Io's Entire Atmosphere Collapses Whenever It Hides In Jupiter's Shadow

An artist's impression of the volcanic plumes struggling to keep up a collapsing atmosphere on Io. SwRI/Andrew Blanchard

Io, the volcanic, hellish moon of Jupiter, has a problem. Its atmosphere, which consists primarily of sulfur compounds, has collapsed – like a punctured balloon, it appears to have rapidly deflated.

Remarkably, this isn’t the first time this has happened, nor will it be the last. Writing in the Journal of Geophysical Research, a team of researchers explain how they’ve observed Io’s atmosphere collapsing for two hours every single day on Io, which lasts for 1.7 Earth days.

Peering through the sizable Gemini North telescope in Hawaii, the team noticed that this cycle of collapse and inflation seemed to occur every time Io moved in and out of Jupiter’s shadow. When shrouded in darkness, Io’s atmospheric sulfur dioxide experiences temperatures of around -168°C (-270°F), down from the relatively toasty -148°C (-235°F) it bathes in in direct sunlight.

This rapid temperature plunge causes most of the atmosphere to freeze and rain down on the surface as a near-uniform blanket of sulfurous frost. When it moves back into the light, this frost sublimates (turns suddenly from a solid into a gas), soars back up into the sky, and “refuels” the atmosphere.

“Though Io’s hyperactive volcanoes are the ultimate source of the sulfur dioxide, sunlight controls the atmospheric pressure on a daily basis by controlling the temperature of the ice on the surface,” co-author John Spencer, a member of the Southwest Research Institute, said in a statement. “We’ve long suspected this, but can finally watch it happen.”

Although this atmospheric collapse sounds quite dramatic and frankly inconvenient to any visiting members of our own species, Io is ludicrously hazardous to life for a whole range of reasons. For one thing, its ephemeral, rarified atmosphere is actually so thin that the moon isn’t able to retain much heat – so at the surface, for the most part, you’d freeze to death within minutes.

Gif in text: A volcanic plume emerging from Io's Tvashtar volcano, as seen by NASA's New Horizons probe. NASA

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