Io’s atmosphere just collapsed, and according to astronomical observations, this isn’t even unusual. According to a study published in the Journal of Geophysical Research, every time Jupiter eclipses Io and blocks its access to the Sun (for about two hours, every day), the surface temperature plummets and the moon’s sulfur dioxide (SO2) collapses.

Artist’s concept of Jupiter’s volcanic moon Io, whose volcanoes create an ephemeral atmosphere during sunlit hours. Image: Southwest Research Institute

Io is one of the most hellish places in the solar system. Jupiter’s moon is the most volcanically active place around the Sun, and that’s not all of it. It’s a cold, frigid place and with a toxic, sulphurous atmosphere. As if that wasn’t even enough, now we know that its atmosphere collapses every single day.

“This research is the first time scientists have observed this phenomenon directly, improving our understanding of this geologically active moon,” said Tsang, a senior research scientist in SwRI’s Space Science and Engineering Division.

The findings were presented in a paper bluntly called “The Collapse of Io’s Primary Atmosphere in Jupiter Eclipse,” in which astronomers used the eight-meter Gemini North telescope in Hawaii and the Texas Echelon Cross Echelle Spectrograph (TEXES).

What basically happens is that for about two hours every day, Jupiter passes between the Sun and Io, blocking sun waves. Without this access to the Sun, Io starts to cool, and fast. Data showed that temperatures drop from -148 degrees Celsius (-235 Fahrenheit) in sunlight to -168 degrees Celsius (-270 Fahrenheit) during eclipse. By the time this cooling fully takes place, the atmosphere is like a punctured balloon – a thin coating around the planet. But as Jupiter’s shadow fades away, the atmosphere starts to re-sublimate, and a new atmosphere develops. Every single day.

“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,” study co-author John Spencer said in a statement. “We’ve long suspected this, but can finally watch it happen.”

It’s interesting to note that although Io’s sulphurous atmosphere is mostly produced by volcanoes, temperature still has a critical impact.

Prior to the study, no direct observations of Io’s atmosphere in eclipse had been possible because Io’s atmosphere is difficult to observe in the darkness of Jupiter’s shadow. The breakthrough was possible due to TEXES. The spectrograph measures the atmosphere using heat radiation, not sunlight, and the giant Gemini telescope can sense the faint heat signature of Io’s collapsing atmosphere.