nothing like a deep breath would even be possible. But the kicker is that its major component is sulfur dioxide, which would react with the water in your lungs to form a strong acid.

Still, the atmosphere is there, which raises the question of how it's maintained despite the intense radiation and magnetic fluxes that might otherwise ionize it and strip it away. One obvious possibility is the sulfur-rich volcanism that reshapes Io's surface. This could provide a steady supply of sulfur-rich material at high enough temperatures to vaporize it, thus constantly replenishing the atmosphere.

If that were the case, it would follow that Io's atmosphere would be thickest over areas with high volcanic activity. Yet searches for this sort of localized atmosphere have produced ambiguous results. Instead, the atmosphere appears to be concentrated on the side farther from Jupiter. The atmosphere also experiences some variability based on which side of Io is receiving sunlight. This has led to the suggestion that the atmosphere may be refreshed when some of the sulfur on Io's surface simply evaporates or sublimates.

To get to the bottom of what's happening, an international team of researchers checked for a phenomenon called "atmospheric collapse." This phenomenon occurs when a body gets too cold for the primary components of its atmosphere to remain in a gaseous form. Once this happens, the components condense on the surface. If the atmosphere is dominated by a single type of gas, then the majority of it can collapse to the surface. Alternatively, if greenhouse gases freeze out first, further cooling can remove other gases from the atmosphere in a sort of chain reaction.

Atmospheric collapse is something that's generally considered hypothetical—something that might happen on an exoplanet or figure into Mars' distant past. But the team here decided to go searching for it on Io and got a pretty good tool to use in the search: Hawaii's eight-meter Gemini North telescope, with hardware that let them zero in on the emissions of sulfur in the moon's atmosphere.

On two consecutive nights, they observed the moon as it orbited from a location where it could receive sunlight to a position where Jupiter eclipsed the Sun. When this eclipse took place, they were able to watch the atmosphere's emissions essentially vanish. Modeling of the atmosphere's behavior indicated that the only way for this to happen is if the atmosphere collapsed, condensing out on the moon's surface.

If Io's atmosphere was being supplied by volcanic activity, then this sort of sudden transition shouldn't take place, since the volcanoes should be providing a relatively constant supply, regardless of whether some of it was freezing out. By contrast, if the atmosphere was supplied by areas on the surface that evaporated or sublimated off material, then the sudden chill caused by the eclipse should cut off this supply, as well as cause the atmosphere to condense out.

The overall picture is one where the atmosphere is in equilibrium with frozen sulfur dioxide on the surface, with sunlight driving cycles of expansion and collapse. But researchers also note that Hubble observations seem to indicate that the atmosphere doesn't simply spring back as soon as sunlight hits the moon again. So, we're still going to have to keep a close eye on Io if we're going to want to understand yet another aspect of this very strange world.

Journal of Geophysical Research: Planets, 2016. DOI: 10.1002/2016JE005025 (About DOIs).