The air on Io goes from gaseous to glacial whenever things get dark
On November 11, 1911, the daytime high in Oklahoma City was 83° Fahrenheit, while the nighttime low dropped to a mere 17°. This 66 degree drop probably felt rather shocking to anyone who spent their afternoon in shorts, but it’s still not as dramatic as the daily behavior of the atmosphere on Jupiter’s moon, Io. In that case, a nighttime shift of merely 35 degrees is enough to cause the entire atmosphere to freeze and collapse as a layer of frost across the moon’s volatile surface, only to thaw out once things have a chance to warm up in the sun again.
Since our relatively cozy atmosphere isn’t freezing solid each night, it’s clear that Io has a few other factors at work each night, starting with what the moon’s night looks like in the first place. As it orbits Jupiter, Io is anywhere from 460 to 507 million miles away from the Sun. That means that even in fully daylight, the moon’s atmosphere only gets up to -235° Fahrenheit. The air then cools in the dark, most of which is actually due to being in Jupiter’s shadow every 1.7 days, rather than simply rotating away from the Sun in the usual day/night schedule. Considering the relatively extended period of darkness, it’s interesting that the temperature only goes drops 35 degrees, although that seems to be enough to turn things to frost.
Part of the explanation for this particular threshold for freezing is the atmosphere’s composition. Io’s atmosphere is primarily made of sulfur dioxide, much of which is spewed in giant plumes from the moon’s many volcanoes. As the most volcanically active location in the solar system, there’s a steady supply of these noxious, acidic gases, which actually sublimate, skipping a liquid phase as they warm up each day. As the atmosphere cycles between gaseous and frosty, a fair amount of particulate finds its way into space, drifting into an orbit around Jupiter rather than Io itself.
Too frosty for life?
While the atmosphere on Io doesn’t seem like a very hospitable place, scientists haven’t completely discounted the idea that the moon could somehow sustain life. The volcanoes can heat pockets of the moon up to 3,000° Fahrenheit, which is a lot of energy that might keep things active enough below the surface for biology to function (where it would also be shielded from Jupiter’s radiation as well.) It’s an unlikely scenario, but microbes living in lava tubes on Earth may be a model for how to survive in a place with such an erratic atmosphere.
Source: Astronomers Watch as Io’s Atmosphere Collapses by Sarah Lewin, Scientific American