An artist's rendition of Io's collapsing atmosphere. (Southwest Research Institute)

Not to get too partisan or anything, but Jupiter's moons are probably some of the coolest worlds in the solar system. Two of its largest — Europa and Ganymede — likely contain subsurface oceans, providing tempting spots to go looking for life. But while potentially habitable waters may be especially intriguing to us earthlings, Jupiter's innermost moon Io is nothing to sneeze at, either.

It's a world practically bursting with intense volcanic activity that has unique interactions with its host planet. Scientists think, for example, that these eruptions — which are the result of the intense gravitational energy Jupiter exerts on its moon — might somehow help create the extravagant auroral light displays detected on Jupiter. And now, a new study shows another unique connection between Io and Jupiter: Io's atmosphere collapses every time it passes into Jupiter's shadow, and scientists just watched it happen for the first time ever. They published their results Tuesday in the Journal of Geophysical Research: Planets.

The LORRI camera aboard NASA's New Horizons spacecraft captured these images of Io's Tvashtar volcano erupting. (Image: SwRI/Andrew Blanchard) (NASA)

Okay, so, let's pause. What does it mean for an atmosphere to collapse?

Scientists already knew that Io's grip on its atmosphere was tenuous at best, and they suspected that it might frequently be lost to space and recreated by volcanic activity. The mostly sulfur-dioxide atmosphere has a surface pressure just one billionth of the one on Earth at sea level.

Every time Jupiter passes between Io and the sun, casting the moon into cold darkness (from a balmy -235 degrees Fahrenheit in sunlight to -270 degrees in shadow, to be precise), that sulfur-dioxide gas cools and drops down to the surface, freezing into a solid. The atmosphere is collapsed and trapped as surface ice for about two hours each day on Io (note: A day on Io is equal to 1.7 Earth days, because that's how long it takes the body to rotate on its axis in relation to the sun. An "hour" is the same on any planet, because it's an arbitrary, purely human measure of time).

And after that two hours, the atmosphere is reborn: The (sort of) warmth of the sun casts itself upon Io's surface once again, causing the newly formed frost to sublimate, or heat from a solid back into a gas.

"This confirms that 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," John Spencer of the Southwest Research Institute, one of the study's co-authors, said in a statement. "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."

NASA recently inserted its new Juno probe into Jupiter's orbit, but the latest findings don't come from that spacecraft (which won't start returning data in earnest until later this month). Scientists used the eight-meter Gemini North telescope in Hawaii and the Texas Echelon Cross Echelle Spectrograph (TEXES) to confirm the atmosphere collapse. The fact that the phenomenon occurs during eclipse — when Io is shrouded in darkness — made it difficult to study with most telescopes, so scientists had to look for changes in the atmosphere's heat radiation signature instead.

But while Juno wasn't involved in the data collection, the new study shows just how much we stand to learn from NASA's probe.

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