When the Sun disappears during an eclipse, not only does the sky go dark, but the air gets cooler as well. The same thing happens on any world with an atmosphere. And on one world, the atmosphere doesn’t just get cooler. Most of it actually freezes.

Io is one of the largest moons of Jupiter, the biggest planet in the solar system. It takes Io a little less than two days to orbit Jupiter. It’s in eclipse for about two hours of each orbit, as it passes through the shadow of the giant planet.

Astronomers at the Southwest Research Institute studied those eclipses with instruments that are sensitive to heat. And they found that during an eclipse, Io’s atmosphere collapses. About 80 percent of it freezes and collects on the surface as frost. When the eclipse ends, the frost vaporizes and rejoins the atmosphere.

There’s no big “whoosh” when that happens, though, because Io’s atmosphere is only one-billionth as thick as Earth’s.

The atmosphere consists mainly of sulfur dioxide. It’s belched out by hundreds of volcanoes that dot the surface. The volcanoes are so powerful that some of the gas shoots hundreds of miles high. Some of it is captured by Jupiter’s magnetic field and swept out into space, where it forms a ring around the planet. But the volcanoes keep spewing out more sulfur dioxide — renewing Io’s thin atmosphere.

And Jupiter is in great view at dawn tomorrow. It looks like a brilliant star a whisker to the upper right of the crescent Moon.

Script by Damond Benningfield