Remember that old sci-fi story, “A Pail of Air” by Fritz Leiber, where the Earth got stolen away from Sol by a dead star whizzing through, and everything froze solid and the Earth’s whole atmosphere snowed out of the sky? New research from the Southwest Research Institute shows that exactly that process is happening on Io, except on a daily basis.
Well, not the stolen-by-a-rogue-star part. None of that yet. But for two hours out of every day on Io, when Jupiter eclipses the Sun and its warmth, the moon’s surface temperature plummets from the positively summery -235F to a frigid -270F. The influx of solar energy stops. And Io’s atmosphere of sulfur dioxide falters, contracts, and then snows right out of its sky. When the solar eclipse ends, sublimation of all that SO2 snow pumps up the atmosphere again. It’s a “constant cycle of collapse and repair.”
Io may be the real-world equivalent of Dante Alighieri’s nethermost circle of Hell. Its surface is a brutally cold, dry place, covered in yellow sulfur and rimed with crystalline sulfur dioxide frost. Racked with constant eruptions, Io is the most volcanically active place in our entire solar system. Tidal heating drives the volcanism: There’s so much orbital momentum being flung around as Io orbits Jupiter that the tidal forces keep the moon’s interior hot, even as the monumental tidal swell cracks the crust and lets basalt lava spout hundreds of miles into the sky. And the high-energy radiation belt in Io’s path means that every day the moon is bombarded with about 36 Sv equivalent. For reference, the lifetime acceptable dose for a human is well under 1 Sv.
Every aspect of this moon is punishing in the extreme, but nobody knew its atmosphere worked quite like this. Before this study, we hadn’t actually even managed to make any direct observations of Io’s atmosphere while it was in eclipse. What made it possible now was the combination of the huge Gemini North telescope and the TEXES system, which can see the atmosphere using thermal imaging when visible-spectrum instruments were blinded by Jupiter’s glare. “Though Io’s hyperactive volcanoes are the ultimate source of the SO2, sunlight controls the atmospheric pressure,” said John Spencer, coauthor. “We’ve long suspected this, but can finally watch it happen.”
Now that Juno has arrived in orbit, there should be a steady stream of data forthcoming on Jupiter and its moons. Instrumentation on Juno makes it possible to get much clearer readings on the gravitational fields, electromagnetic activity, and physical environs around Jupiter, so we’ll know more in the near future. In the meantime, go read some good sci-fi!