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 snow. When it moves back into the light, this snow 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

Far from being a frigid, snow-covered wasteland, though, Io is a synthesis of ice and fire. Thanks to the moon's celestial dance with Jupiter and the nearby moons of Ganymede and Europa, a powerful gravitational force acts on its innards, creating tides within the rock in the way our own Moon creates tides in Earth’s oceans.

This so-called “tidal heating” mechanism manufactures vast volumes of magma, which makes Io the most volcanic object known to science. At present, it is covered in at least 400 active volcanoes, many of which produce lava flows hotter than 1,300°C (2,400°F) that reach lengths of up to several hundred kilometers. Clearly, these would quickly kill any clumsy astronauts that stood in their way.

Thanks to a combination of relatively explosive volcanism, a weak gravitational field, and an incredibly thin atmosphere, eruptions can produce iridescent fire fountains that reach heights of nearly 500 kilometers (310 miles) – around 57 times the height of Mount Everest. Many of these plumes resupply Io’s dynamic atmosphere with its sulfur dioxide.

On occasion, though, these columns of lava often breach the outer shell of the moon’s atmosphere and dissipate out into space as beautiful, sulfur-filled, blue, umbrella-shaped formations. This ejected sulfur is then whisked away by Jupiter’s powerful magnetosphere to the tune of about 0.9 tonnes (1 ton) per second.

Ultimately, this fuels a highly energetic ring of particles called the “plasma torus” that emits a ridiculous amount of ultraviolet radiation, fatal to any form of life within mere moments of encountering it.

So don’t worry about the atmospheric collapse, budding astronauts – from a shield of radiation-pumping doom to lava flows spewing out onto a deathly cold crust, there are plenty of other things there that would kill you first.

Image in text: Auroral glows in Io's upper atmosphere. The blue-purple color making up most of the aurora comes from the volcanically-produced sulfur compounds. NASA/JPL/University of Arizona

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